Handbook for photographic meteor observations

International Meteor Organization MONOGRAPH No 3 HANDBOOK FOR PHOTOGRAPHIC METEOR OBSERVATIONS J¨ urgen Rendtel 2 ...

Author: Jurgen Rendtel.

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International Meteor Organization

MONOGRAPH No 3

HANDBOOK FOR PHOTOGRAPHIC METEOR OBSERVATIONS

J¨ urgen Rendtel

2

IMO Photographic Handbook

Handbook for Photographic Meteor Observations ISBN 2-87355-002-3 Print: André Gabriël, Edegem, Belgium c °1993, 2002 The International Meteor Organization Potsdam 1993, original handbook production by J¨ urgen Rendtel Utrecht 2002, minor textual changes by Marc de Lignie No part of this book may be reproduced in any form without written permission from the publisher.


3

Foreword Observational meteor astronomy differs from most other types of observational astronomy in that the events of interest can not be predicted to occur in a given direction or at a precise time. Although the major meteor showers perform more or less reliably each year, there is still a considerable element of chance if the observer has a limited time to devote to observations. Perhaps it is just this unpredictable nature that makes meteor observing so attractive to many enthousiasts – that spectacular fireball, a new meteor shower, or a true meteor storm from a classical shower, might just occur the next time we observe. As is clearly explained in this Handbook, the amateur astronomer is sometimes in a position to make observations of unique importance. Photography has been the traditional means of securing permanent records of meteoric activity and it remains a major technique today. The equipment to be used and the observing techniques require considerable planning, however, plus a basic understanding of meteor astronomy, if the best results are to be achieved. In the pages that follow, the elements of photographic meteor astronomy are carefully explained. Whether the observer chooses to develop a simple observing facility or a complex one with sophisticated controls, the Handbook will steer him in the right direction. Experience may then indicate areas in which the observer can improve his productivity. Meteor astronomy remains one of relatively few fields where the amateur can make a real contribution to our knowledge of the solar system. The probability of success is greatly increased, however, if the observational program has been planned with care.

Ian Halliday, Ottawa, Canada.

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Table of Contents

INTRODUCTION

7

1. 2. 3. 3.1. 3.2. 4. 4.1. 4.2. 4.3. 4.4. 5. 5.1. 5.2. 5.3. 5.4. 6. 7. 8. 9.

PART 1: FAINT METEORS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lens and film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The limiting magnitude LM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The efficiency number E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Handling black and white photographic material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composition of films and plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification according to speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selection of film types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Warming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Precise timing of the exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rotating shutter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development of the exposed material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Your archive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photographic observations during a meteor storm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of a video camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 9 10 11 11 13 13 13 15 15 16 16 16 17 18 19 21 22 23

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

PART 2: FIREBALL PATROLS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Film and exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cleaning of the optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time of the fireball’s appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical hints for patrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summarizing the equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Handling the exposed films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The archive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fireball networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fireballs on video . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25 26 28 29 31 31 32 33 33 34 35 35


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1. 2. 3. 3.1. 3.2. 3.3. 3.4. 3.5. 4.

PART 3: METEOR SPECTRA A brief historical review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of meteor spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How to build a meteor spectrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The camera and dispersing element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The prism and its mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The orientation of the spectrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Double station work and spectroscopic photography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary of equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of meteor spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37 37 38 38 39 40 40 40 41

1. 2. 2.1. 2.2. 3. 3.1. 3.2. 3.3. 4.

PART 4: METEOR TRAINS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regular trains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Smoke trains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regular trains at night . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dust trains in daylight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dust trains during twilight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of a video camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50 50 50 51 53 53 53 53 53

1. 1.1. 1.2. 1.3. 2. 3. 3.1. 3.2. 3.3. 4.

PART 5: ADDITIONAL EQUIPMENT AND CONSTRUCTION HINTS Rotating shutters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reasons for the use of a shutter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Construction of a rotating shutter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lens heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The spectrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introductory concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The prism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The diffraction grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mounting and drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55 55 55 56 59 60 60 61 62 63

1. 2. 3. 4. 5. 6.

PART 6: DOUBLE STATION CAMERA WORK Computation of the direction of the optical axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intersection of camera fields at a given height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Computation of the region of sky photographed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optimising the camera fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simulating meteor trails for tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65 66 71 71 72 76

1. 1.1. 1.2. 2. 3. 4.

PART 7: MEASURING POSITIONS ON PHOTOGRAPHS Measuring prints of meteor photographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identification of a print . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Format of the prints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measuring original fims or plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The measuring work itself . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78 78 79 79 79 81

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1. 2. 3. 4. 5. 6.

PART 8: PHOTOMETRIC MEASUREMENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Images of stars and meteor trails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The photometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of object colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peculiarities of meteor trail photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83 83 84 85 86 87

1. 2. 3. 4.

PART 9: SHORT GUIDE Faint meteors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fireball patrols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meteor spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meteor trains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

90 91 92 93

1. 2. 3.

PART 10: EXAMPLES & APPENDIX Meteor showers of particular interest for photographic work . . . . . . . . . . . . . . . . . . . . . . Photograph section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

94 96 104


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INTRODUCTION

Meteors are fast moving atmospheric phenomena of short duration. When choosing an appropriate observing method, one has to bear in mind this fact. Currently there are several different methods developed and in use: OPTICAL • Visual observations – naked eye – telescopes/binoculars • Photographic observations • Video observations RADIO • Meteor forward scatter • Radar (meteor backscatter) These methods have advantages and disadvantages and require differing amounts of effort and equipment. Some examples will help to illustrate this point. The cheapest method is the visual observation, using no instrumentation except a precise watch and some means of noting the observed facts. Unfortunately, our brain cannot store the meteor data such that their appearances can be correctly repeated later. This is the main source of uncertainty, affecting both position and brightness estimates. On the other hand, video observations, for instance, require expensive equipment but do allow a more complete analysis of all meteors recorded. All the methods that rely on the optical emission of a meteor are dependent on a clear sky and a low amount of scattered light, but are generally restricted only to nighttime observations. The alternative to this is to use the radio range of the electromagnetic spectrum in place of the optical. Meteor forward scatter observations are somewhat easy to make despite weather or daylight, but generally will not give information about the meteor’s direction or shower association. Also, the reflections depend on complicated geometrical conditions being met between the transmitter and the receiver. An alternative method in the radio range does exist in the form of radar (backscatter) observations. This technique may yield information about shower association and direction amongst other things, but it requires very expensive and technically advanced equipment and special permission to use the appropriate frequencies. Details about the methods briefly mentioned here can be found in other handbooks of the IMO. This text deals with photographic work and the advantages of photographic observations.

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These advantages include the facts that the meteor trails are permanently recorded, information is simply stored, and because the information about the use of photography for astronomical purposes is known. In this book you will find a description of suitable equipment for meteor photography, some suggestions and practical hints for photographic meteor observations and some guidelines for the analysis of successful meteor photographs. The four main topics covered are: (1) Photography of faint meteors in a small field of view by common camera types (2) Photography with all–sky cameras, fish–eye and wide–angle lenses for fireball patrols (3) Photography of meteor spectra (4) Photography of persistent trains For all these methods you will find details on the following covered in this handbook: • cameras • lenses • films • recommendations • additional equipment • practical work • handling of materials Besides these subjects which are dedicated to the practical side of the work, you will find information about the measurement and analysis of data from the photographs, including: • Construction hints for additional equipment recommended earlier • Calculations concerning double–station work • Measurement of positions • Photometric measurements We do not delve into topics where other IMO handbooks cover material in detail, such as astrometric measurements and calculations. Also, we will avoid describing technical solutions that require special equipment or electronic circuits that are not generally available. This may be discouraging to a few advanced amateurs with special skills, but it would be unfair to the average amateur to show devices that work only under certain circumstances. In this respect we will use an approach that gives a description of the task and one example of how to solve the problem, though other solutions do exist. Finally, we will include a list of references and additional bibliographic information that should enable the reader to gain more insight on the techniques that are described in the text and provide greater background knowledge on the topic of meteors in general. We hope you will find this book helpful in your photographic meteor work, and look forward to welcoming your results into the IMO’s Photographic Commission databases. Good luck!

Part 1: Faint Meteors

9

PART 1: FAINT METEORS

1. Introduction The naked eye is able to detect meteors down to approximately +7m under excellent circumstances in the vicinity of the center of the field of view. Video techniques permit detection of meteors to +8m , and radar meteors as well as telescopic ones may be as faint as +11m . Photographic methods can hardly compete with these. Nevertheless it is possible to reach ≈ +4m with modern photographic systems, such as Super–Schmidt cameras or very fast lenses combined with fast photographic emulsions. The photography of faint meteors is necessary for several different goals: – the derivation of precise radiant positions for meteor showers – the determination of meteor atmospheric trajectories (heights, distances, velocities) – the calculation of meteor orbits – confirmation of meteor activity from suspected radiants – activity determination in the rare case of meteor storms But first it is necessary to get a photograph which allows positional measurements, not merely a nice looking picture!

2. Camera The camera body has no great influence overall, since it is effectively only the film holder. Of course, the camera must allow time exposures and the easier it is to handle in the dark the better it is suited for astronomical purposes. In particular, problems may occur with cameras driven by electronics. In this case, it will probably be necessary to switch off most automatic controls (consult the instructions given in the manual first). Long exposures or cold winter nights may cause the operating limits to be exceeded as well. Long exposures are possible only if the camera shutter has either a “B” or a “T” position. (“B” implies that the shutter is open as long as you push the release button.“T” opens the shutter once you push the release button, and it remains open until you push it again.) If you use “B” on your camera, you will need a lockable cable release, while “T” allows exposures without such a device.

10


Sometimes a cable release may cause troubles. Some cable releases are badly affected by humidity or cold, and should be tested before each use. The exposure might otherwise be ended due to a malfunction at an unknown time, and a meteor missed. An alternative is a fixed release which you can mount in the same manner as a cable release, but without a cable (Fig. 1-2). As the conical camera thread is difficult to produce yourself, you might find suitable components for such a device in your local photo shop or you may be able to obtain it from other advanced amateurs.

Figure 1-1: Typical equipment needed to start meteor photography. It consists of a camera (easy to handle in the dark) with a fast lens (see section 3 for details), a stable tripod, and a cable release.

Figure 1-2: Construction of a fixed release as used for many cameras.

3. Lens and film The number of meteors that can be photographed depends on the properties of the film and of the camera. In this section we will quantify this number based on the following two new concepts: – The limiting magnitude (LM ) of the lens for meteors, in other words the magnitude of the faintest photographable meteor. – The efficiency number (E) of the lens. This number allows the comparison of different optics, and defines an expression for the theoretical photographic hourly rate.


11

3.1. The limiting magnitude LM The limiting magnitude is dependent upon the sensitivity of the emulsion being used (g). It is not very useful to try to photograph faint meteors with an ISO 100/21◦ film (ISO is the international standard way of rating films, described in detail in section 4 on pp. 13-14.), even where a very fast lens is available. On the other hand, as the sensitivity of the emulsion increases, the film’s resolution and grain structure get worse. For many films the optimal combination occurs at ISO 800/30◦ , although very recently produced films give excellent results at ISO 3200/36◦ ˙ The focal length (f ) of the camera lens also plays an important role in determining the limiting magnitude. As the focal length increases, the field of view becomes narrower and hence the light of the meteor passes faster over the emulsion grains, so the exposure time per grain decreases. We can state that the LM is inversely proportional to f . Finally, there is one further important factor: the amount of luminosity which enters the camera. The amount of luminosity will depend on the surface area of the lens, hence on the square of the effective diameter of the lens (d2 ). The optical data for a lens is given through its focal ratio, or aperture number r = f /d and its focal length f as in “f /2, f = 50mm”. This means that r = f /d = 2, or, d = f /r = 25mm. Putting all these factors together in one equation we get: sensitivity ∝

g · f −1 · d2

(1)

where: g the sensitivity of the emulsion (first number of ISO noting) f the focal length of the lens (e.g. 50 mm) d the effective diameter of the lens (in mm) Many published relationships are based on investigations by Hawkins (1964). In practice the following formula can be used to determine the limiting magnitude for meteors in pure white light (only valid under perfect sky conditions): LM = 2.512 · log10 (d2 · f −1 · g) − 9.95

(2)

For example let us consider a “normal”lens f /1.8, f = 50mm which is widely used. Its effective aperture is d = f /r = 27.8mm. Used with an ISO 400/27◦ -film we calculate LM = 2.512 · log10 (27.82 · 50−1 · 400) − 9.95 = −0.43 or, for an ISO 800/30◦ -film we find LM = 2.512 · log10 (27.82 · 50−1 · 800) − 9.95 = +0.33. More values for several lenses are summarized in Table 1-1.

3.2. The efficiency number E At a fixed limiting magnitude (LM ) the efficiency number of a camera will only depend on the size of the camera field A, expressed in square degrees. The larger A is, the higher the probability of catching a meteor. First, we determine A by use of the following formula: µ

a A = 2 arctan 2f

¶

µ

b · 2 arctan 2f

¶

(3)

where a and b are the dimensions of the negative (plate) and f is the focal length of the lens, both given in mm. Let us call X the number of meteors that a camera photographs. When a camera with an f /d = 2.8, f = 50mm, abbreviated as f /2.8, f = 50mm lens photographs exactly 1 meteor then another lens with parameters f, d will catch X meteors: X = A · (d2 · f −1 )1.21 · f 0.056 · 10−4

(4)

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It is obvious that this number X will be an important value for the photographer. The number X will allow the photographer to decide which lens offers the highest probability of photographing a meteor. One final formula will yield the photographic sporadic hourly rate N . This relation is again from Hawkins (1964), p. 31: N = A · (d2 · f −1 )1.35 · f 0.056 · 4 · 10−6 (5) Formulae (2), (3), (4) and (5) were used to compile Table 1-1 below (for g = 800). The values given are valid for sporadic activity during “average” nights (in fact, the sporadic activity level varies during the seasons). For the lens characteristics a choice was made which coincides with the equipment used by most IMO members. For comparison we also mention the characteristics for a Super–Schmidt camera. Its construction is described in several books about meteor astronomy, for example in Hawkins (1964). All these equations can not give precise figures but allow the comparison of different lenses and the choice of the most appropriate one. Furthermore, you may get a guess for the success rate. Table 1-1: Camera characteristics; LM is given for an ISO 800/30◦ -film. The value for the 80 mm diameter field (in the first column) refers to the full field of a fish-eye lens (entire sky). Because of the inclusion of near horizon regions an enormous volume of the atmosphere is photographed, but with large extinction and meteor distance from the camera, the values X, N are uncertain for such lenses.

24×36 mm2

58×58 mm2

80 mm ® 6×9 cm2 Super–Schmidt

f mm 28 35 35 50 50 50 75 30 50 75 75 80 30 105 200

f · d−1 2.8 1.8 2.8 1.4 1.7 2.8 4.5 3.5 4.0 3.5 4.5 2.8 3.5 4.5 0.65

d mm 10.0 19.4 12.5 35.7 27.8 17.9 16.7 8.6 12.5 21.4 16.7 28.7 8.6 23.3 307.

LM −1.3 −0.1 −1.0 +0.9 +0.3 −0.6 −1.2 −1.7 −1.4 −0.7 −1.2 −0.1 −1.7 −0.9 +4.1

A sq.deg. 3038 2060 2060 1069 1069 1069 491 7754 3627 1788 1788 1588 20625 1480 2380

X

N

2 4 2 7 4 1 0.3 2.8 1.8 2 1 3.4 7.4 1 553

0.08 0.24 0.07 0.41 0.24 0.06 0.014 0.126 0.084 0.11 0.06 0.19 0.334 0.07 50.7

Within a certain range of focal lengths (15mm . . . 80mm) we may compare the effectiveness of different lenses using the simplified measure E, defined as: E = d2 /f

(6)

where d is the linear aperture of the lens, and f is the focal length, both in mm. If you prefer to use the aperture number r, you may write: E = f /r2 (7) Fast lenses, such as f /2, can attain good limiting magnitudes and modern lenses are in most cases of very good optical quality allowing the required astrometric measurements to be made. Another influence on the rate has not yet been discussed: the direction of the camera. Let us assume an isotropic entry of meteoroids into the Earth’s atmosphere. Meteors in the zenith will then appear brightest because of the smallest distance to the camera. For larger distances, i.e. lower elevations in the sky, the intensity decreases with the square of the distance. Next, the extinction increases towards the horizon when the light transmits larger amounts of the atmosphere. On the other hand,


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the camera photographs a much larger volume if directed to lower elevations. These influences are discussed more quantitatively in section 5 of Part 6 (Double Station Work). Visiting a photographic shop, you will find a great variety of very different films. First, you should bear in mind that the positional measurements are the main purpose of meteor photography. Another item of importance is the brightness of the meteor. The latter can only be derived from black and white negatives, not from color material (either slides or negatives!). Therefore you should preferentially select black and white films. For details on this see section 4 of Part 8 (Photometric Measurements).

4. Handling black and white photographic material Most astronomy handbooks mention astrophotography, but the basic concepts concerning photographic processing are not well covered. Most meteor observers are unfamiliar with photography, and will need to know something about the subject before they can apply it to meteors. Beginners may wonder which materials will be needed and how to get the films developed.

4.1. Composition of films and plates Most amateurs use standard, roll or plate film for meteor photography. These plates or films are composed of a transparent carrier (4) on which a light sensitive emulsion is moulded (2), as shown in Fig. 1-3.

Figure 1-3: Composition of films. The numbers of the layers are explained in the text.

This layer is protected against scratches by a thin layer of hard gelatine (1). Fast films are usually produced with two emulsion layers of different compositions to increase the dynamic range of the exposure span. Before the emulsion layers are moulded, the carrier is covered with an intermediate gelatine layer (3) to improve the contact with the emulsion layers. This is called the substrate layer. Finally we have the anti-halation layer (5), which prevents reflections back from the film base. The emulsion layer itself consists of gelatine and light sensitive salts. The grain structure and distribution of these salts define the photographic characteristics of the film (fast or slow.)

4.2. Classification according to speed In the gelatine of the light sensitive layer we find silver halide crystals of varying sizes. Mean silver halide particle sizes range from ≈ 0.05µm for some special high resolution products to about 1.1µm for a common amateur film and about 1.7µm for a medical X-ray film. To “expose” one of these crystals we need a certain number of photons, and thus a certain amount of light. The larger these light sensitive crystals, the higher the chance of collecting the necessary number of photons and the less light energy required to turn them black, thereby creating a latent image “faster”. The action of just a few photons on the grains renders each of them able to oxidize certain weak reducing agents in aqueous solution, the grains themselves being reduced to metallic silver, thus forming the negative image (this

14


is the reason for the relatively high price of black & white films). The chemical effect of these few incident photons is thus amplified by factors of the order of 109 . The detector quantum efficiency, however, is rather low compared to other optical detectors. It currently lies in the neighborhood of 20% at best. The grain structure of a “fast” film is rough because of large crystal sizes, and therefore the resolution of fast films is poor. Of course, the precise parameters vary between different manufacturers. Fortunately these highly sensitive materials also have advantages such as long possible exposure time-spans. This means that a large difference in intensity can be transformed into differing degrees of blackening as shown later in Fig. 1-5 and described in section 6, p. 19 ff.. A more fine-grained distribution makes the emulsion less sensitive. For these “slow” films, more light is required to create an image, and the possible exposure time-span is also much smaller. With the exception of fireballs, meteors are relatively faint objects. Therefore it is of no use to work with slow films in meteor photography. Using the exposure times common for mean daylight exposures we can make the following sensitivity classification, where ISO stands for “International Standards Organization”, ASA “American Standards Association” (which both use identical numbers) and DIN “Deutsche Industrie Norm” (which works with a different numbering system). The ASA and the DIN use different definitions for the sensitivity of photographic materials. The ISO recommendations include the most appropriate methods for current techniques based on experience obtained with these different methods so far. For example, the ISO standard utilizes the standard method of development from America, the record of a specific adopted contrast determination from Russia, and the determination of the sensitivity-point on the characteristic curve (0.1 above fog) from the German standard. Table 1-2: Classification of films according to their sensitivity.

very low sensitivity material low sensitivity material medium sensitivity material high sensitivity material very high sensitivity material

ISO ISO ISO ISO

up to ISO 12/12◦ to ISO 50/18◦ 64/19◦ to ISO 160/23◦ 200/24◦ to ISO 500/28◦ 650/29◦ and higher 16/13◦

The fine grained, slow emulsions are much more capable of distinguishing details which are close to one another than fast films with their rougher grains. In this regard we speak of a film’s resolution and we define this as a measure of the number of lines per millimetre (ln/mm) that can be distinguished separately under ideal circumstances. The typical resolution of some types of black-white negative emulsions are given below (you will find differences from one type to the next, as well as with the kind of development used). Table 1-3: Resolution of different films, average values.

emulsions for measure-technical goals slow small-frame camera films medium-fast film fast film very fast film

(ISO 25/15◦ ) (ISO 100/21◦ ) (ISO 400/27◦ ) (ISO 1000/31◦ )

2000 350 120 90 75

ln/mm ln/mm ln/mm ln/mm ln/mm


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4.3. Selection of film types Today, highly sensitive films (ISO 3200/36◦ ) with reasonable grain structure (such as the so–called T–grain; T standing for tabular) are available. The trade names for these vary from country to country and also change with time, so it is of little use to sum up all the possible sorts of films presently available. It is up to the photographer to decide which film to buy: always select the fastest film with as fine a grain structure as possible, within your own budget. The general characteristics and advantages have been described above. Manufacturers will usually be able to provide further technical details regarding their own products. Do not store films for long periods because ageing processes – even under favourable storage conditions – reduce sensitivity and contrast, and such degradation will be seen first in situations where you need optimal performance.

4.4. The exposure The so-called panchromatic materials (used by meteor photographers) are sensitive to all visible light wavelengths, but not in the same degree for all colors. As this is the case, the distinction between different colors in terms of different grey levels does not appear on the film. In order to be sensitive to the light of a given color, the silver chemicals in the film must be capable of absorbing the light of that color. A typical problem in meteor photography is the way a photographic image is created during the exposure. In ‘normal’ pictorial photography the photographer exposes the film for a length of time which, from experience, is known to produce a suitable image. A meteor, by contrast ‘writes’ its image through the camera lens on the emulsion very quickly. Hence, the final exposure time is not only dependent on the light sensitivity of the camera’s main lens, but also on the speed with which the projected image of the meteor goes over the grains of the emulsion which is synonymous with the speed at which the meteor moves through the camera field. The faster the motion, the less time the silver grains get to collect the light and thus build up a latent image. In order to expose photographic material there is always a minimum of light required. This may be much light in a short time, or very little light collected over a longer period. At a low light level, however, longer exposure times are required as the film becomes less sensitive to a given light source as the exposure progresses. This is the Schwarzschild-effect. According to the reciprocity law found by Bunsen and Roscoe (1862), the chemical process in film should lead to the same blackening S if one exposes a film with a high intensity I for a short time t or with a weak intensity for a longer time: S = f (I · t) as long as the product H = I · t remains constant. But this is not the case. The photographic effect of light on an emulsion depends not only on the total number of photons received by each silver halide grain, but also on their rate of arrival. This effect is not normally apparent when exposure times are between 0.001 s and 1 s. Outside this range, for example with high intensity short duration flashes or the low light levels of astronomy, a breakdown of the reciprocal relationship between exposure time and illumination occurs. Any emulsion exposed to a high light intensity for a short time (> 1 s) is blackened more than another exposure with a weaker intensity but for a longer time such that they both have the same product H. This is described as low intensity reciprocity failure (see e.g. Malin, 1982). The astronomer Schwarzschild proposed the expression S = f (I · tp ) with the Schwarzschild-exponent p ≤ 1 (Schwarzschild, 1900). For most of the films, p ≈ 0.7 . . . 0.95. It is not a real constant, but depends on the material, the intensity range and the wavelength in question. For X-rays p = 1, for very small amounts of light p → 0. For meteor photography this effect is only partially relevant: the exposure due to a meteor is in the

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order of 10−4 s for a certain grain, and is thus very short. On the other hand, stars and background light expose a given grain for a much longer time. Therefore, the emulsion becomes less sensitive for these sources exposed to the film for long periods of time while meteors will be affected less. This is most important for the long duration fireball patrol photographs (cf. Part 2, Fireball Patrols). These very different exposure durations make it difficult to compare the blackening due to stars and background with those of meteors and thus to estimate meteor brightness by direct comparison with star trails. We return to this point in detail in Part 8 of this Handbook (Photometric Measurements). Table 1-4: Possible exposure times in minutes for different film materials as a function of sky conditions. The sensitivity measure is reduced to the first part of the ISO designation only.

very dark sky, no dust/haze dark sky, no scattered light clear sky, city lights distant hazy sky or nearer city lights

Exposure times for f /1.8, f = 50mm ISO 400 ISO 800 ISO 1600 ISO 3200 40 30 20 15 30 20 15 10 20 15 10 5 10 5 – –

For the purpose of photographing fainter meteors we choose films of high sensitivity, at least ISO 400/27◦ , though preferably ISO 1600/33◦ or even ISO 3200/36◦ . Such films cannot be used properly in lightpolluted areas, hence lower sensitivity films such as ISO 100/21◦ may have to be used to give reasonable results to keep the background light to an acceptable level. For optimal results you will have to experiment with exposure times and film speeds for a given site under different circumstances, but Table 1-4 can act as a good guide for these values. As a rule, during twilight or with moonlight interfering, meteor photography is not practical and you would do better to save the film for more favourable periods.

5. Additional equipment 5.1. Warming What additional equipment is necessary? At most sites you will often need a warming or other protective device for the lens to avoid dew. The idea behind heating the lens is to raise the temperature of the front lens and, more importantly, the air in front of it by some tenths of a degree. This makes the lens slightly warmer than the surroundings and serves to increase the dewpoint near the lens by a small amount. This requires the dissipation of only a few watts of power and it is advisable to use a low voltage power source to prevent accidents on damp nights.

5.2. Precise timing of the exposures If you guide the camera you should make control checks of the positional accuracy from time to time. For an unguided camera, the exact time of appearance of any photographed meteor is required. This is a quantity needed when reducing the measurements of the position of the meteor on the image, without which it is impossible to derive the exact position in right ascension. On a 20 minute exposure, the star trails are 5o long, giving some idea of the size of the error involved. This is unacceptably large for precise measurements. Therefore it is absolutely essential to note the beginning and end of the exposure as well as the appearance time of the meteors in or near the field of the camera with a certainty of at least ± 5 seconds. In only 10 seconds the sky rotates by 2.5 arcminutes in right ascension!


17

In the case of positional measurements it is important to know if clouds were present at the start or end of the exposure since the end points of the star trails are used for reference. Thus, if there are clouds at one or both of these moments, you should note at least 10 stars which were not covered by the clouds, or wait a few minutes to start/end the exposure.

5.3. Rotating shutter If you make astronomical photographs frequently, you will be familiar with different kinds of nonstellar trails. These might be caused by meteors, but also by satellites, airplanes, fireworks or other moving phenomena (even by somebody crossing the camera field with a cigarette!). Often it is hard to decide which object is which on the image. Some examples are given in Part 10 at the end of this Handbook. Careful observation of the camera field during the exposure should provide the necessary information, but an alternative way to overcome this situation is by the use of a rotating sector or rotating shutter usually located in front of the lens (Fig. 1-4). More details will be given in Part 5 (Additional Equipment). Normally such a shutter is constructed with two wings or blades. The interruption frequency once the blade is rotating should lie between 8 and 50 breaks per second. For positional measurements, particularly in the case of double station meteors (see Part 6 of this Handbook), the interruption frequency chosen will be dictated by the shower under investigation. A higher frequency is useful for shower meteors with a high entry velocity, such as the Perseids and Leonids, while a lower rotation rate is more appropriate for slow meteors, e.g. the α-Capricornids or the κ-Cygnids. A proper choice should produce enough shutter breaks in the trail of a fast meteor, while ensuring the breaks are well separated from each other for a slower meteor. In practice, this is often not achievable. For example, in August we have fast Perseids as well as slow meteors from the α-Capricornids, the Aquarid-complex and the κ-Cygnids all present at the same time. Essentially, the choice is made by deciding which shower is more important to observe at the time. In November, when both the Taurids and Leonids are active, a shutter suitable for Taurids giving a rate of about 15 breaks per second will leave a typical Leonid meteor trail divided into only five breaks. Consequently, a value of about 15-25 breaks per second is suitable for “average” velocity meteors, while about 25-50 breaks per second are recommended for showers like the Leonids. The very high shutter breaks are only useful in combination with very sharp optics.

Figure 1-4: Rotating shutter used for meteor photography.

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A rotating shutter interrupts the exposure periodically which in turn reduces the amount of background light and also decreases the density of the star trails, though these will still appear as continuous lines on the negatives. This will also be the appearance for nearly all satellite trails. By contrast, meteors move considerably during just one revolution of the shutter, hence meteor trails will appear as dashed lines on the photograph. The length of each segment of ”light” along the broken trail is proportional to the instantaneous angular velocity of the meteor at that point. The shape of the rotating shutter can be varied depending on the observational circumstances at your site. If you have a bright sky background, you may wish to use shutter wings with an angle of about 120◦ . In this case 13 of the meteor trail is exposed on the frame, while 32 of the trail is ”blanked” due to blockage by the shutter. This reduces the data available for meteor trail photometry (ie. to derive a light curve), but will not affect the accuracy of information needed to derive a radiant association. Conversely, under favourable circumstances it is desirable to use a rotating shutter with 60◦ wings (or even less). The disadvantage of this setup becomes apparent when bright meteors (fireballs) are photographed as the exposed parts become too bright and the light may ”spill-over” to the breaks. Unfortunately, the use of a rotating shutter will not eliminate all identification errors. You may find some trails of airplanes look like interrupted meteor trails, or that a rotating (blinking) satellite may cause a similar trail. Thus, even when using a rotating shutter one should visually watch the area covered by the camera while it is operating. Some examples of different non-meteor trails are shown in the appendix containing photographs (Part 10). Practical experience has shown that the use of synchronous motors for rotating shutters is a must. The frequency of such motors is only dependent on the power frequency (60 Hz in North America and 50 Hz in Europe). Even when these motors are used it is very important to occasionally check the speed of the shutter as they can be retarded by residues of oil and dust covering the axle of the motor. A good test for this utilizes a stroboscopic device. In this test, the shutter is lit by a lamp whose frequency can be changed continuously. By finding the flicker frequency of the lamp such that the shutter is seen to be fixed, you can find the shutter’s rotation frequency . Depending on the number of shutter blades, you will find several lamp frequencies which a fixed appearance – in this case the lowest frequency which leaves the shutter fixed is the true frequency. Finally it should be noted that a rotating shutter also helps prevent dew formation on the lens by circulating the air in front of the lens. However, do not rely on this and use proper lens heating.

5.4. Getting started Appropriate equipment should include: (1) A camera which allows long exposures, is easy to handle in the dark and which is not influenced by dew and cold (2) A fast lens (for example f /1.8, f = 50mm) (3) A warming device for the lens (4) A rotating shutter with a synchronous motor giving about 15 breaks per second; for meteors of high velocity showers, such as the Leonids, 25 breaks per second During the observation you should note the following information for each exposure: (1) Precise time for the beginning and end of the exposure (use UT only to avoid confusion) (2) If possible, exact time of appearance for all bright meteors (about 1m or brighter) (3) Notes about other moving objects crossing the camera field (4) Some notes about the sky conditions, specifically about clouds near the start or end of the exposure (remember to note at least 10 stars which are not covered by clouds at these moments to allow position measurements to be made later) These notes should be written up in a book kept only for this purpose, or single sheets put together in one file in order to allow easy identification and provide information years later.


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6. Development of the exposed material Nothing can be seen on film after an exposure but undeveloped film or plate. Only during the development process does the image become visible. There are many different developer chemicals available. The variety in choice is a reflection of the many different purposes of photography. The developer will influence the sensitivity, the overall contrast, resolution, and fog formation on the final photograph. For meteor photography, sensitivity is the single most important aspect, while resolution is also of importance when it comes to the measurement of the negatives. In spite of the fact that commercially available developers are of good quality with regards to their composition as well as their reliability, it often happens that two users of the same developer, working in the same way, obtain different results. The reason for this is that many factors influence the developing process. The development of almost all negative films is a function of the time the film spends in the developer, the developer’s temperature, its concentration, as well as the agitation applied to the developing tank. To get good results, one has to assume the information provided by the manufacturer is accurate and gain some trial and error experience. For continuous development, closed developing machines with constant feeds can be used, but it is also possible to use small tanks with an internal spiral to hold the film in the so-called “tumble” method. In order to get the process running smoothly with regards to time and temperature, a few things are important: – accurate time reading – calibrated thermometer and measuring glasses – agitation of the plate or the film during the developing process around in the developer (always according to the instructions of the manufacturer). – caution with the use of tap-water: its composition is not the same everywhere and may contain some impurities. It is recommended that such water be boiled before use to remove some minerals or better yet use distilled water. It should also be noted that some combinations of film type and developer are more sensitive to changes in these circumstances than other combinations. Try some combinations and find your favourite one. Films that have passed their expiry date often give a slack image, while insufficient darkness during the development will create fog. Preference of a liquid or powder developer is a personal choice. For a beginner, the liquid may be better as it is easier to use, but the main criteria should be the best possible improvement of the film’s sensitivity by the developer, the creation of a smooth gradation in darkness levels, and a minimum increase in the grain size. Thus, ask for a fine-grain developer with good sensitivity. It is not possible to get out any more detail at this stage of developing than that obtained by the latent image over the exposure time. It is of little use to sum up all the developing products available from manufacturers. In many journals you will find tables comparing different developers and their resulting characteristics for certain films. Your local photographic shop may be able to give you good advice in this matter. It should be emphasized, however, that meteor photographs should ideally not be given to commercial photo shops. If you take this risk it may mean that all you receive in return is another (new) film as they may throw the original away when they are unable to decipher any obvious pictures on your film. In good developing shops it is possible to specify “development only” or “print all”. Nevertheless, it remains risky to give films to commercial developing shops. Therefore the only safe method is to develop the films yourself or have another amateur who is acquainted with the aims of meteor photography do this.

20


When developing your own film the primary points to consider are: (1) We want to obtain precise positions. ⇒ As some of the high sensitivity films already show a relatively large grain size, we should use a developer which does not further increase this. (2) We wish to analyze rather faint meteors. ⇒ The developer we chose should work to further increase the film’s sensitivity, hence the smallest activated regions, exposed with the smallest amount of light I · t must be developed into detectable blackening (in Fig. 1-5 the lower ends of the characteristic curves). (3) We have many objects (stars, meteors) of very different brightness on our film. ⇒ Therefore we should try to transform all these differing light intensities into different grades of blackness on the negative (Fig. 1-5). These three facts suggest fine grain equalizing developers as the best overall choice for developers.

Figure 1-5: Characteristic curves of different slope (steepness) achieved by different types of development: (a) with a rapid developer, (b) using a “standard developer”, and (c) with an equalizing fine-grain developer. Note the different ranges of I · t which can be transformed into different amounts of blackening S. Although the curve (a) seems to look best at first, (c) is more suitable for analysis, particularly for photometric measurements as it has a larger dynamic response in I · t for a given change in dark level (i.e. a shallower slope).

While the developer is of considerable importance, the fixing bath is not. You should, however, ensure that sufficient fixation and final watering occurs (at least 20 minutes) to avoid damage when you store the films. More information on the techniques of film development can be found in any of the recommended photographic texts in the reference list at the end of this book.


21

7. Your archive After a relatively short time you will have amassed a large collection of photographs and negatives. In order to find a specific negative it is necessary to establish a storage and retrieval system. First, you must decide whether you will store all the photographs or only those showing meteor trails. In the latter case we strongly urge you to store the preceding and the following exposures in order to uncover any instrumental effects which may only become apparent at a later time. Before you throw an image away, you should check it carefully. Consider also the fact that each image of the sky may become of interest to somebody searching for a phenomenon perhaps years later. The probability that any one photograph contains important information is not very high, but it is nevertheless finite. The notes written during or after the observation should be either directly assembled with the negatives (e.g. in protective envelopes), or connected to the negatives based on an identification system which allows the association of any negative with its data unambiguously. For example you might note numbers and/or letters along the edge of negative strips or at the edge of large films using a permanent ink pen. One example for different sized films is given in Fig. 1-6. Although prints of negatives can be made many times and then associated with their original data, it is advisable to note this data on the reverse side of the prints. If you need a particular photograph, say for a publication or identification with another observation, you may then find it easily.

Figure 1-6: In your archive the negatives should be marked according to a specific system for identification. These examples, for 35 mm-film and films of 120 (60 mm) format demonstrate one possible scheme.

For storage we recommend keeping film in strips of 5 or 6 exposures each. Avoid storing the film in rolled form as this can easily cause scratches on the emulsion. Paper or plastic bags suitable for negatives should be employed. Furthermore you can always note additional information on the outside of the bags. This is of assistance when searching for particular exposures. A stout wooden or metal container should house your records when you have prepared them for storage to avoid damage by dust, accidental spills etc.

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8. Photographic observations during a meteor storm At this point in the Photographic Handbook we describe a possible solution for scientifically useful photographic observations during extraordinary events – meteor storms. If the number of visible meteors becomes too large to be counted, we generally speak of a meteor storm. This, however, is not a well-defined term. It is nevertheless a problem of great interest to determine the actual activity level during these events. Visual observers cannot accurately count hundreds or thousands of meteors per hour. Radio techniques may have problems in recording this phenomenon due to saturation effects caused by the superposition of signals from many meteor trails simultaneously. For the investigation of the meteor stream responsible for such storm events it is very important to obtain reliable magnitude data as well as precise numbers of meteors per time unit (Koschack, 1993). During periods of normal shower maxima the number of photographed meteors is far too small for an activity analysis. But perhaps you are familiar with the famous 3-minute-exposures taken during the 1966 Leonids containing some 40 meteors? Generally, during such a meteor storm we try to record as many of the fainter meteors as possible. Therefore, we may follow the guidelines established in this chapter for capturing faint meteors during periods of normal activity. The aim of determining numbers of shower meteors and their apparent magnitudes, however, introduces some additional constraints. A major goal in studying meteor storms is to calculate spatial number densities (flux densities) in the meteor stream. Thus we need a reliable record of meteors per magnitude range per time unit. Because of the factors involved in the ability of a lens-film combination to photograph meteors of different angular velocity and brightness, there are several, mainly geometrical reasons for the following restrictions: – shower meteors move faster the closer one gets to 90◦ distance from the radiant – shower meteors move faster near the zenith than near the horizon – the film will record fainter meteors of low angular velocity, or only bright meteors of high angular velocity Of course, there is no possible camera field which is not affected by these factors. But we may minimize their influence. The camera should be pointed 180◦ away from the radiant in azimuth. If a wide angle lens is used, the lower limit of the field should be 10. . . 20◦ above horizon. For a standard lens of f = 50 mm the elevation of the optimum field center depends on the elevation of the radiant hRad . The most suitable elevation of the field center hfc is recommended in Table 1-5:

Table 1-5: Elevations of the field center hfc for meteor storm photography using a standard lens depending on the radiant elevation hRad . The azimuth of the camera should be opposite to the radiant.

hRad 0◦ 20◦ 40◦ 60◦ 90◦

hfc 90◦ 80◦ 70◦ 60◦ 45◦

It cannot be predicted in advance which exposure time will be most suitable. If the activity exceeds the ability of visual counting, perhaps a 10 minute exposure is appropriate. In the case of a further increase in activity, 2 minutes may be long enough.


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For each of the photographs the following data are required in order to permit a scientific analyses to be made: – date – the exact beginning and end of the exposure (±1 s; in UT) – the approximate field center in α, δ – site location and its geographic coordinates – focal length and speed of the lens – film type (sensitivity, format) – observer For the analysis of such events it is expected that the IMO will establish a research group devoted to each outburst. Therefore, look for instructions given through the IMO journal WGN or other information regarding the data handling, who to send the information to and which data are required.

9. Use of a video camera If you have access to a video camera you are encouraged to use it for meteor work as well. Although normal color camcorders are limited to about +2m , which is roughly comparable with the meteor limiting magnitude achieved with a standard lens (cf. Table 1-1 on page 12), the precise timing and frame by frame analyses possible with video equipment compensates for sensitivity limitations. Better sensitivities can be obtained with special high sensitivity monochrome video cameras, or with image intensified video cameras (Hawkes, 1990a). Use the largest aperture possible with your video camera lens. If it is a zoom lens, you will want to select a fairly wide angle. Once you have selected a zoom setting, do not change it during the course of the observations. Set the focus to manual at infinity, as some types of automatic focus mechanisms will not operate properly when aimed at an almost black sky. For most purposes you will not want to use the electronic shutter available on CCD video cameras since the sensitivity will be further impaired. Some observers may, however, want to use this feature to specifically look for wake (see Part 4 of this Handbook: Meteor Trains). In this case be sure to note the electronic shutter speed used (e.g. 1/1000 s). Turn the time display to on and set it to the finest time increment possible. Synchronize your clock time to a standard time signal. If no video time signal is possible with your camera, briefly blank the picture (by covering the lens) at several recorded times. Recording a short wave radio time signal on the audio track of the video recording offers another timing option (or more simply, using a microphone to place time markers on the audio track according to the time indicated on an accurate and calibrated clock). Use high quality video tape, and in most cases it is preferable to use the highest recording tape speed possible (e.g. SP in VHS, Beta I or Beta II). Unless a clock driven equatorial mount is available, use a firm tripod with a fixed direction. Select an observing direction in the same way suggested for photographic work, but adjust it as necessary to make sure that a minimum of three stars are visible in your field of view. It will assist with photometric corrections if, at the beginning or ending of your observing period, you record several minutes with the same camera settings but with the camera skewed at angular rates roughly corresponding to that of the expected shower. Note the identifications of the stars used in the test.

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Immediately after the observations make a copy of the video tape. It is acceptable (perhaps even preferable) to make this copy on a slow tape speed (e.g. SLP in VHS or Beta II), since frame by frame advance is better on most machines with slow tape speeds. In making the copy of your tape use the video in and video out connectors, rather than the RF modulated signal. Be sure to use shielded cables intended for video work in making the copy. Carefully review the tape at least once (preferably twice) to make a listing of all meteor occurrences. This will make it easy for others to complete the analysis of your observations. For each meteor note the following: 1. time (UT) to the nearest second 2. position of meteor on the screen 3. apparent direction of motion 4. apparent angular speed (approximate) 5. approximate apparent luminosity, in magnitudes Send this information to the VMDB responsible as soon as possible.

References and bibliography: Bunsen R.W., Roscoe H.E., 1862: Photochemische Untersuchungen. Poggend. Annalen (2) 117, 529.(in German) Halliday I., Griffin A.A., 1982: A Study of the Relative Rates of Meteorite Falls on the Earth’s Surface. Meteoritics 17, 31–46. Hawkins G., 1964: Meteors, Comets & Meteorites. McGraw-Hill Book Co. Koschack R. (Ed.), 1993: Handbook for visual meteor observations. (IMO monograph series, no.2) Kresák L., Kresáková M., 1965: The variations in frequency of bright photographic meteors. Bull. Astron. Inst. Czechosl. 16 81–88. Malin D.F., 1982: Photography in Astronomy. Phys. Bull. (U.K.) 33, 206–210. Rendtel J., 1990: Fireball Rates. In: P. Sp´ anyi and I. Tepliczky (Eds.): Proceedings IMC 1989 Balatonf¨ oldv´ ar, 25–28. Rendtel J., Knöfel A., 1989: Analysis of Annual and Diurnal Variation of Fireball Rates and the Population Index from Different Compilations of Visual Observations. Bull. Astron. Inst. Czechosl. 40, 53–63. Schwarzschild K., 1900: On the deviation from the law of reciprocity for bromide of silver gelatine. Astrophys. J. 11, 89–91 Vanmunster T., 1981: Handboek Simultane & Fotografische Meteoorwaarnemingen. VVS Werkgroep Meteoren (Belgium; in Dutch) Wetherill G.W., ReVelle D.O., 1981: Which Fireballs are Meteorites? A Study of the Prairie Network Photometric Meteor Data. Icarus 48, 308–328.

Part 2: Fireball Patrols

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PART 2: FIREBALL PATROLS

1. Introduction Fireballs are rare phenomena. Analysis of more than 5000 hours of visual observations suggests a mean annual rate of one sporadic meteor brighter than 0m every 2.7 hours and one meteor brighter than −3m in about 300 hours (Rendtel, 1989). Of course, there are significantly more bright meteors concentrated within or near the core regions of meteor showers such as the Perseids or Geminids, but it is known that most meteoroids of cometary origin (and even the somewhat dense Geminids associated with the asteroid 3200 Phaethon (Halliday, 1988)) completely disintegrate before reaching about 50 km altitude. In contrast, the most interesting and dynamic events are the so-called potential meteorite-producing fireballs. These are all meteoroids entering the Earth’s atmosphere at a velocity of less than about 25 km/s and reaching luminous end heights less than 25 km altitude. This group naturally includes actual meteorite falls as well (Wetherill and ReVelle, 1982). The probability of such events occurring in the evening hours is about four times greater than in the morning (Halliday et al., 1984). This is because the point in the sky toward which the Earth is moving at any moment (called the apex) culminates from a given location at 6h local time. As meteors from radiants in this part of the sky have entry velocities typically higher than in the evening sector (since we tend to encounter meteoroids ”head-on”) there is a dearth of meteorite-producing fireballs in the morning (Fig. 2-1). Thus, while a higher entry velocity does imply a higher amount of kinetic energy and thus also a brighter meteor, the luminous trail starts at a higher altitude and as a result the meteoroid fully ablates at a higher altitude. Therefore we expect more bright meteors in the morning hours, but more meteorite-producing fireballs in the evening sector. A typical event of the latter kind was the meteorite dropping fireball of 1992 October 9. Its radiant was almost on the south-western horizon and it entered the Earth’s atmosphere at 19:49 local time, lasted for nearly 30 seconds and resulted in a meteorite fall at Peekskill, N.Y., U.S.A. (Brown, 1992). The major goals of fireball photography are: (i) the recovery of fallen meteorites (ii) the calculation of a fireball’s atmospheric trajectory (iii) the determination of a fireball’s heliocentric orbit (iv) securing photometric information that can reveal information concerning energy transformation along the trajectory

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Figure 2-1: Earth’s rotation and the direction of Earth’s movement along its orbit. At 6 h local time an observer is situated on the “front side” of the Earth where the average collision velocity is higher than any other time during the day, while around 18 h local time meteoroids enter the Earth’s atmosphere “from behind”, and thus at lower relative velocities than in the morning hours.

2. Equipment Bearing in mind the rarity of these events and the data we wish to record, the photography of fireballs, particularly of possible meteorite-producing events, requires two principal things: – coverage of a very large field of view, ideally the whole sky, and – imaging the event at a scale sufficient for accurate astrometric measurements. To permit the entire sky to be imaged, the “classical all-sky camera” was invented. It uses a convex mirror with a normal camera mounted above the mirror such that the camera can photograph the whole sky (Fig. 2-2). This has some disadvantages, for instance: (i) the camera body blocks the zenith area (cf. also the examples given in the appendix of photographs, Part 10). (ii) reflection from the mirror surface reduces the amount of light available for forming an image on the film in the camera and thus decreases the speed of the whole system. (A mirror that is used regularly reflects less than 80% of the incident light. The effective equivalent ”speed” of such a camera system is about f /11.) (iii) it requires construction of large and cumbersome apparatus to hold the camera body. (iv) the mirror surface has to be recoated every 2. . . 3 years to maintain acceptable reflectivity, a procedure that can be expensive. A better alternative is to use fish-eye lenses. They overcome those disadvantages inherent in the classic “all-sky” set-up, with lenses for 6 × 6-cameras such as Zeiss-Distagon or the Russian Zodiak, (both f /3.5, f = 30mm), being of excellent quality and permitting astrometric measurements to be made down to the horizon. Ceplecha et al. (1979) and Ceplecha (1987) have also found that the photometric accuracy of these lenses is satisfactory over most of the visible field. But, a problem with the 6 × 6-camera is that it does not make use of the whole field of view presented by the lens. In general, cameras ”capture” only the central part of the field that is optically available from the lens (Fig. 2-3).


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Figure 2-2: General construction of an all-sky mirror camera. The reflecting layer (usually aluminum perhaps with a protective layer of quartz) of the mirror is located on its surface. Therefore be careful to avoid scratching or touching the mirror surface. The distance between the camera and the mirror surface should be adjusted so that the whole mirror is photographed. Remember when focussing that the mirror’s base is ≈ 0.8 . . . 1.0 m away from the camera. Here we have to focus the lens to this distance, not “∞”.

Figure 2-3: Cameras only make use of the central part of the optically available field. A good, fast lens for a camera supporting a 24 × 36mm2 format may be used in combination with 6 cm format film. It requires construction of a “film-holder”. Of course there is a decrease in brightness toward the limb of the field, an effect called “vignetting”, which can cause problems with photometric measurements in that region. Also, the accuracy of astrometric measurements decreases with distance from the optical axis. However, this is a cheap alternative to buying a lens designed for large format films.

For example, an f = 35mm lens for an ordinary small frame camera produces a circular image that is larger than the 24 × 36mm2 area passed through the camera body and available on the film used in

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such a camera. At the very least you would want to have a field of 44 mm diameter (since for fireball photography you wish to capture as much of the image from the lens as possible) with the same image quality you would expect to find within the 24 × 36mm2 field. For such cameras it is in fact possible to make use of a field that is up to 60 . . . 80 mm in diameter (Fig. 2-3). The amount of light will decrease toward the outer circular edge of the photo and the distortions will increase depending on the type of lens used. A 6 cm format film would be adequate to capture the full-field of such a lens. The fish-eye lens mentioned above will deliver a circular image of 80 mm diameter to the film, so we would want to use a film that is large enough to capture this full area (e.g. a 9 × 12cm2 sheet). This would require the construction of a suitable “camera” box. The camera shown in Fig. 2-4 is an example that has been used successfully for several years. It also includes some additional equipment (see figure caption).

Figure 2-4: Camera-box for a fish-eye lens in combination with a 9 × 12cm2 film sheet which makes full use of the 80 mm field. The construction requires a precise rectangular mounting of the lens (1) and the film cassette (5). Be careful that no stray light gets into the box. Since a shutter in front of the lens would partially obstruct the field, it can be of small size and included in the camera box, together with its motor (3, 4 respectively). More details are given in section 4 of this Part of the Handbook (pp. 30–31).

We may therefore conclude that there are two principal ways for setting up a fireball patrol – all-sky cameras (mirror cameras) or cameras with a wide-angle/fish-eye lens. The latter should be used with an appropriate film format. If you do not have a mirror camera or a fish-eye lens, you may still meaningfully contribute to our knowledge of fireballs. Here it may be most profitable to arrange a cooperative venture with other photographers at distances of 20 . . . 50 km from your site. In this arrangement a certain area of sky will be covered from all the stations to improve the odds of photographing multi-station meteors. (For details see Part 6 “Double station work”.) Such networks using small frame cameras and wide-angle lenses have worked successfully in parts of Germany, for instance, for several years now. Cooperative ventures have also been successful during campaigns around the time of major shower maxima in places like the Netherlands and in Southern France.

3. Film and exposure The choice of film for fireball patrols involves requirements which are very different from the case of faint meteor photography already covered in Part 1 of this Handbook. As an example, the all-sky cameras of the German fireball network (part of the larger European Network, EN) make a single all-night exposure per night. Usually, fish-eye lenses allow exposures of up to 7 hours duration in areas that are only slightly disturbed by city lights before fogging becomes a problem. Exposures of the order of several hours length are typical for fireball patrols. Bear in mind that the primary goal is not “beautiful pictures” but images that permit astrometric measurements and photometry. Again,


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black and white film should be used NOT color film. The reasons for this will be explained in the chapter covering photometry (Part 8 of this Handbook). Since there is no need for highly sensitive film for fireball work (as the meteors to be captured on film are so bright), the finer grain structures of slower films make this type of film the preferred choice, as it allows precise positional measurements. This increased resolution acts to offset the somewhat poor scale (ie. large scale) intrinsic to the short focal length of wide-angle or fish-eye lenses. With the combination of a lens covering a very wide field of view and a medium-sensitivity film we can reach adequate astrometric accuracy. Table 3-1: Suitable exposure times (in hours) for a system consisting of a rotating shutter equipped with 90◦ wings and a lens-warming device for dark sky sites and exposure times for the same system with moonlight or other interference (given in brackets).

ISO ISO ISO ISO

50/18◦ 100/21◦ 200/24◦ 400/27◦

all sky mirror >15 (10) >15 (8) 13 (6) 10 (4)

fish eye f /3.5 14 (7) 10 (5) 7 (3) 5 (2)

fish eye f /5.6 >15 (10) 14 (7) 10 (4) 8 (3)

wide angle f /2.8 11 (5) 8 (3) 6 (2) 4 (1)

Table 2-2: Astrometric accuracy achievable with different combinations of films and lenses (in arcmin).

ISO ISO ISO ISO

50/18◦ 100/21◦ 200/24◦ 400/27◦

all sky mirror 40 60 90 120

fish eye f = 30mm 1 2 3 5

wide angle f = 20mm 2 3 4 5

lens f = 35mm 0.5 1 2 3

A photograph taken on a night with a full-moon and some haze looks terrible. In all likelihood only the trail of the moon will be recorded. If a fireball occurred, you must hope to find the start or end points of some bright star trails as reference points or make use of reference points on the horizon. In such cases, the accuracy of the position of the meteor trail decreases dramatically.

4. Additional equipment A rotating shutter is generally of higher importance in fireball work than it is for faint meteors. For the analysis of fireball photographs, knowing the velocity of the fireball is essential and may be determined only with the help of a shutter producing breaks at regular intervals. The velocity as a function of distance along the trajectory of a bright fireball is a vital quantity that plays a role in determining the form of diverse phenomena such as the processes of energy transformation, fragmentation (due to atmospheric drag) and the fireball’s (luminous) end height. The shutter also reduces the influence of any background light. The interruption frequency should lie between 8 and 25 breaks per second. Experience shows that 90◦ wings are most suitable for this. Except some rare very slow and/or very bright fireballs, the breaks are readily distinguishable and photometry of the visible parts of the trail is also possible using this setup. Many fireball network stations use 12.5 breaks per second that is at the lower end of the range given. The reason for this choice becomes clear when we consider that fireball patrols are designed to secure photos for meteorite-like events that should enter the Earth’s atmosphere at a low velocity. The long exposures require that the air in front of the optics is warmed. This applies equally to the large mirrors of the all-sky mirror cameras and for the large front lenses of fish-eye or wide-angle lenses.

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Another effect that may cause problems is because the films, chiefly the larger formats (6 × 6cm2 , 6 × 9cm2 or 9 × 12cm2 sheets), can spring out of focus as they cool. If you warm them a small amount, they tend to curve in the other direction (Fig. 2-5).

Figure 2-5: Distortion of large films due to temperature effects. The amount of curvature is strongly exaggerated for clarity.

By warming the emulsion you also reduce the Schwarzschild exponent p (cf. section 4.4 of Part 1, p. 15). While this is to be avoided in “normal” astrophotography as it makes faint objects harder to detect, this is helpful for fireball work. In our case the prominence of objects exposed for a long time is reduced (stars, background) while the short-lived fireballs are unaffected. Another problem may arise due to the weather conditions at some sites. In general, you do not remain with your camera during the whole lengthy exposure, particularly if the moon is shining or dust and haze decrease the limiting magnitude. This leaves open the possibility that rain may suddenly occur. While clouds “only” finish the sky exposure and add background brightness (especially in streetlit areas), rain can badly affect the optics and the camera as well. In light-polluted areas you can make use of the fact that clouds, which are at relatively low altitudes, appear significantly brighter than the cloudless night sky, and thus a suitable electronic “clear-sky indicator” can be constructed. Such a device is described in detail by Mostert (1982). Although the text is in Dutch, detailed information about the electronics is given in the form of circuit diagrams. Other examples can be found in electronics magazines and journals. In dark, non-light-polluted areas clouds appear as dark patches in the sky without any stars. Any cloud-detector must then make use of the visibility of a certain star, but to the author’s knowledge such a device has not been put to use yet. Detectors for rain and/or humidity have been constructed by different people at various times. These detectors have had a bad track record, as exposures may be finished by dew or affected by rain before the closing mechanism is triggered. Although risky, it seems the best method is to get reliable weather information (e.g. from the nearest meteorological office or an airport situated not too far away) and to make regular checks of sky conditions in questionable cases. For a privately run fireball patrol camera it does not seem appropriate to operate by the rules established by the German part of the EN that runs mirror-cameras: these are switched on every night independent of the weather . . . To maintain a regular fireball patrol and to make life easier for the operators of all-sky survey meteor cameras, there are some additional tools that are easily accessible, and are also very helpful. As an example we will describe some devices and procedures developed by the EN that has been running successfully now for more than 30 years. For example, each all-sky camera operated within the German part of the “European Network of Fireball Photography” is equipped with a special closing device that is triggered by a programmable clock. These mirror cameras make one exposure during each night of the year.


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To manage this properly, the tasks of the operators are (a) to transport the film (b) to set the time for the next night’s exposure The clock switches on the camera via a magnetic coil that moves a wire release. The camera is then kept open until the end of the scheduled interval.

5. Cleaning of the optics If you use your camera regularly for fireball patrol work lasting the entire night it will become wet due to rain at some point. Also, you will find that more and more dust accumulates on the front lens after several hundred hours of use. At this point the front lens has to be cleaned carefully. We place great stress the last word of this sentence! Camera optics are covered by thin protective layers to reduce reflections (which is why they appear metallic-brown in reflected light). These layers are very sensitive to scratches. Some small grains that have collected on the lens could be silicates, and thus of comparable hardness to the glass itself and certainly far harder than the protective layer. Besides damage to the reflection-reducing layer each scratch increases the amount of stray light entering the camera. These are the reasons for being extremely careful when cleaning the lens. As a rule, a small amount of dust on the front lens has less effect than any scratches. However, from time to time you must clean the optics. You should do this immediately after your lens has been in the rain since you will find the task much more difficult after drying has hardened the wet dust coating onto the lens. Presented here is a cleaning method that has been used safely for many years. First, use a soft brush (hair-pencil) that is specifically designed for the cleaning of optics to remove the larger grains. Next wipe very gently with cotton-wool and pure alcohol. Use each cotton-wool swab only once to lift away the particles from the lens. After doing this until no particle deposit is visible, you will see a surface that looks smeared and inhomogeneous. As the alcohol vaporizes, the lens will cool slightly. Next, take another fresh, dry swab, breathe onto the lens and wipe the moisture away applying gentle pressure. This has to be repeated until, once again, the whole lens looks clear and homogeneous. As already pointed out, this should be done only with care and only when absolutely necessary (after about half a year of operations, following a rain or until the first accident). If in any doubt, consult a specialist in this field. A good camera shop can provide advice on lens care. In principle these hints are also valid for cleaning the large surfaces of the mirrors used in the all-sky camera. If no protective layer has been placed on the mirror, however, you may remove portions of the reflecting layer and thus seriously damage the mirror. This may require a new aluminization, which requires a special vacuum device and is expensive. Again, if you are unsure how to proceed contact an optical specialist. Astronomical telescope makers may be helpful for this mirror problem.

6. Time of the fireball’s appearance Unfortunately, the photograph is unable to record the time at which the fireball appeared, but the knowledge of this precise moment is necessary for the computation of the initial heliocentric orbit. The time of appearance of a fireball is needed only for the determination of the right ascension of the radiant, not for the measurement of the trail or the reduction of the atmospheric trajectory. The radiant position is essential in determining the meteoroid’s origin and is one of the most valuable aspects of fireball photography. In the evening hours there is a chance that eyewitnesses may report a bright fireball, though as the night proceeds this probability progressively decreases as does the chance that meteor observers may be outside under good sky conditions nearby.

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Occasionally, however, you may be lucky and witnesses have reported the event and its time of appearance. In such a case the Fireball Data Center (FIDAC) of the IMO may be able to provide the time of the fireball’s appearance. Another possible source for the time of a bright fireball is through amateurs who have an automated radio forward scatter system. A long enduring reflection may be related to a fireball (Knöfel, 1993). For more details see the IMO’s Radio-Handbook. Records of long enduring radio echoes are also stored in the FIDAC files. If you are observing while the camera is in operation, you should set your watch to a radio time signal, the most precise of which are time signals from special stations. In most parts of Europe you may use the DCF 77 station in connection with a transmission-guided clock, while in North America the short-wave signal from the WWV time station may be employed. For other regions contact astronomical observatories in the vicinity for information or consult an almanac that annually publishes lists of suitable time signals and details of their broadcasting schedule and frequencies. Usually, however, the fireball patrol camera operates while no observer is active, i.e., on cold winter nights, during moonlit periods, during hazy nights or when the amount of cloudiness is variable and precludes other forms of observations. A bright fireball causes a large portion of the sky to be illuminated. Often its brightness increases to a certain level and in the middle and latter portions of its luminous trajectory intense flares occur. These sudden increases in brightness may be used by timing devices. Actually, there are only few such devices in use (Mostert, 1982). Their light sensitive receiver can be either a photomultiplier, a photodiode or a phototransistor. For these devices the sensitivity must be regulated in such a manner that the normal sky brightness (perhaps plus moonlight or terrestrial light sources nearby) does not produce a signal. Any additional light exceeding this threshold intensity, which is arrived at by adjustments in accordance with the local conditions and the device parameters, should give a signal and record the time of the light’s onset. These are very general remarks only. There have been several different attempts to overcome this problem, none of them entirely successful. A general problem can occur in densely populated areas or near electric railway lines. In these regions there may be many artificial flashes that could potentially give an erroneous signal to a timing device. However, these flashes are normally of much shorter duration than those from meteors and may therefore be distinguished from bright fireball flares. If no precise time of appearance is known for a fireball, the position in right ascension is uncertain by the angle corresponding to the duration of the exposure. For the case of all-sky or fish-eye photographs you may use features on the horizon to figure out the horizontal coordinates that will then permit the calculation of the atmospheric trajectory. But, in such cases it is impossible to find the orbit of the meteoroid and thus we cannot obtain any information concerning the origin of the particle.

7. Practical hints for patrols Since a major goal of fireball photography is the calculation of fireball trajectories and meteoroid orbits, users of wide-angle lenses (instead of all-sky or fish-eye cameras) should contact each other to choose appropriate camera fields that intersect at the meteor level (about 80 km altitude for fireballs) and which are of comparable size at that height. Details of the calculation of double station camera fields) are described in sections 4 and 5 of Part 6 of this Handbook. It is worth photographing even during dusty or hazy nights or when the moon is present. Although the images look terrible (from an aesthetic and photometric point of view) a very bright fireball may still be captured. During partially cloudy periods you should survey the night sky. If there are clouds at the beginning or end of the exposure, note at least 10 stars that are not covered by clouds at these times. This will guarantee usable reference stars for astrometric measurements. If it becomes totally clear later, you may interrupt the exposure for about half a minute. The breaks in the star trails may then be used


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as reference points, provided the precise time of this interruption is determined and noted. If possible, mount your camera so that the orientation is unchanged from night to night. This will permit the determination of positions if reference stars are missing (with a strongly-reduced accuracy however) or may be helpful in identifying long star trails. For a situation where no star trails can be measured, a fixed object may be used as a reference. This could be an antenna on a roof or anything else on the image.

8. Summary of the equipment Appropriate equipment consists of: (1) an all-sky mirror + camera OR a fish-eye lens + camera with suitable film size OR wide-angle lens + camera, possibly with an enlarged film format (2) an appropriate fixed mounting for the equipment to ensure that even strong winds do not harm the camera or alter the direction of the equipment (3) warming devices for lens and film (4) a rotating shutter, preferably with a synchronous motor giving about 15 breaks per second (5) a device for fireball timings (6) a cloud detector, perhaps with twilight dimmer-switch (7) a timer for the beginning and end of exposures. (5), (6), (7) are helpful, but not essential. During the observation you should note the following information for each exposure: (1) Precise time of beginning and end (use UT only to avoid confusion) if you use an electronic timer, check the accuracy! (2) Sky conditions, particularly clouds at the moment the exposure starts or ends; note about 10 stars that are visible at these specific times if clouds are present.

9. Handling of the exposed films You will not normally know which events, if any, appeared during the exposure(s) and were secured on film. You can expect some fainter meteors (about 0m ) to have been recorded as well as very bright fireballs. Also, the sky conditions may have been different from one exposure to the next. To permit astrometric measurements and photometry, it is best to obtain a fine grain and a smooth weak gradation (shallow slope on the characteristic blackness curve). Both goals can be achieved by using fine grain developers. A smooth and weak gradation produces sufficient density for somewhat faint meteors or the beginning of the brighter fireballs and transforms nearly the whole range of brightness into different degrees of blackening (Fig. 2-6). Sometimes, however, a large part of a fireball trail may be overexposed, or the fainter parts and / or fainter meteors do not appear on the image.

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Figure 2-6: Characteristic curve for a weak gradation with the transformation of a huge intensity range into measurable differences of blackening. Although the range in S is normally larger for higher gradation, weak gradation allows better results over large intensity intervals.

Fixing is less important, but for safety fix for at least twice the time that is needed to clear the film. Otherwise, the image will not be fixed totally. Careful attention also should be taken to bathing the film in water after the processing. Half an hour usually is a recommended bathing time for images that will be stored, otherwise brownish areas may ultimately form and the emulsion can be destroyed.

10. The archive Although the number of successful photographs from a fireball patrol is somewhat low, you should archive all negatives. If you use films such as 35mm or 60mm, they should be stored in strips whose length is determined by the size of the negative bags you can buy. Each strip has to be clearly marked to allow later identification and to avoid the possibility of mixing them up. Times of exposures and additional remarks should be noted in a file or book that is used only in connection with the fireball patrol. If you are using single film sheets, you should write a unique identity number, the date, start and end of the exposure directly onto the film sheet (emulsion) using a permanent ink pen. It is helpful to mark photos with fireballs in the list/book also on the negative itself, or to keep such images separately after noting that they are not included in the general stock of images. You should then also separate the data of the successful photographs.


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Figure 2-7: Possible identification systems applicable for fireball patrol photographs. For 120 roll film or small scale 35 mm film the space on the negative is limited (unless you include a blank image after each night’s series to include notes – a rather expensive method). A reference number will allow you to associate the notes from a notebook with the negative. Large film sheets like those used with fish-eye lenses leave enough space to note all the exposure data as well as a reference number. The diary may contain even more information about the exposure and the circumstances, e.g., about failures or unusual conditions.

As there is much trouble and money invested in obtaining each nightly patrol photo it is best to keep all the negatives, even those without any fireballs found. Occasionally it will happen that you did not see, for instance, a very short trail or a partially obstructed trail that is later detected on other photographs or reported by visual observers. Additionally, the patrol photographs may be of interest for other events, e.g. a nova or the sudden brightness variation of variable stars that are accidentally recorded on such survey images.

11. Fireball networks A photograph of a fireball from a single station has limited value, but it may help to improve a fireball trajectory determined from visual observations. For a meteorite fall, a single photograph may put useful constraints on the associated fireballs’ trajectory. Single-station photometry (cf. Part 8) gives relative information about the brightness, but the real distance to the observer remains unknown. Calculations of the trajectory require photographs from at least two stations that are separated by several tens of kilometers. For fireball patrols it is useful to have a complete network established. Preferably the network should consist of fish-eye camera-equipped stations or all-sky cameras, but other cameras with wide-angle lenses are also possible. In the latter case, camera directions should be arranged in a certain way to guarantee that the fields of view from different stations intersect at the likely level of fireball first appearance as described in Part 6.

12. Fireballs on video There are two spectacular fireball events which have been recorded on movie film and video respectively: an Earth-grazing fireball on August 10, 1972 (Jacchia, 1974), and the fireball associated with the Peekskill meteorite fall of October 9, 1992 (Brown, 1992). In both cases witnesses instantly used their film/video cameras to follow the fireball, and these recordings permitted further analyses of the fireball trajectory. But, both events were recorded by accident. Of course, it also would be possible to

36


establish a fireball patrol based on video cameras. The camera would require a wide angle lens, and somehow it must be arranged so that adequate tape is supplied for an entire night. The information about the appearance of a fireball would then be recovered from the tape, which requires either a visual inspection of the tape or a very fast computer system that can automatically search video tapes image by image for non-stellar light. In fact, these constraints and the cost of a video-fireball patrol reduce the use of video to accidental events, particularly during daylight or twilight when photographic patrols are not active. On the other hand it also might be useful to use video cameras near the peak of major meteor showers, in this case following the hints given in section 9 of Part 1 (Faint Meteors), pp. 23–24.

References and bibliography: Brown P., 1992: Meteorite-dropping fireball over the eastern USA: Peekskill, NY, October 9, 1992, 23h 50m UT. WGN 20, 222. Ceplecha Z., 1967: Photographic data on fireball of −17th magnitude from Jan 16, 1966. Bull. Astron. Inst. Czechosl. 18, 233–238. Ceplecha Z., 1987: Geometric, dynamic, orbital and photometric data on Meteoroids from photographic fireball networks. Bull. Astron. Inst. Czechosl. 38, 222. Ceplecha Z., Boˇcek J., Jeˇzková M., 1979: Photographic data on the Brno fireball. Bull. Astron. Inst. Czechosl. 30, 220. Ceplecha Z., Boˇcek J., Jeˇzková M., Porubˇcan V. and Polnitzky G., 1980: Photographic data on the Zvolen fireball (EN270579, May 27, 1979) and suspected meteorite fall. Bull. Astron. Inst. Czechosl. 31, 176–182. Ceplecha Z., Boˇcek J., Nováková M. and Polnitzky G., 1983: Photographic data on the Traunstein fireball (EN290181, Jan 29, 1981) and suspected meteorite fall. Bull. Astron. Inst. Czechosl. 34, 162–167. Ceplecha Z., Jeˇzková M. and Boˇcek J., 1976: Photographic data on the Leutkirch fireball (EN300874, Aug. 30, 1974). Bull. Astron. Inst. Czechosl. 27, 18–23. Halliday I., 1978: The Innisfree meteorite and the Canadian camera network. J. Roy. astr. Soc. Can. 72, 15–39. Halliday I., 1985: The Grande Prairie Fireball of 1984 February 22. J. Roy. astr. Soc. Can. 79, 197–214. Halliday I., 1988: Geminid fireballs and the peculiar asteroid 3200 Phaethon. Icarus 76, 279–294. Halliday I., Blackwell A.T. and Griffin A.A., 1984: The frequency of meteorite falls on the Earth. Science 223, 1405–1407. Halliday I. and Griffin A.A., 1982: A study of the relative rates of meteorite falls on the Earth’s surface. Meteoritics 17, 31–46. Jacchia L.G., 1974: A meteorite that missed the Earth. Sky & Telescope 48, 4–9. Kn¨ ofel A., 1993: Visual, radio and photographic fireball over NE-Germany. FIDAC news 1, 1-2. Mostert H.E., 1982: Wolkendetektor. Radiant 4, 18. (in Dutch) oldv´ ar, Hungary. Eds.: P. Sp´ anyi and Rendtel J., 1990: Fireballs rates. In: Proc. IMC 1989, Balatonf¨ I. Tepliczky, 25–28. ReVelle D.O. and Rajan R.S., 1979: On the luminous efficiency of meteoritic fireballs. J. Geophys. Res. 84, 6255–6262. Wetherill G.W., ReVelle D.O., 1981: Which fireballs are meteorites? A study of the Prairie Network photographic meteor data. Icarus 48, 308–328.

Part 3: Meteor Spectra

37

PART 3: METEOR SPECTRA

1. A brief historical review The first photographic meteor spectrum was obtained accidentally in 1897 by Pickering at the Harvard Observatory. The first successful meteor spectroscopy program was set up in 1904 in Moscow. A quarter of a century later, however, only 8 spectra had been obtained globally. Photography of meteor spectra has never been very popular. As late as 1946, not even 100 spectra had been obtained worldwide. Fortunately, intensive observing programs were eventually set up and by 1961 more than 400 spectra had been recorded. Countries which lead the way in the acquisition of meteor spectrograms were the former U.S.S.R., Canada, the USA, the Czech Republic and Slovakia, where major efforts were made to investigate this aspect of meteor astronomy (Millman, 1980; Millman, 1983). The introduction of very fast emulsions brought revolutionary progress in all aspects of meteor photography, including meteor spectral studies. The earliest spectra were all taken on blue sensitive plates. After 1932 ortho plates were introduced, which are also sensitive to green and yellow light. Details in the red part of the spectrum were obtained from 1934 onwards. However, infra-red meteor photographs were not obtained until 1952! Since bright meteors are more likely to occur around the time of meteor stream maxima, this is the most favourable period to devote to capturing meteor spectra. It should come as no surprise to learn that 40% of all meteor spectra are Perseids and 12% Geminids! Indeed, very little spectral information has been gathered concerning minor showers and the sporadic background. The quality of spectra varies widely as well, from very faint ones which show only one line, to cases where 150 and more lines can be measured. Today the “records” in meteor spectroscopy belong mostly to the Ondˇrejov Observatory in the Czech Republic: the deepest ever photographed meteor spectrum (down to 20 km height); the maximum number of spectral lines for one spectrum, (over 1000 in the visible region); the greatest dispersion ever for a meteor spectrogram (0.5 nm/mm) and the spectrum of the brightest meteor, that of a fireball of −21m absolute. This is the result of a systematic observing program with long-focal length spectral-grating cameras, which has been going on each clear night since 1960 (Ceplecha, 1991).

2. Classification of meteor spectra A simple classification system for meteor spectra was introduced by P. M. Millman and was based on the first 24 spectra ever photographed (Millman and McKinley, 1963, p. 747). This classification scheme is based on the identification of the strongest lines in two regions of the spectrum; the orangegreen region and the blue-violet region which correspond respectively to 500–600 nm and 350–450 nm.

38


The four major spectral classes can be defined as follows: Type Y: the H- and K-lines of Ca II make up the brightest group in the blue-violet. Type X: when the definition for type Y is not valid, then the D lines of Na I or the lines of Mg I at 518 nm or 383.8 nm respectively are the brightest lines in the orange-green or blue-violet. Type Z: if the definitions for Type X and Type Y do not hold, then the lines of Fe I or of Cr I are the brightest in the orange-green or the blue-violet regions. Type W: none of the characteristics of the Types X, Y, or Z occur. This classification has proved mainly to reflect the velocity of the meteoroid: X belongs to the region of 15 to 20 km/s, Z belongs to the 30 km/s region, Y belongs to the 60 km/s region, and W to meteoroids of unusual composition. The following elements have been observed in meteor head spectra (Ceplecha, 1967): (i) neutral atoms (indicated by the “I”): Fe I, Mg I, Na I, Ca I, Mn I, Cr I, Al I, Ni I, Ti I, H I, O I, N I, (ii) ionized atoms: Ca II, Mg II, Si II, Fe II, N II, O II. Here “II” stands for single ionization (e.g. Ca+ ), a “III” for double ionization (e.g. Ca2+ ). There are some additional elements (Co I, Sr I, Sr II, Ba I, Ba II, Si I) which are sometimes identified. In addition, molecular lines and bands can occasionally be identified (N2 , FeO, MgO, CaO, CN, C2 and others). A spectrum depends partly upon the composition of the meteoroid and its density, but mostly upon its geocentric velocity (and thus the classification by meteor stream) and also upon its height in the atmosphere. There is no great difference between the meteor head and wake spectra, if considering element identifications only. As the wake spectra show substantially less excitation than the head spectra, only the lines corresponding to the low-energy states are recorded (Ceplecha, 1967). Data obtained from spectra are complementary to all other results obtained from meteor photography, such as trajectories and orbital elements, as well as to the visual work from which different stream characteristics can be derived. The spectra, however, provide a unique link to meteorite research. Put together, all this research permits the study of the interrelation and evolution of meteoroids, asteroids and comets.

3. How to build a meteor spectrograph 3.1. The camera and dispersing element In principle, any camera can be converted into a spectrograph by placing a dispersing element (prism or grating) in front of the lens. More details about the construction of a meteor spectrograph can be found in section 3 of Part 5. Prisms produce one non-linear spectrum. Gratings have the advantage of linear dispersion, but they produce many spectra of different orders overlapping each other. Much of the light from the meteor ends up in the useless zeroth order (the ”normal” meteor image with no lines visible). Modern gratings are blazed (meaning the grooves that form the grating lines are constructed to a controlled shape) and this concentrates the majority of the light in a specific order. In general, though, prisms are easier to obtain and of lower price. We have already mentioned that only lenses with a long focal length (at least 100 mm) are useful for obtaining spectra with a prism. The reason for this is to ensure that a scale sufficient for analysis of the spectrum is achieved. If the dispersion (defined as the spread of the spectral wavelengths of light per unit length on the film) is too small, the lines are not well separated from each other. A large dispersion scale can be achieved using a prism with a large refracting angle, though this also means that the light must pass through a large quantity of glass, leading to light absorption. Alternatively,


39

we may use a lens of longer focal length. A prism made of glass with a high refractive index n and a refracting angle α = 25◦ . . . 30◦ combined with an f = 100 mm lens can produce useful results. Because the camera field must be sufficiently large, a small frame camera is not suitable. An old 6 cm × 9 cm bellows camera or a 9 cm × 12 cm plate camera are very well suited to meteor spectroscopy. Such cameras can often be found for very reasonable prices second-hand.

3.2. The prism and its mounting The prism employed should have a large deviation angle made of a glass with a high refractive index n. The position and orientation of the prism in front of the camera lens is very important. Care must be taken to ensure that no direct light falls on the lens, which has to be completely covered by the prism. With a deviation angle α of the prism amounting to about 45◦ , the angle γ between the bottom of the prism and the camera lens should be about 25◦ . For a smaller angle α, the angle γ decreases.

Figure 3-1: Camera with prism — construction principle.

It is convenient to fit the prism and camera into a wooden box; this prevents light reflections on the prism from being projected through the camera lens. This box can also house some heating elements, as experience shows that large prisms are easily covered with dew. An automatic camera for meteor spectroscopy using a prism has described by Degewij (1965). If a Rowland grating is in use, it is not necessary to use a lens with a long focal length. In this case a small frame camera with a wide angle lens will suffice. The use of a diffraction grating has the advantage that a direct image of the meteor occurs on the film and that higher orders of the spectrum may also be recorded. The disadvantage is that the Rowland grating has to be large enough to cover the entire camera lens, a condition that increases the price of the grating considerably. The focal length of a lens is normally given for a certain wavelength in the visual range. With a spectrograph we are dispersing the light. Therefore, we obtain parts of an image containing separate wavelengths and consequently different focusing points. Since meteor spectra contain many lines towards the blue part of the spectrum (see the analysis described in section 4 below), it may be useful to focus on the Hydrogen Balmer lines prominent in early type stars or other lines in the blue region. For these wavelengths f is slightly shorter, the lens should be adjusted to a distance of “less than ∞”. Trials with terrestrial light sources may be useful.

40


3.3. The orientation of the spectrograph When the spectrograph is aimed at the sky, the orientation of the instrument requires special attention. In many cases this aspect of meteor spectroscopy is completely forgotten or ignored, the consequence being that the very few bright meteors that appear in the camera field are photographed but without a spectrum! Therefore, the spectrograph must be oriented relative to the radiant in such a way that a bright meteor from that radiant will occur perpendicular to the dispersion direction of the spectrograph, in other words, parallel to the long-axis of the grating. Otherwise, the meteor will fill up its own spectral image. An example is presented by Russell (1990). Generally the magnitude threshold for a spectrograph is 2m . . . 3m brighter than with the same lens used for direct imaging.

3.4. Double station work and spectroscopic photography As with an ordinary camera, a spectrograph can be aimed at a given point in the sky. Aiming the camera requires that the optical axis of the camera be fixed (i.e. no clock drives to follow the motion of the sky), so the grating or the prism must be rotated relative to the radiant in such a way as to have any stream meteors travel perpendicular to the dispersion direction. During a double station project of several hours, the position of the prism, or the grating, has to be corrected regularly as the radiant will move relative to the camera position, which is fixed in azimuth and elevation. This should be taken into consideration when building the mounting of the spectrograph. It is also desirable to add a rotating shutter in front of the spectrograph. In this way, it will be possible to record the spectral lines of the meteor wake in the interruptions on the trail produced by the shutter blades. Otherwise the wake radiation will be inseparable from the head radiation, which makes a detailed spectrum analysis difficult.

3.5. Summary of equipment The appropriate equipment should consist of: • a camera, preferably with f = 80 . . . 120 mm lens and large film format • a prism OR a diffraction grating which is well mounted in front of the lens and oriented such that the dispersion of the expected shower meteors is optimal • a heating system for the whole optics • a rotating shutter in front of the prism/grating The data to be noted before or during the exposures are: • focal length of the lens • data concerning the prism (deflecting angle) or the grating (lines per millimeter), respectively • start and end of each exposure (use only UT to avoid confusion) • region of the sky photographed • orientation of the prism / grating • film used • time and magnitude of brighter meteors (if seen)


41

4. Analysis of meteor spectra A meteor penetrating into the Earth’s atmosphere causes excitation and ionization of atoms and molecules of the air as well as of the atoms of the meteoroid. Different states of excitation correspond to discrete levels of energy. Thus, we will find several monochromatic images of a given meteor (i.e. each spectral line), parallel to each other and of different intensity, while the spectra of stars contain continuous parts and absorption (but rarely emission) lines. Since the direction of travel of the meteor cannot be foreseen, the scale of the wavelength discrimination is unknown, and we have to calibrate it for each photograph separately. Therefore we need a “reference standard”. A spectrum of a star of spectral type A will serve this purpose quite well. It contains bright lines of the Balmer series (hydrogen spectrum). In the following paragraphs we briefly describe how to measure and calculate meteor spectra. All measurements of spectra have to be carried out perpendicular to the monochromatic images of the object (star, meteor). You may measure positions with a measuring microscope or any other device allowing precise coordinate determination in the required direction. The procedure described below gives the calibration of the dispersion relation, i.e. the amount of dispersion as a function of the wavelength. This has to be done for a given combination of lens and dispersing element (prism or grating). For this, we need an object in the same field providing well defined lines of known wavelength. Such a source can be either a suitable reference star, preferably one of type A, or a terrestrial emission lamp providing enough lines in different parts of the spectrum. For example, a mercury-vapour-lamp, which may be in use for street illumination, will suit this purpose (Fig. 3-2). The high pressure sodium-vapour lamps (orange light) emit light mainly from a doublet line which is broadened by the high pressure and hence does not allow the accurate measurement of line positions. In any case, you will need a reference table containing the wavelengths and some information about the relative intensity of the spectral lines appearing in the light source used for calibration. The example given here is from Bakulin (1973, pp. 311-316). The described procedure is somewhat complicated. We mention other options below. For the procedure we now describe, five reference stars with strong hydrogen lines were chosen.

Figure 3-2: Examples for spectra of a mercury-vapour emission lamp obtained with a simple prism of 45◦ refracting angle and an f = 50mm lens (above) and with a transmission grating used with an f = 135mm lens (below), respectively. In the first case the camera was moved in vertical direction to get broader lines. The latter example shows the zero-order image (left) as well as the first-order spectrum.

42


Figure 3-3: Lines in stellar spectra and the coordinate x used for the described measurements. The point 0 may be edge of the image. Table 3-1: Measurements of the positions of hydrogen lines in five star spectra.

No. of star line Hβ Hγ Hδ Hε Hζ Hη coord. of center

III

VII

IX

XIII

XIV

1.2606 1.0761 0.9635 0.8915 0.8432 0.8093 1.0070

1.6142 1.4278 1.3166 1.2456 1.1945 1.1656 1.3597

3.4276 3.2488 3.1393 3.0669 3.0231 3.9897 3.1811

1.3362 1.1530 1.0406 0.9687 0.9218 0.8888 1.0841

0.7710 0.5818 0.4686 0.3956 0.3466 – 0.5127

Firstly, in each reference spectrum the coordinates of the main (known) lines are measured in mm from the edge of the image. In order to define the scale for the wavelengths we now calculate the distances of the lines from the centre as shown in Fig. 3-3. Table 3-2: Definition of the scale as shown in Fig. 3-3.

line Hβ Hγ Hδ Hε Hζ Hη

III 0.2536 0.0691 –0.0435 –0.1155 –0.1638 –0.1977

VII 0.2545 0.0681 –0.0431 –0.1141 –0.1652 –0.1941

IX 0.2465 0.0677 ... ... ... –0.1914

XIII 0.2521 0.0689

XIV 0.2538 0.0691

–0.1953

position x 0.2530 0.0686 –0.0432 –0.1153 –0.1631 –0.1946

wavelength in ˚ A 4861.6 4340.5 4102.0 3970.3 3889.2 3835.5

The next step in this example is to find a connection between the average deviation of a given spectral line from the position of the star’s image wihtout any dispering element. In the case of a diffraction grating this is the zeroth order (cf. Part 5, sections 3.2 and 3.3). We may call this the “white” image of the star. For the determination of a relation between the distance from this “white” image to the position of a line of given wavelength, we choose three lines. Table 3-3: Three lines chosen for calibration from Table 3-2.

line Hβ Hγ Hη

x 0.2530 0.0686 –0.1946

λ, ˚ A 4861.6 4340.5 3835.5


43

By using the following equation of dispersion (called Hartmann’s in the literature) we may define the scale for all further measurements on a given film or plate by solving for the unknown constants using the calibration lines (ie. data given in Table 3-3): c0 (1) k0 − x with λ being the calculated wavelength from the photograph, and c0 , k0 and λ0 being constants for the photograph to be derived from the measurement, i.e. we have to find a solution of three equations with three unknowns. x is the measured relative position of a spectral line. Therefore, we need at least three spectral lines for the procedure (but more is preferable). From the values in Table 3-3, the following constants can be calculated: λ0 = 1665.4˚ A, k0 = 1.19963, and c0 = 3025.6. Then we obtain the differences given in Table 3-4. In fact, the differences for the three lines used for calculation should be zero. With these values we may check the whole measurement done so far: λ = λ0 +

Table 3-4: Measured and expected wavelengths.

x, mm 0.2530 0.0686 –0.0432 –0.1153 –0.1631 –0.1946

calculated λ 4861.6 4340.5 4099.8 3966.4 3885.6 3835.5

“catalogue” λcat 4861.6 4340.5 4102.0 3970.3 3889.2 3835.5

difference ∆λ, ˚ A ±0.0 ±0.0 +2.2 +3.9 +3.6 ±0.0

The differences found are rather small. If you want to derive precise quantities it is necessary to minimize these errors according to the next equation: ∆λ = ∆λ0 +

c0 ∆c0 − ∆k0 k0 − x (k0 − x)2

(2)

This leads us to a system of six equations using the values calculated before: 0.0 = ∆λ0 + 1.06∆c0 − 3399∆k0

(3 − 1)

0.0 = ∆λ0 + 0.88∆c0 − 2365∆k0

(3 − 2)

2.2 = ∆λ0 + 0.80∆c0 − 1959∆k0

(3 − 3)

3.9 = ∆λ0 + 0.76∆c0 − 1750∆k0

(3 − 4)

3.6 = ∆λ0 + 0.73∆c0 − 1629∆k0

(3 − 5)

0.0 = ∆λ0 + 0.72∆c0 − 1556∆k0

(3 − 6)

There are enough equations to solve this whole system. The system of six equation has to be solved by the least squares method (there are only 3 unknowns so a best ”fit” has to be made). You may find basics of linear regression in a variety of books. As an example we mention Taylor (1982). We derive the following, corrected values of the constants for the equation of dispersion using the least-squares method: ∆λ0 = +5.18 ∆ c0 = +2.16 ∆ k0 = +0.00235 and, consequently, the dispersion relation to be used is: λ = 1680.2˚ A+

3002.4˚ A 1.1968 − x

(4)

44


Usually, such a corrected equation permits the meteor spectra to be reduced in an accurate way. The amount of measuring error can be expected to be larger than the remaining error included in the formula. A dispersion relation of the form: c0 λ = λ0 + (5) (k0 − x)a leads to sufficient accuracy, usually with a = 1. In practice, you may also have a number of “suspected” lines in a meteor spectrum, which then can be used for calibration lines. Then you need to measure only the meteor spectrum and treat the lines as in the above example. You may have to run through the procedure several times. If only one line is measurable (and this one line must always be the “suspect” line), the constants λ0 and c can be taken from the calibration spectrum (ie. a star or lamp) and k can be computed from the “suspected” line. In fact, k depends only on the origin of the coordinate system.

Figure 3-4: Example of a meteor spectrum (from Halliday (1967), p.93).

Knowing the dispersion relation, we now can start measuring the meteor spectrum. Since a meteor spectrum is a line emission spectrum, we are essentially obtaining an image of the meteor in each separate line. The appearance of any given line, its intensity or its disappearance, gives information about the meteoroid, the ablation process as well as the high atmosphere. We determine the distances between the spectral lines in the direction c (ie. a direction normal to the lines). Furthermore, we need to know the angle ψ between the dispersion direction and the meteor trail, because we have to consider the corrected distances b in the dispersion direction, through the relation c b = c sec ψ = (6). cos ψ This is for normal dispersion, which allows us to use the relation determined above. The most intense lines in a meteor spectrum are generally the H and K lines from ionized Calcium (Ca II). These are situated towards the violet end of the spectrum and are quite easy to identify. We may use the coordinates of one of these twin lines as the origin of our coordinate system for measuring the meteor spectrum. Depending on the length of the meteor trail, we will measure the line positions at several points along each line, say near the beginning and the end, and perhaps at flares or other easily identifiable points. This is demonstrated in Fig. 3-5.


45

Figure 3-5: Coordinates and angles used for the measurement of a meteor spectrum. The symbols are explained in the text along with the equations and some accompanying instructions. From this figure it becomes obvious that the angle ψ should be as small as possible. This can be realized, however, only for meteors of a given shower since their directions are known in advance while the directions of sporadic meteors are random. As discussed before, this is achieved by orienting the grating or prism so its longaxis is perpendicular to the expected direction of motion of the meteor. The differences ∆λ otherwise appear to become smaller (smaller ∆c, but the same ∆b), and small wavelength differences ∆λ become unresolvable as in the case ∆λ1 in the right hand scenario.

We continue to describe the method begun above. The following example is adapted from the Russian Handbook (Bakulin, 1973). At some chosen n positions, an , along the trail (numbered 1 to 5 in our example, Fig. 3-6), we measure the coordinates bn of the spectral lines. In this example, we choose l = 8 lines in order to demonstrate the procedure. The coordinate a is perpendicular to the dispersion direction b, while c follows the meteor’s direction. Table 3-5: Sample of l = 8 spectral lines measured as described above. The positions number n = 1 − 5 are measured in the direction a, the positions of the lines are then given in bn [mm] as shown in Fig. 3-6.

position in a [mm] line number n a = 3 b= 6 8 9 10 12 15 18

1 2.1428 1.8444 1.9136 1.9665 1.9854 2.0332 2.1167 2.1875 2.2575

2 2.1737 1.8808 1.9496 2.0070 2.0258 2.0742 2.1570 2.2252 2.2922

3 2.1967 1.9108 1.9732 2.0354 2.0540 2.1050 2.1824 2.2540 –

4 2.2227 1.9483 2.0130 2.0680 2.0905 2.1366 2.2157 2.2882 –

5 2.2716 2.0092 2.0730 – – 2.1959 2.2790 2.3470 –

line intensity – 1 2 10 10 3 1 5

46


Figure 3-6: Calculation of the “average meteor” from the measured positions of spectral lines.

Next, we choose lines that were measured at all positions along the trail. In our example, these are the lines 3, 6, 10, 12 and 15. Their coordinates bn are averaged now, i.e. we calculate the “average meteor”. For example, the point n = 1 with the coordinate a1 given in the table above, was measured at b3,1 = 1.8444 (line 3, position 1), b6,1 = 1.9186 (line 6, position 1), b10,1 = 2.0332 (line 10, position 1), b12,1 = 2.1167 (line 12, position 1) and b15,1 = 2.1875 (line 15, position 1). The average position is b1 = 2.0191. This point we define as the beginning of the coordinate parallel to a, and set it to b01 = 0. The other average coordinates bn are b2 = 2.0574, b3 = 2.0854, b4 = 2.1204 and b5 = 2.1808. Since b01 = 0, we find b02 = b2 − b1 = 0.0383, and furthermore b03 = 0.0663, b04 = 0.1013 and b05 = 0.1617. As shown in Fig. 3-6, we now have defined the “average meteor” and have shifted the coordinate b and may also find the average positions of the spectral lines in the b0 coordinate system. With these figures we may reduce the values determined in Table 3-5 to average positions, as shown in Table 3-6. We also “suspect” lines 8 and 9 to be the Calcium Ca II doublet, with line number 8 = Ca K with λ = 3934˚ A. With the help of the dispersion relation, we now may identify the other lines. We put this value for line number 8 into the equation λ − 1680.2 =

3002.4 1.1968 − x

i.e. 3934 − 1680.2 =

3002.4 1.1968 − x

(7),

(8)

and find x = −0.1351. From our measurements we obtained the coordinate b8 = 1.9678, and we now need to know the values of x for the other lines n as well as to calculate their wavelengths from the dispersion relation. For each line n we now may find xn = bn − const

(9).

From the first case we calculate const = b3 − x3 = 1.9678 + 0.1351 = 2.1029 for the identified line 8. Now we find the other xn , and consequently also their wavelengths.


47

Table 3-6: Final calculations after line number 8 assumed to be Ca K (3934 ˚ A).

line number 3 6 8 9 10 12 15 18

1 1.8444 1.9136 1.9665 1.9854 2.0332 2.1167 2.1875 2.2575

2 1.8425 1.9113 1.9687 1.9875 2.0359 2.1187 2.1869 2.2539

3 1.8445 1.9119 1.9691 1.9877 2.0352 2.1161 2.1877 –

4 1.8470 1.9117 1.9667 1.9892 2.0353 2.1144 2.1869 –

5 1.8475 1.9113 – – 2.0342 2.1173 2.1853 –

bn 1.8452 1.9120 1.9678 1.9874 2.0348 2.1166 2.1869 2.2557

xn −0.2577 −0.1909 −0.1351 −0.1155 −0.0681 +0.0137 +0.0840 +0.1528

λ 3744 3844 3934 3968 4054 4218 4378 4556

We now present a table of selected lines which were found in meteor spectra (Tab. 3-7). A longer list is given by Ceplecha (1966) and Ceplecha (1971). Table 3-7: List of spectral lines frequently found in meteor spectra and their relative intensities. The identification of the lines (numbers) in our example is also given. Lines marked with an asterisk appear bright in spectra of fast meteors, such as the Perseids, but much fainter in spectra of slow meteors.

Laboratory data ˚ λlab , [A] atom/ion intensity 3719.9 Fe 10 3734.9 Fe 8 3737.1 Fe 9 3745.6 Fe 8 3749.5 Fe 8 3820.4 Fe 9 3825.9 Fe 8 3829.4 Mg 10 3832.3 Mg 11 3838.3 Mg 12 3859.9 Fe 11 3886.3 Fe 9 + 3933.7 Ca 40* 3968.5 Ca+ 35* 4030.8 Mn 10 4045.8 Fe 10 4063.6 Fe 9 + 4131.0 Si 1* 4226.7 Ca 11 4254.4 Cr 9 4271.8 Fe 10 4274.8 Cr 8 4289.7 Cr 7 4307.9 Fe 10 4325.8 Fe 10 4383.5 Fe 14 4404.8 Fe 11 + 4481.2 Mg 15* 4920.5 Fe 3

ident. number 2 3

8 9

12

15

Laboratory data ˚ λlab , [A] atom/ion intensity 4923.9 Fe+ 2* 4957.6 Fe 4 5012.1 Fe 1 5018.4 Fe+ 3* 5110.4 Fe 1 5167.3 Mg 17 5172.7 Mg 25 5183.6 Mg 28 5208.4 Cr 10 5227.2 Fe 5 5269.5 Fe 14 5328.0 Fe 12 5371.5 Fe 9 5397.1 Fe 5 5405.8 Fe 6 5429.7 Fe 6 5434.5 Fe 4 5446.9 Fe 4 5455.6 Fe 4 5528.4 Mg 2 5615.7 Fe 1 5890.0 Na 40 5895.9 Na 35 6156.8 O 1* 6162.2 Ca 1 + 6347.1 Si 6* 6371.4 Si+ 3* 6495.0 Fe 1 6562.9 H 2*

ident. number

48


Another way to relate measured coordinates x of dispersion into wavelengths λ is through the use of an interpolation polynomial, especially for gratings. (A simple linear dispersion is a good first approximation for grating spectra.) You should have several lines already identified (or with “suspected” identifications) and make a fit using a polynomial of degree somewhat less than the number of identified lines. You can check your preliminary identifications of lines and add more identifications by the fit obtained with this polynomial. You can use this procedure iteratively, up to a degree of about 6 to 8. Then you can use the final polynomial to try to identify all spectral lines in your meteor spectrum (Ceplecha, 1991). A meteor spectrum photographed with an optical transmission grating is usually off the optical axis of the camera. The diffracted rays of different wavelengths lie on the surface of a cone, the axis of which is parallel to the grating grooves. The section of the diffraction cone intersected by the focal plane is the diffraction hyperbola, along which the spectral record is distributed. Absolute wavelengths can be found from such a spectrum if it includes the zero-order image. An excellent example of a fireball spectrum was provided by Jiˇri Boroviˇcka. It is reproduced and explained in detail in the photograph section in Part 10 of theis Handbook. A method for wavelength determination from spectra obtained through diffraction gratings is described by Ceplecha (1961). The spectrum must be photographed with a rotating shutter. The measurements of one break in three different lines then define the diffraction hyperbola, and thus the connection between the coordinate system of the plate and that of the grating. The coordinate system of the grating directly determines the position of individual lines, so if we know the precise number of grooves per millimeter and the direction cosines of the zero-order image of the meteor, wavelengths of all lines can be computed without previous identification (Ceplecha, 1961; Ceplecha and Rajchl, 1963). Finally, we note that a list of 189 line identifications in a meteor spectrum can be found in Ceplecha (1966) and a list of 990 lines identified in a fireball spectrum is given by Ceplecha (1971). The photometric calibration of a spectrum is of interest if line intensities should be determined. However, this is a very specialized task and is not further described here. The basic procedure involves finding the characteristic curve of the film as described in Part 8 (Photometric measurements). This can be done from zeroth order star images for grating spectra. To permit the comparison of line intensities, the correction for the sensitivity of the film to different wavelengths must be applied. The spectral sensitivity curve of a given emulsion can normally be obtained from the manufacturer or may be measured on a known spectrum.

References and bibliography: Bakulin P.I., 1973: Astronomical Calendar. Nauka, Moscow [in Russian]. Barron R., Russell J.A., 1967: Some unusual spectra of meteors from the Palomar 18-Inch Schmidtfile. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 119–127. Bumba V., Valniˇcek B., 1954: The meteor spectra of the Perseid shower 1953. Bull. Astron. Inst. Czechosl. 5 108–111. Bumba V., Valniˇcek B., 1955: Das Spektrum des Meteors 1954 Dezember 14. Bull. Astron. Inst. Czechosl. 6 18–20. (in German) Ceplecha Z., 1961: Determination of wavelengths in meteor spectra by using a diffraction grating. Bull. Astron. Inst. Czechosl. 12, 246–250. Ceplecha Z., 1966: Complete data on iron meteoroid (Meteor 36221). Bull. Astron. Inst. Czechosl. 17, 195–206. Ceplecha Z., 1967: Meteor spectra. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 73–83. Ceplecha Z., 1971: Spectral data on terminal flare and wake of double-station meteor no. 38421 (Ondˇrejov, Apr. 21, 1963). Bull. Astron. Inst. Czechosl. 22, 219–304.


49

Ceplecha Z., 1973: Evidence from spectra of bright fireballs. In: C.L.Hemenway, P.M.Millman and A.F.Cook (eds.): Evolutionary and Physical Properties of Meteoroids. Washington (NASA SP-319), 89–102. Ceplecha Z., 1991: personal communication Ceplecha Z., Rajchl J., 1963: Meteor spectra with high dispersion. Smithson. Contr. Astrophys. 7, 129–153. Degewij J., 1965: Een automatische Meteoorspektrograaf. De Meteoor 21, 63–66. (in Dutch) Degewij J., 1967: Lyridespektrum. Hemel en Dampkring 65, 161–166. (in Dutch) Evans S.J., 1992: Meteor photography. J. Br. Astron. Assoc. 102, 336–342. Fritzová L., Rajchl J., 1957: Two meteor spectra. Bull. Astron. Inst. Czechosl. 8, 148–150. Halliday I., 1967: The influence of exposure duration and trail orientation on photographic meteor spectra. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 91– 104. Halliday I., 1987: The spectra of meteors from Halley’s comet. Astron. Astrophys. 187, 921–924, Harvey G.A., 1974: Four years of meteor spectral patrol. Sky & Telescope 47, 378–380. Hirose H., Nagasawa K. and Tomita K., 1967: Spectral studies of meteors at the Tokyo Astronomical Observatory. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 105–118. McCrosky R.E., Posen A., Schwartz G. and Shao C.-Y.: Lost City Meteorite – Its recovery and a comparison with other fireballs. J. Geophys. Res. 76, 4090–4108. Millman P.M., 1967: A brief survey of upper-air spectra. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 84–90. Millman P.M., 1980: One hundred and fifteen years of meteor spectroscopy. In: I.Halliday and B.A.McIntosh (eds.): Solid Particles in the Solar System. IAU-Symp., 121–128. Millman P.M., 1983: Current trends in meteor spectroscopy. In: R.M.West (ed.): Highlights of Astronomy. 6, 405–410. Millman P.M., McKinley D.W.R., 1963: Meteors. In: B.M.Middlehurst, G.P.Kuiper (eds.): The Solar System IV: The Moon, meteorites and comets. Chicago, 674–773. Poole L.M.G., 1978: The decay of luminosity in the trains of moderately bright meteors. Planet. Space Sci., 26, 697–701. Russell J.A., 1980: Correlation of height and forbidden oxygen line strength for Perseid meteors. In: I.Halliday and B.A.McIntosh (eds.): Solid Particles in the Solar System. IAU-Symp., 129–132. Russell J.A., 1981: Spectral-height relations in Perseid meteors. Astrophys. J. 243, 317–321. Russell J.A., 1990: Dissimilarities in Perseid meteoroids. Meteoritics 25, 177–180. Skopal A., 1983: Characteristic curve of meteoric spectrograms obtained by an optical diffraction transmission grating. Bull. Astron. Inst. Czechosl. 34, 173–178. Taylor J.R., 1982: An introduction to error analysis. University Science Books, Mill Valley, CA.

50


PART 4: METEOR TRAINS

1. Introduction Bright meteors, particularly fireballs, often leave an “afterglow” along their trail, called a train. Such trains have been described extensively in the past (e.g. Beech, 1987). High velocity cometary-type meteors especially produce persistent trains, but little is known about the mechanisms behind these phenomena. When a meteoroid enters the Earth’s atmosphere its surface atoms sublimate and are turned into ions through collisions with atmospheric particles, which will themselves be ionized simultaneously. Ions of both the meteoroid and the atmosphere are then present along the trajectory. While most of these atoms exist in excited states for about 10−8 seconds only, there are also some atomic states that may persist for times longer than several seconds (so-called metastable levels), perhaps associated with some molecules (N2 , NO, NO2 , . . . ?). These appear bright against the night sky. However, much mystery still surrounds the physics of train phenomena (Ceplecha, 1991). A large meteoroid entering the atmosphere also distributes a substantial amount of material along its trajectory as it ablates. This may lead to the formation of the so-called smoke trains, which do not emit light, and are composed of dust particles. Both types of trains occur at altitudes above 20 km. At these heights strong winds exist that will cause the trains to distort within a short period after formation. Thus observations of trains also may yield information about the upper atmosphere as well as about the amount of material brought into the atmosphere by the meteoroid.

2. Photography 2.1. Regular trains Ceplecha Even regular meteor observers are surprised by bright meteors or fireballs. Normally data about the brightness, time of occurrence, colors, and perhaps sound of a fireball are recorded in detail. Only for very bright and/or long enduring trains might someone think to take photographs of the train. To secure a train on film you might try the following while you are visually observing. Keep a camera with a high sensitivity film (cf. section 4 of Part 1: Faint Meteors) and a very fast lens with an already-opened shutter near at hand, protected from exposure by a suitable cover. When a bright

Part 4: Meteor Trains

51

event leaving a train occurs, you remove the cover and expose the film for about 20 . . . 40 seconds. For very bright and long enduring trains, take a series of photographs to record the variations in brightness and shape. Each exposure should be between 10 and 30 seconds long. Guiding the photographs is not necessary. Generally, observers are not prepared in the manner we have just described, so chance plays a major role. Some instructive examples are presented in Hirose et al. (1967), p. 110, in Cook and Hughes (1957), in Keller and Schmidbauer (1988) and also in the Part 10 of this handbook (Fig. 10-12). Experiments conducted during the 1989 and 1991 Perseid shower suggested there were very few meteors leaving suitably bright trains that could be photographed. As a result, the time for the operator to remove the cover from the lens was comparable to the duration of most of the trains. As already mentioned, train photographs of very bright fireballs, the trains of which were visible for minutes, do exist.

2.2. Smoke trains Since smoke trains do not emit light, they appear dark, thus they are not visible in the night sky. There are two possible ways of observing them. First, they may be visible accompanying daylight fireballs as smoke trains. Of course, it is not feasible to establish a regular survey for daylight events, but if you do accidentally see a bright fireball during the day, you should note if this kind of train is formed, and if so try to photograph it. In this circumstance, the rules of “normal” (daylight) photography are valid, and no special instructions are necessary. To be sure that the structures in a daylight smoke train are measurable, take photos with different exposure times, both over- and underexposing the image. The second way of seeing such trains occurs in twilight (Fig. 4-1). Since smoke trains appear below 80 kilometers altitude, they are still lit by the Sun, even in late twilight. Consequently, they appear bright. The conditions for the visibility of smoke trains as sunlit clouds after sunset or before sunrise are similar to those for noctilucent clouds.

Figure 4-1: Higher altitudes in the atmosphere are still lit by the Sun even when it is below horizon, causing twilight phenomena and also illuminating noctilucent clouds (at h = 83 km) and smoke trains left by meteors. The figure does not consider atmospheric refraction, which increases the duration of visibility somewhat (cf. Gadsden and Schröder, 1989, pp. 149–153). Geometrically, with the Sun being 9◦ below the horizon, the zenith Z of the observer at O is still lit by direct sunlight, while at a depression angle of 19◦ a near-horizon area (around H) receives sunlight.

52


Noctilucent clouds are frequently seen at latitudes north of 50◦ N during the period May-August as well as south of 50◦ S in the months of November-February. These clouds are situated at a mean height of 83 km. For the clouds to receive sunlight, the sun must not be more than 16◦ below the horizon at this altitude. The diagram shows the time interval during the night on different months when the depression angle of the Sun permits these clouds to be visible for three different latitudes (Fig. 4-2). These times correspond roughly to the periods at night when smoke trains may be lit, so you can find which times permit observations of smoke trains in the region of the twilight arch. If a smoke train is observed you may make a series of exposures of about 10 . . . 20 seconds duration (depending on the brightness of the sky and the train). If you are not sure about the optimal exposure time to use, try several, but not less than 3 seconds and not longer than 2 minutes. In choosing film types, high sensitivity films are ideal as they keep the exposure times short. If the exposure is too long, the shape of the train will distort during the exposure and the image of it will appear blurred. It is then hard to measure structures contained in the train. Both types of trains were recently observed accompanying a daylight fireball which appeared shortly before sunset, with the trains visible at heights between 30 and 10 km (Ceplecha, 1991). An excellent example of a very bright smoke train observed in the twilight, is reproduced in Part 10 (Fig. 10-13).

Figure 4-2: Diagram of the visibility of noctilucent clouds which occur at an altitude of ≈ 83 km. During the indicated twilight period (white) the Sun is between 9◦ and 18◦ below horizon. This is also the most favourable time interval for the observation of sunlit fireball trains. The same diagram, shifted by 6 months, is valid for the same latitudes in the southern hemisphere. For ϕ ≥ 65◦ and ϕ ≤ 40◦ the time spans for effective observations are short.

Part 4: Meteor Trains

53

3. Equipment The equipment required to photograph trains is different depending on the train type.

3.1. Regular trains at night 1. camera with a very fast film and lens 2. shutter already open, but covered (e.g. by a dense piece of cloth) 3. when a bright meteor appears and leaves a persistent train you remove the cover and expose the frame for a length of time dependent on the brightness of the train (at least 20 . . . 40 seconds) 4. if the train lasts for more than one exposure, continue with exposures of about the same duration.

3.2. Smoke trains in daylight 1. “normal” daylight photography; slight underexposure may be best 2. try to make a series of photographs and note the time of appearance of the fireball (if seen) as well as the time of the train exposures. 3. note azimuth and zenith distance of the train.

3.3. Smoke trains during twilight 1. camera with a fast lens and film 2. try to make several exposures of different durations, depending on the film and lens used(try at least 10 . . . 20 seconds) 3. note the time of the fireball’s appearance (if seen) and the time(s) of the exposures taken. 4. note azimuth and zenith distance of the train.

4. Use of a video camera These phenomena can also be recorded with a video camera. The dust trains in daylight or twilight do not require special hints. One advantage of a video camera over a still camera is the time mark which is included on each frame. Even if the time is not precisely adjusted to a time signal, you will still record the relative time scale of the changes within a train. Regular trains during the night may be much more difficult to record because of their faintness.

54


References and bibliography: Beech M., 1987: On the trail of meteor trains. Q. Jl. R. astr. Soc. 28, 445–455. ˇ Ceplecha Z., 1967: Meteor spectra. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 73–83. Ceplecha Z., 1991: personal communication. Cook A.F. and Hughes R.F., 1957: A reduction method for the motions of persistent meteor trains. Smithson. Contr. Astrophys., 1-No. 2, 225–237. Gadsden M. and Schröder W., 1989: Noctilucent clouds. (Physics and Chemistry in Space 18). Springer. Hirose H., Nagasawa K. and Tomita K., 1967: Spectral studies of meteors at the Tokyo Astronomical Observatory. In: L.Kres´ ak and P.M.Millman (eds.): Physics and Dynamics of Meteors. IAU-Symp., 105–118. Keller P., Schmidbauer G., 1988: Helle Leuchterscheinung in der Faschingsnacht. Sterne und Weltraum 27, 249. (in German) Terentjeva A., 1990: Main problems of visual meteor observation. In: P. Sp´ anyi and I. Tepliczky (eds.): Proceedings IMC 1989, Balatonf¨ oldv´ ar, Hungary, 53–57.

Part 5: Additional Equipment & Construction Hints

55

PART 5: ADDITIONAL EQUIPMENT AND CONSTRUCTION HINTS

1. Rotating shutters 1.1. Reasons for the use of a shutter It is possible to derive the mean velocity and deceleration of a meteor, required for the computation of a heliocentric meteor orbit, if we know how long the luminous appearance lasted. As the duration of a meteor trail is less than one second in most cases, it is rather impractical to use a stop watch to estimate this time and we have a more accurate device, which can be easily constructed, called a rotating shutter in any case. The shutter is a disc with two or more open sectors which is placed before the camera lens and once in rotation will interrupt the exposure at regular intervals. The rotation speed and shutter blades are designed to give several interruptions during the time of a meteor’s appearance which will produce a dashed line on the negative as the meteor’s trail. Star trails will remain as unbroken arcs, however, unaffected by this process.

1.2. Other concepts In principle, also other possibilities to interrupt the exposure at regular time intervals are applicable. At the IMC ’88 Steyaert (1988) reported about the use of an LCD (Liquid Crystal Display) instead of a rotating disc. A LCD of 5 × 5 cm2 was mounted in front of the camera lens. The reaction of such a LCD slows down with the temperature. Therefore it was heated to about 15◦ C. The advantages are the light construction, low power consumption, high accuracy when used with an oscillator, safety of use, lack of mechanical vibrations, and the possibility to easily changing the frequency and the ratio between dark and transparent phase (the latter is equivalent to the change of the angle of the shutter blades described below). Principially, such a shutter could also be integrated into the camera. There are no applications of LCD shutters known for regular use so far. Perhaps the disadvantages are not neglectable: the transmittance is of the order of 45% only, i.e. there occurs a loss of about 0.9m in meteor brightness. The breaks are reported to be not sharp, especially when approaching the upper limits of the frequency (25 s−1 ).

56


1.3. Construction of a rotating shutter MOTOR A bicycle dynamo suits this purpose when linked to the electricity mains by a 6 Volt alternating current transformer. The dynamo, which has to be started by hand, behaves as a synchronous motor with a number of rotations per second equal to the frequency of the electricity mains (50 Hz) divided by the number of pole pairs of the dynamo. Most common types of bicycle dynamos have 4 pole pairs, which makes the number of rotations per second equal to 50/4 = 12.5. If the shutter has two blades (sectors), this corresponds to 2 × 12.5 = 25 interruptions per second. To find out how many pole pairs your bicycle dynamo has, you can pass 6 Volts through it and take it in one hand. With the other hand you turn the little tyre wheel through 360◦ . You will feel some resistance (shocks) at certain points, twice as many as there are polar pairs. A dynamo with 4 polar pairs will show 8 points of resistance in one revolution, for instance. It should be pointed out that there are sometimes problems with this kind of motor in regular use. However, experience shows that such bicycle dynamo motors often lead to problems if used for regular work as in the case of fireball patrols. Alternatively, you may use a synchronous motor working with a 220 Volt alternating current. Preferably such motors should give 375 rotations per minute (equal to 6.25 rotations per second). Equipped with a two sector-shutter you will then obtain 12.5 breaks per second on your film. When using a relatively high voltage like this with your camera set-up an earth connection for the whole camera is recommended (in darkness and under damp conditions this is for your own safety). Normally, such synchronous motors have to be started by hand, and problems may occur if the power supply is interrupted which may especially happen on expeditions. THE SHUTTER DISC The diameter of the shutter disc must be as large as possible in order to give the blades sufficient tangential speed of the blades relative to lens. When the blades cover the lens very quickly breaks of the meteor trail will become much sharper. Care must be taken if a rotating shutter is working and operations of the camera are necessary in the dark, like starting a new exposure. Some training is recommended to avoid injuries due to the rotaing shutter blade. A suitable diameter for a dynamo driven shutter is about 30 cm. Larger shutter may need to be started with an electric drill, to get the shutter to the right rotation frequency by use of a friction connection. Starting a shutter is often not without some problems. It is possible to have to start over and over again as the motor must be brought to the exact rotation frequency before it will operate. The most useful material from which to produce the disc for a rotating shutter from is beyond doubt an aluminum plate of less than 1 mm thickness. On this aluminum plate you should draw a circle with a diameter of 30 cm divided into 4 equal sectors. With a hacksaw the disc and the two open sectors can be cut out. In this case the shutter will have two blades of 90◦ but it is possible to choose other angles for these blades. Most photographers use rotating shutters with 60◦ blades. It is very important to work very precisely and carefully when sawing out the open sectors. When the weight is unequally distributed over the shutter, its accuracy will decrease due to vibrations. Next, the sharp edges should be filed down and one side of the shutter should be painted matt black (use black board paint). This black side has to be aimed towards the camera lens. Right at the central point of the shutter you should drill a small hole where the shaft of the dynamo will be placed. It is essential that this point corresponds to the point of equilibrium of the shutter in order to avoid vibrations once in use. The advantage of using a shutter like this is that the exposure time can be extended before the same degree of fogging due to moonlight or light pollution is attained. For a 60◦ -blade rotating shutter for instance, the exposure time can be extended by more than 30%; for a 90◦ blade rotating shutter by over 50% (this is possible because of the low intensity reciprocity failure, described in detail in Part 1). Many amateurs have different types of shutter discs; for instance a 60◦ and a 90◦ blade shutter. The


57

most common disc (60◦ ) is used for nights without light interference, the other one for less favourable nights (with streetlights or the moon). Probably a 90◦ shutter is most suitable for fireball patrols when exposures will exceed several hours. 60◦ blades will leave shorter breaks and are therefore preferably applied for fast moving meteors, like the Leonids. In the case of bright meteors at a low angular velocity, breaks may become overexposed and thus badly visible. HOW TO PLACE THE SHUTTER DISC ON THE MOTOR AXLE Attaching the disc on to the dynamo may cause some trouble. Many photographers successfully use one of the methods shown in Fig. 5-1. For example, the shutter disc is placed between two screws if the axle of the motor allows to put a thread on its top (about 6 mm are needed), as shown in Fig. 5-1 (right). The disc-hole must not be too large.

Figure 5-1: Mounting of the shutter disc on the motor axle depending on the axle diameter. The thread to fix the shutter blade must be arranged according to the actual sizes; the measures given here should be regarded as examples.

If you use a synchronous motor with a thin axle, you must find out a method to mount the shutter disc yourself properly. It must be well connected with the motor axle and well centered. Although it is possible to glue the shutter disc to the motor axle, a removable connection is preferable. Perhaps a screw may fix the blade well (Fig. 5-1, left). Remember that synchronous motors often do not start automatically when you switch them on. Thus it is recommended to prepare a possibility to start them by hand, for example the motor axle can be prolonged to be easily reached. This must be considered especially if the motor and shutter are placed within a self constructed camera box (Fig. 5-2). MOUNTING THE ROTATING SHUTTER The camera and the rotating shutter must be mounted separately to avoid vibrations. The use of two different tripods is strongly recommended; one for the camera and one for the rotating shutter. The latter can be home-built. The major enemy of a rotating shutter is the wind. Wind has a decelerating effect on the blade. Consequently the actual number of rotations can be far below the theoretically assumed frequency. Therefore, try to protect the disc of the rotating shutter, for instance by placing it into a half open box. The camera has to be mounted in such a way behind the shutter blades that the entire lens is covered at every rotation. Please pay especial attention to this when using wide angle lenses. As already shown in Part 2 (Fireballs) it is possible to include the shutter inside the “camera” itself (Fig. 5-2). This avoids problems due to the wind and because you may use a very thin and quite

58


small shutter disc the vibrations are minimized too, providing the disc is not centered badly. Indeed, this set-up requires a very precise shutter balance, since optics, film, and rotating shutter are not separated, and all vibrations will badly affect the quality of the image.

Figure 5-2: Construction where the rotating shutter is within the camera box. Note the prolongation of the motor axle outside the box for starting the shutter. Although the blade may be quite small and does not weigh a lot, it must be well centered to avoid vibrations.

COMPUTATION OF THE TIME DURATION OF A METEOR Once a meteor photograph with a shutter-interrupted meteor trail is obtained, it is easy to determine the duration of the meteor’s appearance. Let N be the number of streaks, T the number of revolutions per second of the shutter and B the number of blades, then the duration d (in seconds) is given by: d=

N (T · B)

Example: on the meteor trail we counted N = 30 streaks, the shutter (2 × 60◦ ) we used made 25 revolutions per second: d=

30 = 0.6 s (25 s−1 · 2)

Visual observers may have estimated a longer duration as only the brightest part of the trail may have been photographed. STABILIZATION OF THE ROTATING SHUTTER In order to reduce the uncertainty of the duration of a meteor, we may develop a device to stabilize the rotation frequency of the shutter. A rotation stabilizer (for a shutter motor on battery current) is based on light detection. A light sensitive transistor catches the light of an IR LED placed above it on the opposite side of the blades. The light beam is interrupted periodically by the shutter blades, which creates pulses. The duration T of these pulses is inversely proportional to the rotation frequency. These light pulses are compared to an electronically generated pulse of constant frequency. When the generated pulse turns out to be shorter (longer) than the measured pulse, the motor is running too slow (resp.too fast). A solution is to have more (less) electric current in the motor, to compensate for the discrepancy in the rotation frequency. Care should be taken with small rotating shutters as sudden increases in electric current can cause some damage. This problem will not occur with rotating shutters with a sufficiently large moment of inertia (sufficiently heavy shutter disc). Whether you use a frequency stabilisation or not, it is recommended to check the rotation’s speed by using a stroboscopic lamp. There are such lamps available which will allow a shift in the frequency


59

which permits the determination of the precise frequency of rotating objects. Such rotating objects seem to stand still if the frequencies of the light emitted and the rotation are identical. Use such a lamp only for checking to avoid danger for your fingers which easily may be put between the rotating shutter blades.

Figure 5-3: This pattern can be copied and placed at the shutter disc. If lit with a stroboscopic lamp, the rotation frequency can be checked.

HOW TO BUILD A STABILIZER FOR A ROTATING SHUTTER The accuracy of the rotation frequency of the shutter is very important when deriving the velocity of the meteoroid afterwards. The velocity is in turn essential for heliocentric orbit calculations. In order to obtain these final results we need to know the rotation frequency as accurately as possible. Moreover we need to be certain that this rotation frequency remains stable during the entire period of the observation. Some ingenious devices have been developed by amateurs in the past to ensure this and people who are interested in such devices should contact the IMO Photographic Commission. See also the device description below. In most cases observers will make use of a feed connected to the electricity net. Normally the voltage is constant, although this may vary strongly, especially in areas with sparse population. A lot depends on the distance of the user from the closest high voltage distribution centre. Measurements during multiple station photographic campaigns in Europe have indicated that voltages may vary by ±30V. A steady voltage is essential for the tension delivered to the shutter by the motor. Too high a voltage is very bad for the motor (wear and tear) and may even set the motor alight. In some regions the voltage varies constantly and in such a case it is simply impossible to keep to the correct rotation frequency of the shutter. The best solution is a portable generator with a continuous display of the number of interruptions per unit time. A digital display allows the photographer to keep an eye on the reliability of the rotating shutter. Ignoring these aspects of meteor photography may result in a major disappointment if it turns out that the assumed constant rotation frequency of the shutter was not constant at all. Computing orbital elements from such poor photographic data is virtually useless.

2. Lens heating Many observing sites are damp at night and this can be a disaster for optical equipment. While optics manufacturers produce dew protection devices for telescopes, most photographers do not have equipment to keep dew off lenses. Most meteor photographers therefore build their own lens heaters, which are mostly very efficient. The commonest lens heaters dissipate about 20 Watts close to the lens of the camera and in this way the temperature near the lens is kept slightly higher than the surrounding air temperature. This is

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just enough to keep dew off the lens, without creating any warm-air turbulence. There are many possible ways to construct lens heaters. Some examples include: (1) Take a film bobbin from a 120 roll film and wind up about 5 meters of wire of resistance 2.5 Ohms per meter on it. Protect the bobbin and the wire with sticky tape and use a 6 Volt power source. This heating element feels warm in the hand. (2) An alternative is to link a number of small resistors in series until the required temperature is reached. The chain of resistors can be placed in a plastic tube if necessary. A 6 Volt power source will do here as well. (3) Another good way to keep the dew off is to attach a tube of brown paper to the lens house with an elastic band. The tube projects a few centimeters in front of the lens, and is supposed to work well. The heating elements have to be held by 2 or 4 pieces of elastic in front of or under the camera lens. This rather primitive fastening has the advantage that it can very quickly be made ready for use.

3. The spectrograph 3.1. Introductory concepts As you may know, visible light covers only a very small part of the electromagnetic radiation spectrum. White light is composed of radiation with different frequencies or wavelengths λ expressed in nanometers (nm) between about 400–700 nm (see Fig. 5-4). In spectroscopy you often will find the unit ˚ A(for ˚ Angstrøm) instead of nm, with 1 nm = 10−9 m, −10 ˚ ˚ 1 A= 10 m, thus 10 A= 1nm.

Figure 5-4: Electromagnetic spectrum.

A given atom or ion can remain stable at a well-defined energy level. When an atom is modified from a higher to a lower energy level, an amount of energy will be given out as radiation of a specific wavelength (emission). This wavelength typifies each kind of atom or ion if the energy level transition occurs. As a given wavelength is absorbed, so the atom or ion changes to a higher energy level. Likewise the luminosity of a meteor is composed of different wavelengths from which we can learn more about the composition of the meteoroid as well as the composition and state of the Earth’s upper atmosphere. In order to study this we need to separate the meteor’s light into its various wavelengths. This is possible when we put a prism or a diffraction grating before the lens of the camera. Some advantages and disadvantages of the use of a prism or a grating are discussed in Part 3 (Meteor


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Spectra), on p. 37 ff. Either dispersing element can be applied successfully, but a prism is the cheaper alternative. The probability of photographing a good spectrum however, is much smaller than with a more expensive diffraction grating.

3.2. The prism When using a prism with a refracting angle α = 45◦ , the limiting magnitude for photographic meteors will be reduced by 3m to 4m compared to the direct imaging. This decrease in magnitude can be reduced somewhat when a prism with a refracting angle α = 25◦ . . . 30◦ , perhaps made from a glass with a higher refractive index n is used. The amount of absorption will be less because of the shorter path length through the glass. Hence the decrease in meteor limiting magnitude will be of the order of 2m to 3m . This means mainly fireball spectra can be photographed. Regarding the limiting magnitude and the spectral resolution we have to find a compromise: a wide angle lens f ≤ 30 mm will concentrate the light into a rather narrow trail. The spectral resolution however is poor, and details of the spectrum will be lost. Features of a spectrum can be improved by using a lens with a long focal length, say f ≥ 80 mm. In this case the field of view becomes rather small and the chance of photographing a spectrum decreases again. A focal length f ≈ 100 mm combined with a 6 × 6-format camera, or an even larger film format, can compensate for this. For such tasks old-fashioned cameras may be very useful.

Figure 5-5: Light refraction in a prism.

Fig. 5-5 shows the refraction of light in a prism. The following equations are valid (cf. also Fig. 5-5): sin i = n · sin r r + r 0 = aa i + i0 − aa = δ Where n is the refractive index and aa is a characteristic number for the sort of material the prism is made from. The value δ is called the deviation relative to the angle of incidence. Since the index of refraction n depends on the frequency or wavelength, then the deviation angle δ will also depend on this wavelength. When the incident light beam (for instance of a meteor) is composed of different frequencies, every component will assume a separate deviation angle δ. This effect is called dispersion D: dδ D= dλ Our meteor spectrograph needs an n-value as large as possible. The best materials are in order of suitability: silicate flint-glass, boric flint-glass, quartz, and silicate crown-glass.

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3.3. The diffraction grating The use of a diffraction grating is based on the principle of interference, where wavelengths are intensified or weakened. Such gratings are made of optical glass with a thin transparent film smoked onto one side. The less expensive gratings have parallel lines etched in this film by using a diamond or a laser beam. The number of etched lines varies from 20 to more than 1000 per millimeter. Due to the often poor geometric quality of such cheap gratings we cannot expect first class images. Moreover, the transparency is very poor, which is most unfavorable for the limiting magnitude (and thus also for the possibility of catching a spectrum). Better, but much more expensive, gratings are made with fine laid-up threads (for instance Rowland gratings). The much better geometric quality and the high degree of transparency improve the probabilities of capturing a spectrum considerably. When light of different wavelengths falls on a grating, the various components form diffraction maxima at different angles ϑ (Fig. 5-6).

Figure 5-6: Light is diffracted in a grating with a distance a between its slits (lines) by the angle ϑ, which depends on the wavelength λ. The light passing through each slit then interferes with the light passing through the other slits. The result is a series of spectra as shown in Fig. 5-7.

Figure 5-7: Light passing through a grating gives (i) a bright zero-order image, where all wavelengths coincide (“white”), and (ii) spectra of higher order n with decreasing intensity. Since the dispersion D increases with the order number n, the orders overlap. (Note that for simplicity the lens behind the grating necessary for the image is not shown in the figure.)


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A series of maxima for all wavelengths of a given order makes up a spectrum. In this way we have spectra of the first order, the second order, the third order, etc. The longer the wavelength, the larger is the deviation for a given order. Thus the deviation is larger for red light than for blue. With a prism this works the other way round. The dispersion D of a grating is given by :

D=

dϑ n = dλ a cos ϑ

(Where n = 0, ±1, ±2, . . . ; a is the distance between lines on the grating) This shows that the higher the order, the larger the dispersion becomes. For order 0 (D = 0) all wavelengths fall together (see the example of a grating spectrum given in Fig. 3-2, p. 41). A diffraction grating has several advantages over a prism: it is independent of the dispersive characteristics of the material, the image is richer in details and the limiting magnitude of the more expensive Rowland grating is much more favourable (Mv = 0.5 for zero order). The less expensive gratings are however not very sensitive and require very bright meteors to yield any results. If somebody intends to really specialize in meteor spectroscopy it is worthwhile investing in a Rowland grating. The larger cost will pay off when the grating can be used regularly. More about meteor spectroscopy, especially procedures regarding their analysis, can be found in paragraph 4 of Part 3 (Meteor Spectra), p.40 ff..

4. Mounting and drive Almost nothing has been said about the equipment to place the camera and the additional devices on. Generally, it is sufficient to use stable tripods for all purposes. But the photographer might wish to obtain images with the stars being points. Photography of a field with both a guided and an unguided camera allows to determine the time of a meteor’s appearance. The image of a star moves over the film with an linear velocity vs which depends on the declination δ of the star. This velocity is zero at the poles. The linear velocity vs of a star at a declination δ is

vs =

2πf cos δ Tsid

with Tsid = 86164 s the Earth’s sidereal rotation period. Hence it moves by l mm within a time t [s]

t=

l l Tsid = vs 2πf cos δ

and we find the time lapse between the beginning of the exposure and the meteor’s appearance as shown in Fig. 5-8.

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Figure 5-8: Principle of the determination of the time of a meteor’s appearance from a guided (left) and an unguided (right) photography. The guided image gives the position relative to the stars, e.g. the differences ∆α. This difference ∆α has to be applied to the unguided image. Then we may find the time passed between the beginning of the star trail and the moment the meteor appeared. (The same procedure, of course, holds for the end of the exposure.)

Sine ce the usual durartions of the exposures are of the order of hours, the mounting must be well adjusted to the north or south pole. The guiding normally has to run without control by an observer. Therefore you should have tested a mountiong and guiding system intended for meteor work before using it regularly for example for a fireball patrol station. Otherwise the stars become trails of which we do not know the moving direction during the exposure. Construction of guidings and mountings are described in many amateur astronomer’s manuals and are not further outlined here. We want to note, that such a pair of cameras – guided and unguided – is regularly used at the Ondˇrejov Observatory for fireball patrol, and it allows to determine the appearance of fireballs with an accuracy of some seconds (except cases where the fireball appeared at low elevation angles or in hazy skies).

References and bibliography: Steyaert C., 1988: LCD shutters in meteor photography. In: J. Lanzing (ed.): Proceedings of the International Meteor Conference 1988. H.A.S.A. (Netherlands), 23–24. Vanmunster T., 1986: Handboek simultane & fotografische meteoorwaarnemingen. VVS Werkgroep Meteoren, Belgium. (in Dutch)

Part 6: Double-Station Work

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PART 6: DOUBLE-STATION CAMERA WORK

1. Computation of the direction of the optical axis Different methods exist for computing where to direct a camera for double-station photography. The following section describes one possible way to get some results. We assume meteors produce their maximum luminosity at about 90 km above the Earth’s surface. This means we have to observe the same area at 90 km height in the atmosphere from two or more geographic locations. Mostly we take a fixed point at 90 km altitude above a set location on the Earth which is at about an equal distance away from both observing stations. (e.g. at about 50 km distance). This point, called the projection point, can be determined with the help of an atlas or with a topographic map. First of all the geographic coordinates of this projection point (ϕp , λp ) have to be determined. For each observing station we determine the height h and the azimuth Az at which the camera has to be aimed, to be sure to photograph the area at 90 km above this projection point. A few formulae using trigonometry allow us to solve this problem. An example will clarify the working procedure: Step 1: Calculation of the angular distance from observing station to projection point. Since the computations are analogous for each observing station we need work with only one station here. X = arccos(sin ϕp · sin ϕo + cos ϕp · cos ϕo · cos(λp − λo )) Y = 60 · 1852m · X X is the angular distance on the spherical Earth’s surface between the two places (in ◦ ). As one arc minute corresponds to 1852 meters we find Y in meters. Step 2: Computation of the elevation angle of the camera. Once we know the distance Y , it is very easy to determine the elevation h at which we have to aim the camera. The sketch below makes it clear that : tan h =

90km 90km , h = arctan Y Y

Step 3: Computation of the azimuth Az of the camera direction. The azimuth is found as follows: sin (270◦ − Az) = cos (270◦ − Az) =

B 00 60 · 1852 · (ϕp − ϕo ) = Y Y

A00 cos (ϕp − ϕo )/2 · 60 · 1852 · (Lp − Lo ) = Y Y

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where tan (270◦ − Az) = B 00 /A00 and thus Az = 270◦ − arctan (B 00 /A00 ) The values of sin (270◦ − Az) and cos (270◦ − Az) allow the determination of the right quadrant where the azimuth is to be found. We remind readers who left school some time ago of the following rules: sin + and cos + then 0◦ ≤(270◦ −Az) ≤90◦ sin + and cos − then 90◦ ≤(270◦ −Az) ≤180◦ sin − and cos − then 180◦ ≤(270◦ −Az) ≤270◦ sin − and cos + then 270◦ ≤(270◦ −Az) ≤360◦ or use the following circle:

Figure 6-1: Sign of the sin and cos functions.

Some pocket calculators allow the conversion of an angle into the right quadrant automatically. Step 4: Computation of the declination and right ascension. To point the camera exactly it is recommended to convert the h and Az to right Ascension α and declination δ, in order to find the selected area to be photographed in a star atlas. The declination we compute from: δ = arcsin (sin ϕo · sin h − cos ϕo · cos h · cos Az) Once we know δ, we can compute the hour angle τ = (θ − α): sin τ = cos τ =

cos h · sin Az cos δ

cos Az · cos h + cos ϕo · sin δ sin ϕo · cos δ

Analogous to the computation of the azimuth, the values of sin(θ − α) and cos(θ − α) allow us to determine the right quadrant for τ = (θ − α). Once we have obtained τ = (θ − α) we can derive the right ascension of the camera direction when we know the local sidereal time θ. The local sidereal time can be obtained from any good astronomical yearbook or by a simple computational procedure (see for instance “Norton’s Star Atlas”; Tirion, 1981). Worked example: The Perseids were observed on 12 August 1981 from two stations, one being Brustem in Belgium (ϕ1 = 50◦ 480 2900 , λ1 = 5◦ 130 5600 ) and the other being Mechelen also in Belgium (ϕ2 = 51◦ 000 5900 , λ2 = 4◦ 290 5900 ). The mean time of the photographic project was planned to be 1h UTC. Since the Perseid radiant is in an easterly direction at the mean time of the photography, the projection point was chosen such that both observers photograph an area of the sky in a southerly direction. We chose a projection point at ϕp = 50◦ 350 and λp = 4◦ 450 . Step 1: X = arccos(sin 50.808 · sin 50.583 + cos 50.808 · cos 50.583 · cos(5.2322 − 4.75)) Y = 60 · 1852m · 0.378 = 42031m


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Step 2: 90000 tan h = 42031 h = arctan 2.14 = 65◦ Step 3: A00 = −33782m B 00 = −24974m and tan(270◦ − Az) = 0.73925 sin(270◦ − Az) = −0.59445 cos(270◦ − Az) = −0.80413 and since both sin and cos are negative then: 180◦ ≤ (270◦ −Az) ≤ 270◦ , e.g. (270◦ -Az) = 216.4737◦ so Az = 270◦ −216.4737◦ = 53◦ 320 Step 4: δ = arcsin(sin 50◦ 480 sin 64◦ 590 − cos 50◦ 480 cos 64◦ 590 cos 53◦ 320 ) δ = arcsin(0.54332) δ = 32◦ 550 Furthermore sin(θ − α) = 0.405213 cos(θ − α) = 0.914409 and hence both sin and cos are positive values, so we get 0◦ ≤ (θ − α) ≤ 90◦ and thus (θ − α) = 23◦ 540 For the local sidereal time we found 22h 42m 22s or θ = 340.5939◦ so α = −23◦ 540 + 340◦ 360 = 316◦ 420 Summarizing the result: Observing station 1 Brustem h = 65◦ , Az = 54◦ α = 317◦ , δ = 33◦ As an exercise, compute the camera position for the observer in Mechelen. You should find the following results: h = 60.4◦ , Az = 340◦ , δ = +22.6◦ and α = 351◦ . If you have a country-wide network with many observing stations which each have several cameras we strongly recommend you to program the organization of a multiple station project on a PC. Always check your starting data as an error in the setting up of the camera directions will result in disappointed photographers when they find out their photographed meteors were not simultaneous, despite all the careful planning!

2. Intersection of camera fields at a given height and determination of the camera directions To determine the direction at which a camera has to be aimed for simultaneous photography, most people consider only the intersection of the optical axes at a height h, typically 90 km for most meteor photography. This method does not guarantee the cameras will not be pointed close to the horizon. The following numerical-graphical method prevents this. We assume that cameras with stereographic projection of the image (such as reflex cameras), with a focal length f (mm) and rectangular negatives a(mm) ×b (mm) are used.

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Figure 6-2: Field of view of a camera equipped with a lens of focal length f (mm). The format of the negative is a (mm)×b (mm). This gives a field in the sky of 2α × 2β degrees.

From Fig. 6-2 (right) we see, that the semi image angle α (vertical) is given by: tan α =

a 2f

The semi image angle β (horizontal) is then given by: tan β =

b 2f

Example: consider a small-frame camera with a standard lens: a = 24 mm, b = 36 mm, f = 50 mm then: tan α = 0.24 and tan β = 0.36, or α = 13.5◦ and β = 19.8◦ , respectively. The camera will then image a field of 2α × 2β, which is 27.0◦ by 39.6◦ in this example. The optical axis is pointed at an elevation η (◦ ), whereby the edge b is parallel to the horizon. We work relative to a flat Earth, which makes our results less accurate for η − α < 25◦ (to be avoided in practice anyway because of light pollution or haze close to the horizon, the excessive distance to the meteor, etc.) In Fig. 6-3 the field of the camera is shaded. The camera is located at point W at the Earth’s surface. The surface at the height level h photographed in the atmosphere can be reconstructed when the dimensions b1 , b2 , d1 and d2 are known, therefore we need to compute these. The formulae are: p = −h · tan η

(1)

p · do = −h2

(2)

h tan η

(3)

d1 =

h tan (η + α)

(4)

d2 =

h tan (η − α)

(5)

do =


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Figure 6-3: Camera field in the atmosphere at height level h. R denotes the camera field center in the sky. The lower part shows the situation projected onto the Earth’s surface.

The computation of b1 and b2 is more complicated. First compute: bo = Next: tan γ =

bo tan β = do − p sin η

tan β · h sin η

1 tan η

(6)

1 = tan β · cos η + tan η ¶

µ

1 + tan η b1 = (d1 − p) · tan γ = h · tan γ tan(η + α) µ ¶ 1 b2 = (d2 − p) · tan γ = h · tan γ + tan η tan(η − α) µ

b1 = b0 (d1 − p)/(d0 − p) = b0 µ

b2 = b0 1 +

(7)

(d1 − d0 ) 1+ (d0 − p)

(d2 − d0 ) (d0 − p)

(8) (9)

¶

(8a)

¶

(9a)

The formulae (8a) and (9a) are better than (8) and (9) for numerical calculations when η is close to 90◦ (p very large). It is also interesting to know the surface area S (in km2 ) of the camera field: S = (d2 − d1 ) · (b1 + b2 )

(10)

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Figure 6-4: Intersection of camera fields in the meteor level between 80 and 120 km shown as a vertical cut. (from Vanmunster, 1986, p. 37)


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An example may clarify this method: the camera from the example above is directed at an elevation η of 60◦ . We compute the results for a height h = 90km and h = 70km, respectively.

(2) (3) (4) (5) (6) (7) (8) (9) (10)

p do d1 d2 bo tan γ b1 b2 S

h = 90km h = 70km −155.9 km −121.2 km 52.0 40.4 26.7 20.7 85.4 66.4 37.4 29.1 0.18 32.9 25.6 43.4 33.8 4473 km2 2715 km2

3. Application The aim is to optimise the intersection of the camera fields (at the mean height of bright meteors) for photography from two or more observing stations. Since both azimuth and elevation must be determined, it is very useful to draw the horizontal projection of the camera fields on a transparent sheet for a few heights h. This method allows us to verify which cameras have to be mobilized at the different stations in order to cover a well-defined part of the sky. Remember however that some other factors will still play a role in the determination of the positioning of cameras, such as differences in the light sensitivity of the cameras. Therefore coordination of multiple station projects needs to be centralized by local workers.

4. Computation of the region of sky photographed When a multiple station project is prepared only one point is given to direct the camera; the point at which the optical axis must be aimed (R). Where several cameras are used at one place, it is interesting to know whether some camera fields are overlapping and if so by how much. Therefore the corners of each camera field are computed as projected onto the celestial sphere. In such a spherical projection, the camera field is of course no longer a rectangle (Fig. 6-5).

Figure 6-5: Shape of a camera field projected onto the celestial sphere.

The required calculations remain simple as shown in the following method. When we determine the efficiency E of a camera for meteor photography we find that the size of the camera field is given by: A = 2 · arctan

a b · 2 · arctan 2f 2f

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For a camera with f = 50 mm and film dimensions 24 mm×36 mm (a×b) the camera field A measures 39.6◦ ×27◦ . Assuming that the camera is mounted horizontally, we can obtain the upper limit of the camera field (this is the elevation of points 1 and 2 in the figure above) by adding 27◦ /2 or 13.5◦ to the elevation of the direction point R. Analogously we can determine the lower limit by reducing the elevation of R by 13.5◦ (= elevation of points 3 and 4). Example: when the elevation of the direction point R is 60◦ , then the elevation of points 1 and 2 (upper limit) is 73.5◦ and of the points 3 and 4 (lower limit) 46.5◦ . Now we have still to determine the azimuth of the corners of the camera field. From Figure 6-6 we see that the angle on the azimuth scale becomes larger as we approach the horizon, in other words, the angle on the azimuth scale increases by a factor sec h = (cos h)−1 . The value that has to be added to (or subtracted from) the azimuth value of R equals sec h · (39.5◦ /2).

Figure 6-6: Camera field on the celestial sphere. The indicated points are explained in the text.

Example: For the points 1 and 2 we found an elevation of 73.5◦ . The angle on the azimuth scale between the ‘central meridian’ and the upper corners of the camera field are found to be (39.6◦ /2)· sec 73.5◦ = 69.7◦ . Assuming that the azimuth value of the direction point R was 200◦ , we find an azimuth for corner 1 of the camera from 200◦ −69.7◦ =130.3◦ and for corner 2 from 200◦ +69.7◦ = 269.7◦ . Exercise: Compute the azimuths of the other corners. Results: Corner 3: h = 46.5◦ ; a = 171.2◦ Corner 4: h = 46.5◦ ; a = 228.8◦

Of course when several cameras are used at one observing station, it is useful to arrange to minimize the overlapping regions of camera fields on the sky via the above method!

5. Optimising the camera fields As the basic principles of double station photography have now been explained we can find out whether or not every meteor within the camera field will be caught at the two stations. Let us look once again at Fig. 6-3. The bottom of our camera field photographs a region of the sky which is much closer to the horizon than the other parts of the image. Meteors that appear in the bottom region of the photo occur at a much larger distance away from the observer than the meteors caught in the center. Due to this greater distance these meteors appear fainter and the probability of photographing them decreases considerably. The loss in luminosity due to the distance relative to a


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standard distance of 100 km is given by: ∆m = 2.5 · log (A2 /1002 ) where A is the distance to the meteor in km. The composition of the gases in the atmosphere through which we observe will also reduce the meteor’s luminosity, due to absorption. The absorption can be approximated by the following equation (valid for a pure terrestrial atmosphere): ∆m0 =

0.38 = 0.38 · csc (η − 1) sin (η − 1)

Where η is the elevation for which ∆m0 is to be determined. For a polluted atmosphere ∆m0 will be greater as the ‘constant’ value of 0.38 will no longer be valid. In populated areas, the ideal values of 0.38 will never occur, but will be at least 0.6 to 1.0! The absorption value varies strongly from one region to another and from one night to the next. The total loss in luminosity ∆m for a photographed meteor thus becomes: ∆M = ∆m + ∆m0 . The real loss in hourly rates is also determined by the mass distribution of the meteoroids, described by the so-called population index r. For sporadic meteors, r varies mostly around ≈ 3.0. For shower meteors, r is generally smaller: 2.0 ≤ r ≤ 3.0. The factor which reduces the number of meteors recorded C due to the loss of luminosity is written as: C = r−∆M . It is obvious that the lower the elevation of the camera, the fewer meteors we will photograph due to the loss in luminosity. There is, however, also a gain. The lower we aim the camera towards the horizon, the larger the intersection volume with the meteor layer in the atmosphere becomes. The photographed volume at 100 km height V increases by the following factor as a function of the camera elevation η: V =

1 = csc η 3 sin η 3

In order to find out whether or not we gain anything by selecting a given elevation of the camera, we compute T : T = csc3 η × r−(0.38 csc η−1)−5 log sin η) T = 1 for a camera centered at the zenith. T < 1 means that fewer meteors will be photographed than overhead. T > 1 means that more meteors will be photographed than in the zenith. For a perfectly transparent sky (absorption coefficient = 0.38) it turns out we gain in terms of photographic hourly rates for most shower meteors (r < 3.0) by aiming the camera at a less than zenithal elevation (e.g. 30◦ ). For meteor streams which are richer in faint meteors it turns out the optimal elevation is around 50◦ . In the case of shower meteors we also have to consider systematic differences in angular velocity dependent on the direction and distance to the radiant (cf. section 8 in Part 1.) In most cases however one will work where the sky is far from perfectly transparent, as most observers do their observational work close to their homes, often near towns or industrial areas. Because of the large absorption coefficients it is not recommended to aim a camera at a low elevation in these cases. At most observing stations the photographic hourly rate will decrease as the zenith distance of the camera field increases. This explains why photographers can have very different degrees of success with the same kind of optics and film, but at different sites.

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In double station photography it is important to try to photograph an area in such a way that the absolute limiting magnitude for meteors in both camera fields is, as far as possible, identical. Remember, however, that: – The camera fields must cover about the same volume in the atmosphere. – The distance to the other stations of a network must be about equal. At this stage it becomes clear that the optimization of a camera network becomes a complex organizational job! Even when the above criteria are fulfilled there is still one important element to be considered. There is no guarantee that the photographed double station meteors will yield good results, as for instance, the trail of the meteor and the two observing places may lie in the same plane. The angle of convergence of the two photographed meteors will then be zero. Such a case is a most annoying problem, giving very poor computational results. In view of this problem we need to take the geometric conditions of occurrence for shower meteors into account. The best way to improve our camera positions is to examine the entry angles of the shower meteors we intend to try to photograph by simulating the actual circumstances.

Figure 6-7: Two photographers aim their cameras, equipped with identical lenses, at the same point R in the atmosphere. Observer W1 aims at an elevation of 40◦ , W2 at 90◦ . In this case we may question the usefulness of a double station campaign: in the example, the camera at W2 photographs only a very small part of the camera field of W1 (dashed).


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Figure 6-8: The camera fields for two stations, one in Switzerland, the other near Munich in Germany, as used during an actual observing campaign as projected onto a map. As can be seen, the camera fields cover large areas above France, Switzerland, Austria, Italy and Germany. Corners of three fields are marked (1A–E, 1a–d, and 4a–d), which correspond to the positions indicated in Fig. 6-9.

Figure 6-9: As seen from the Swiss station Jungfraujoch, the camera fields were projected at 0h UT onto a star map. This is helpful when positioning the cameras at night. Also the area photographed from Munich has been indicated (dashed field), which shows only a small part of this camera field is common with the corresponding one at Jungfraujoch.

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6. Simulating meteor trails for tests We will not generate meteors at random, but will instead select where we will let the meteor appear (1), from which radiant it radiates (2), the time (3) and its absolute magnitude (4). With these four starting elements we can begin the simulation. The procedure, step by step is as follows: – We compute the azimuth a and elevation hRad of the radiant at the selected time and for the geographical position ϕb , λb above where the meteor starts at height hb . – We compute the position of the ending point he of the meteor. From the length l(km), the azimuth and the elevation of the radiant: he = hb − l × sin hRad l cos hRad sin aRad 1852 · cos ϕe · 60 l cos hRad cos aRad ϕe = ϕb ± 1852 · 60 note: for some showers the mean values for l and hb can be found in research papers. – Calculate from he , ϕe , λe and hb , ϕb , λb the azimuth and elevation for each observer W1 and W2 at locations ϕ1 , λ1 and ϕ2 , λ2 . – Compute: (Wix are vectors) λe = λb ±

W1b = (cos ab1 cos hb1 , sin ab1 cos hb1 , sin hb1 ) W1e = (cos ae1 cos he1 , sin ae1 cos he1 , sin he1 ) W2b = (cos ab2 cos hb2 , sin ab2 cos hb2 , sin hb2 ) W2e = (cos ae2 cos he2 , sin ae2 cos he2 , sin he2 ) – Compute the angular length L(◦ ) of the meteor seen from both observing stations. At W1 : L1 = arccos(W1b W1e ) (vectorial product) at W2 : L2 = arccos(W2b W2e ) – Compute the apparent magnitude Ma seen from W1 and W2 : derive the mean distance from observer to meteor A and the elevation at which the meteor appear for that observer. Next compute the magnitude correction due to the distance, ∆m, and the loss in luminosity due to absorption ∆m0 . The apparent magnitude Ma then becomes: Ma = Mabs + ∆m + ∆m0 . – Calculate the angle of convergence σ: u1 = W1b × W1e u2 = W2b × W2e u1 · u2 σ = arccos |u1 | − |u2 | As a final visual presentation we can project these meteors on to a map. We translate (a, h) of the meteor’s position to (α, δ) and project (α, δ) as (x, y) coordinates onto a gnomonic map. This will give a much better insight into all the elements, such as angular length L(◦ ) of the meteor, its angular distance from the radiant, the angle of convergence between the meteor trails as seen from different stations, etc. With such a method it would even be possible to simulate an entire observing campaign. In the case where conditions for a double station project need to be respected, it is recommended to direct the camera at about 30◦ from the radiant as at such a distance from the radiant the angular velocity and trail lengths of the stream are most favourable.


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Figure 6-10: To get an impression about the parallax of meteor trails photographed from two stations separated by 25 km, we show an example of a Perseid meteor of −2m . The exposures were taken on 1975 August 13 between 22h 40m and 22h 50m local standard time. The meteor appeared at 22h 44m . We superposed both meteor trails into one frame to show the parallax. The trail photographed by the easterly station appears west of the other trail. It is interrupted by a rotating shutter with 25 breaks per second.

References and bibilography: Boroviˇcka J., 1990: The comparison of two methods of determining meteor trajectories from photographs. Bull. Astron. Inst. Czechosl. 41, 391-396. Ceplecha Z., 1961: Multiple fall of Pˇribram meteorites photographed. (1. Double station photographs of the fireball and their relations to the found meteorites.) Bull. Astron. Inst. Czechosl. 12, 21-47. Ceplecha Z., Spurny P., Boˇcek J., Novákov´ a M., Polnitzky G., Porubˇcan V., Kirsten T., Kiko J., 1987: European Network fireballs photographed in 1978. Bull. Astron. Inst. Czechosl. 38, 211-222. Hawkins G.S., 1957: The method of reduction of short-trail meteors. Smithson. Contr. Astrophys. 1, number 2, 207-214. McCrosky R.E., 1957: A rapid graphical method of meteor trail reduction. Smithson. Contr. Astrophys. 1, number 2, 215-224. Tirion W., 1981: Sky Atlas 2000.0. Cambridge (Mass.). Vanmunster T., 1986: Handboek simultane & fotografische meteoorwaarnemingen. VVS Werkgroep Meteoren, Belgium. (in Dutch) Whipple F.L., Jacchia L.G., 1957: Reduction methods for photographic meteor trails. Smithson. Contr. Astrophys. 1, number 2, 183-206. Whipple F.L., Wright F.W., 1957: Methods for the study of shower radiants from photographic meteor trails. Smithson. Contr. Astrophys. 1, number 2, 239-248.

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PART 7: MEASURING POSITIONS ON PHOTOGRAPHS

1. Measuring prints of meteor photographs A beautiful meteor photograph may have some aesthetic value to a meteor photographer, but if the result is to also have a scientific value it is of great importance that all the data is carefully recorded. The photographer may help to speed up the reduction work for the IMO by identifying some background stars on the print and by preparing some of the measurements.

1.1. Identification of a print Identify a print as complete as possible, to avoid any doubts in the case of double station photography. Most of the information can be written on the back of the print. Date: Observer: Observing site:

Exposure time: Meteor time of appearance: Visual magnitude: Shutter Number of interruptions Film number Negative number: Remarks:

19 .... /..... /..... (yy/mm/dd) ................................................................... ................................................................... ◦ 0 00 E/W; ◦ 0 00 N/S; λ= ϕ= height = m above sea level h m s UT; h m s UT Begin End h m s UT ........ m ........... breaks per second; type of shutter ......................... ........... , meteor duration ....... s ..................... .....................

Part 7: Position Measurements

79

1.2. Format of the prints For further processing of the data we need two prints: – A print which covers the entire negative – An enlargement of the region around the meteor. For instance: – The entire photo printed to 12 cm×9 cm. – Detail of 12 cm×9 cm, with an enlargement factor of about 3 or 4. The print of the entire photo must allow the determination of the plate center (the centre of the frame rectangle). When the camera has been directed to a pre-defined point in the sky, for a double station project, this point should coincide with the plate center. Aiming a camera accurately without using an azimuth or equatorial mounting is thus rather difficult. A print with details near the meteor allows the determination of specific stars in that area. For the measurements bear in mind that the image of an all sky mirror camera is a reflected image and thus the orientaion is different from a “common” image of the sky. For the measurements this is no general problem, but it must be borne in mind. For calculations you have to remember to reverse one axis (see below).

2. Measuring original films or plates Of course, each copying process reduces the accuracy of an image. On the other hand, copying allows us to enlarge the scale and makes measurements possible without expensive measuring devices. These devices are needed to measure distances on the original images. The required accuracy is of the order of some 10−3 mm. Coordinate measuring machines are specially designed for this purpose. If you have access to such a device, it is to be preferred over measurements on prints. When placing original plates or films in such a machine, be careful to avoid damages to the emulsion. Regarding the measurement itself, follow the instructions given for the particular device used.

3. Comparison stars In order to compute the correct position of a meteor, a number of comparison or reference stars need to be known. The minimum number of stars is three: in such a case it is not possible to get any idea about the errors on the measurements themselves. Therefore, in practice, normally 5 or 6 reference stars are chosen. These reference stars have to be distributed evenly around the meteor. They should not be on or near one line, as this gives rise to large uncertainties. An example of a good and a bad choice is given in Fig. 7-1.

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Figure 7-1: Examples for a good and bad choice of comparison stars for position measurements.

Figure 7-2: Trails of stars having nearly identical declinations may overlap and are therefore not suited as reference stars for position measurements.

A reference star must not be a component of a double or multiple star. The term “multiple star” must be considered in the widest context, including stars that are between 0.30 to 30 apart in the sky including official multiples as well as “true” multiple star systems. It is often not possible to separate the components on the photo, and this may result in errors of the order of the distance between the components. A star that overlaps another star trail on a non-guided exposure may not be used (stars of about the same declination). This is especially necessary for long exposure times (in excess of 15 minutes). In the neighbourhood of the pole there are no problems. The star trails here are very short. As a result even more fainter stars can appear on the photo. Any inaccuracy in the measurement will, however, result in a large error in Right Ascension, though the absolute angular error remains unchanged. Of course, the equatorial coordinates of the reference stars, α and δ need to be known. Therefore we need both a star atlas and a star catalog. For this, the “Atlas of the Heavens” of Beˇcvar or “Norton’s Star Atlas” or “Sky Atlas 2000.0” may do. The first of these depicts the sky down to limiting magnitude 7.5, the second one to 6.5. One of these atlases should be used in combination with the “Atlas of the Heavens - Catalogue 1950.0” (to visual magnitude 6.25) or “Sky Catalog 2000.0”. The numbers in this catalogue are those of the Boss General Catalogue (GC). Mentioning the GC-number is sufficient as identification. Another numbering system often used is the Flamsteed identification, for instance. GC 4973 = 50 Per, 5.59m . There are other more elaborated atlases and catalogs, but the above mentioned ones are more generally known and widespread among amateurs.

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4. The measuring work itself Once the reference stars have been chosen, we then have to measure their rectangular coordinates (and also the position of the meteor) relative to some randomly chosen rectangular x and y axes.

Figure 7-3: Axes on the print. The bold frame marks the entire print. The point 0, or (0, 0) with its coordinates, is the origin of the coordinate system.

Mostly, the axes are chosen along the edges of the print, with the zero point in the bottom left hand corner. The x and y values are then measured to 0.1 mm accuracy. For instance: x = 80.6 mm ,y = 45.2 mm. For measuring a clear plastic ruler with divisions every 1 mm is very suitable though the tenths of a mm have to be estimated. For measuring the x-coordinates, the zero point of the ruler is positioned on the left edge of the print (y-axis). and the ruler itself is kept as parallel as possible to the bottom edge of the print (x-axis). This can easily be achieved by fixing the print truly square to a drawing board. Similarly for the y coordinates the zero point of the ruler is positioned on the bottom edge (x-axis) and the ruler kept parallel to the left edge (y-axis). The fact that the measuring staff may not be kept perfectly parallel to the edge of the print produces only a minor error. When the edges of the print are not straight (badly cut), or they are not perpendicular, then the print should be glued on a sheet of paper, to give another rectangular x, y coordinate system. The rest of the procedures are described in detail in IMO’s “Photographic Astrometry” brochure (Steyaert, 1990). A blank IMO astrometry form is given on the next page.

References and bibliography: Steyaert C., 1990: Photographic Astrometry. (IMO Monograph No 1). The International Meteor Organization.

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IMO – The International Meteor Organization The Photographic Meteor DataBase – PMDB ASTROMETRIC FORM Vol.: rec.: . Date (dd/mm/yy) Visual reference Site of observation

/

/ Time of meteor (hh/mm/ss) Magnitude Meteor shower

Latitude ϕ = Longitude λ = Elevation h = Location

Camera

lens f =

Exposure

start (UTC) end (UTC)

0

00

◦

0

00

/

mm

Observer

focal ratio d/f = 1 :

h

m

s

h

m

s

ISO

film identification negative number after developing minutes at

for

◦C

breaks/second

type: DC / AC / Synchr / LCD ◦

blade angles

breaks measurable along the trail

Reference stars Catalog: GC / HD / SAO / . . . Designation

Number

x begin

y begin

◦,

Estimated plate center α =

x end

δ=

y end

◦

Points on meteor trail Identification

Remarks

x

UTC

m

Emulsion Developed in Shutter

◦

/

y

Identification

x

y

Part 8: Photometric Measurements

83

PART 8: PHOTOMETRIC MEASUREMENTS

1. Introduction As we have seen, major information stored on the film concerns the position of meteors and reference stars. Furthermore, the film transforms certain amounts of light into different blackenings or densities. The creation of a photographic picture was described in detail in Part 1 Faint meteors. Film development suggestions have been given too, for instance in Part 2 on fireballs. Now we want to deal with a method to find out the brightness of a meteor from the photographic density along its trail. Such investigations were carried out many years ago (e.g. Millman and Hoffleit, 1937). The principles in these remain valid, of course, but the characteristics of the films have changed greatly. There are several general textbooks about astronomical photometry, for example Sterken and Manfroid (1992).

2. The image of star and meteor trails Firstly, we must think about the differences between an image of a star and a meteor. This difference is enormous if the camera is guided. Then stars are appearing like points of various size, whereas meteors are trails. As we have already seen in the description of the photographic process in section 4.4 of Part 1 “Faint Meteors” (p. 15) and section 4 of Part 2 “Fireball Patrols” (p. 29), there are substantial differences if a grain of the emulsion is permanently lit by a faint source (star) or if it is exposed for a very short moment by a relatively bright meteor. In the case of guided photographs the magnitude determination of a meteor may be affected by a number of systematic influences as well. However, most meteor photographs are obtained by unguided cameras. In this case we compare geometrically similar features, i.e. trails caused by “moving light sources” of very much differing apparent velocity. Here we continue describing their analysis. In principle, the procedure is the same for all kinds of images. The star image moves over the film with an linear velocity vs dependent on its declination δ and the focal length f of the lens: vs = v(δ, f ) (1) The nearer to the poles, the slower the star image moves: vs =

2πf cos δ Tsid

(2)

with Tsid = 86164 s the Earth’s sidereal rotation period. Consequently, the trails appear shorter and denser (“blacker”). Or, in other words, more fainter stars will become visible on the film.

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Consequently, we have to reduce the star magnitudes mr to a reference declination, say δ = 0◦ . This provides the information what star at δ = 0◦ would have caused the blackening S detected at the film. The difference in apparent magnitude ∆m(v) due to velocity v can be described by ∆m(v) = mr (δ = 0◦ ) − ms (δs ) = −2.5p log

vr (δ = 0◦ ) vs (δs )

(3),

where the parameter p is called the “Schwarzschild exponent” described in detail in Part 1 of this Handbook (“Faint Meteors”), p. 15. For common black and white emulsions we may assume p between 0.7 and 0.8. Substituting eq. (2) for vs , with cos 0◦ = 1, we obtain mr (δ = 0◦ ) − ms (δs ) = −2.5p log

1 cos δs

(4)

or, to calculate the “corrected magnitude” we need, mr (δ = 0◦ ) = ms (δs ) − 2.5p log

1 cos δs

(5).

This fact must be considered when the characteristic curve is determined as described below (cf. section 5, the measurement and Fig. 8-2). If we refer to the magnitude of a comparison star ms , we now mean its reduced value mr (δ = 0◦ ). By contrast, a meteor causes only a very short exposure of each crystal on the emulsion. This leads to a much fainter image of the meteor trail. The difference ∆m(v) is ∆m(v) = mM − ms = −2.5p log

vM vs

(6).

To make a photometric measurement means to determine the magnitude of a given object by comparing it with objects of known magnitudes. The main problem of meteor photometry is the comparison of star trails with very briefly exposed meteor trails.

3. The photometer A photometer is a device which may • measure the absolute density of parts of a photographic emulsion, or • allow a comparison between the amount of light transmitted through the film and a previously defined transmission value (e.g. a grey cone). The advantages of comparison measurements are considerable when compared with absolute measurements. There is no need to measure the precise amount of blackening as a figure, because we want to find out which amount of light along the meteor trail caused the same blackening like a certain star of another magnitude. Thus we need only the relative parameters for a meteor and reference stars. The most common photometers work after this principle. Normally, there is a diaphragm into which you place the part of the film to be analysed. This diaphragm can be altered in size, so that the amount of light passing through the film can be changed until it equals the amount of the comparison light beam (Fig. 8-1). In this way you determine the “effective blackening” of the film, since a brighter object does not cause a “blacker” silver grain (which is impossible), but more blackened silver grains. In the case of very bright objects you will find an overexposure of a certain region of the film. This may cause shutter breaks to be smeared out or details of the fireball to be undetectable.


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Furthermore, a shutter with a sector angle of α◦ reduces the blackening of the star trails by ∆m(sh) since this exposes the emulsion only when the shutter does not cover the lens: ∆m(sh) = ms − ms,sh = −2.5 log

360◦ αsh

(7).

Figure 8-1: Principle of a photometer. The light beam of a stabilized lamp is splitted into two beams. One of these passes a filter of defined absorption, which can be adjusted for different purposes. The other beam passes the emulsion. Both amounts of transmitted light are detected at the comparison instrument. The size of the changeable diaphragm has to be changed until the both intensities are identical. This way we determine the “effective blackening” S (i.e. the intensity and size of the image) and the size of the diaphragm is the measure of S. Other principles, e.g. directly measuring the transmission of light, are also possible, but comparison measurements are to be preferred.

Then, we may calculate the ideal meteor magnitude mM from a star trail showing the same blackening on the image, considering the meteor’s angular velocity vM and the shape of the shutter blades (their wing angle α): 360◦ vM + 2.5 log (8). mM = ms − 2.5p log vs αsh The “Schwarzschild-exponent” p plays an important role, but there are other effects to be considered. The light sensors in photometers may be photo resistors, photomultipliers, or other semiconductor receivers. Each of these sensors has a certain spectral range in which they work optimally. The lamps used for the transmission measurements are then chosen according to the characteristics of the sensor. Since only the photographic density of black and white films are measured, there are no changes in the spectral composition of the light after it is transmitted through the film. If you try to measure the brightnesses of objects from color materials, not only does the meteor trail have its own color, but also the colour may change along the trail. Furthermore, comparison stars showing different colors will create additional problems. Finally, color films itself do have a color “mask”, which affects the light sensor of a photometer as well. Consequently color films must not be used for photometry. Of course, the color differences of comparison stars cause some differences on black and white films, too, but knowing the color index of the stars used and the behaviour of the film–lens combination, it is possible to calculate precise V -magnitudes, as described next.

4. Influence of object colors As you may know, an object can appear at a different brightness in different ranges of the spectrum. For instance, Aldebaran or Antares as red stars are brighter in the red part of the spectrum than in

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the blue part. Alternatively, Rigel as a “blue star” for example, appears brighter in blue than in red. Therefore, a system of magnitudes was defined by the IAU, differentiating between U (ultraviolet), B (blue), V (visual, or yellow), I (infrared), as well as further infrared regions (J, H, K, . . . ). Thus the color of a star or other object can be described by the difference of its brightness in different, neighbouring spectral channels. This is called the color index. The most frequently used color index is the B − V , e.g. the difference of the blue and the visible (yellow) channels. According to the definition of the magnitude scale, B − V > 0 characterizes a “red object”, B − V < 0 is a “blue object”. For our purposes the V magnitude is of main interest. Most black and white films give V to a good approximation without applying additional filters. Several combinations of common black and white films and filters have been tested and the differences to the V -magnitude were found to be negligible, especially if it is possible avoid to using comparison stars of intense red colour (values of B − V larger than 0.7). It was found that meteors have a negative B − V color index (Hajdukov´ a, 1974).

5. Measurement Before starting with the photometry of the meteor trail, we must choose a number of reference stars. We should avoid double stars, i.e. all stars which are not clearly separated with the lens used. The reference stars should cover a wide magnitude range, since we need them to construct a characteristic curve for the film. If possible, we choose stars in the vicinity of the meteor trail. Then we avoid errors due to differences in sky conditions. If the time of the meteor’s appearance is known, we should refer to the point of the star trail which was exposed at that moment. Otherwise, we may measure near the middle of the star trail. Each measurement should be repeated about 3 to 5 times in order to find the accuracy of the obtained value. Thus we get a number of points defining a density-magnitude relation (Figure 8-2), where ms is the reduced star magnitude according to eq. (5). Next we start measuring the meteor trail. This procedure depends on the photometer. If a device for scanning is available, we can adjust the negative so that only the trail is scanned. In the case of wide angle or fish eye images, the meteor trails may be curved, and we have to adjust the negative after examining short parts of the trail. Care must be taken with this as we are trying to obtain a magnitude profile along the path, so the measurement should be done as continuously as possible.

Figure 8-2: Construction of a characteristic curve from photometry of reference stars. The magnitudes have to be corrected for different declinations of the stars according to eq. (5).


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If no scanning equipment is to hand, we need adjust the trail so, that a shift in one coordinate only will allow the following of the trail. Then we must measure point by point, shifting the measuring table in between each measurement. It is not easily possible to recommend a step width for this procedure, however, it should be short enough to separate all the characteristic features of the brightness variations. On the other hand, the resolution of the emulsion is of the order of 20 . . . 50µm, and the step width may be longer than this. The measured densities may then be transformed into relative magnitudes. After this we have to apply the above-mentioned formulae to calculate the true meteor’s magnitude. As a rough guess, we may expect the meteor to be about 7m brighter than a reference star causing the same photographic density.

6. Peculiarities of meteor trail photometry Photometric methods have been primarily developed for stars, and thus for point-like objects rather than for trails. The kind of diaphragm in the photometer is also of importance. If your photometer does not use a circular diaphragm, you must ensure that the orientation of the trails is comparable for each trail, whether of star or meteor. The main point is to use always the same orientation (Fig. 8-3).

Figure 8-3: Influence of the orientation of the star trails in the measuring square diaphragm of a photometer. It is necessary to always use the same orientation for the entire photometry of both stars and meteor trail.

If we look at the intensity of the blackening of star trails on long exposure photographs, we may find that they differ from the beginning towards the end. Initially, the film’s sensitivity seems to be larger. Obviously, there is some influence of the pre-exposure to the sky-background later in the trails, and from this we would expect also that meteors appearing at the beginning of an exposure appear brighter than others appearing later in the exposure. If the background density of a film is greater than D = 0.1 then the meteor has to be brighter to be detected. Malmström (1985a) points out that this phenomenon is sometimes, erroneously, called the “loss in film sensitivity” or “reciprocity failure”. In reality it is a loss in the low intensity part of the dynamical range of the emulsion. The change in limiting magnitude ∆mp for trailed point sources, stars or meteors, with increasing sky density is: ∆mp =

(D − 0.1) −0.32

where D is the background density on the plate (Malmström, 1985b). For a 4-hour exposure the limiting magnitude of trails decreases by about 1m to 2m for background densitites between 0.5 and 0.7 which easily occur on fireball patrol photographs obtained at average sites. This difference ∆mp increases after about 1 hour (Fig. 8-4, right diagram). If the time of the meteor’s appearance is known, we should compare the meteor trail density with that of the star trails recorded at the same time. Note that this phenomenon will not occur with short exposures. For example, there was no effect

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measurable on 10-minute exposures (Fig. 8-4, left diagram; Rendtel, 1979). Also Russell (1986) found that the effect is less important than described by Malmström (1985). According to his investigation, changes in the distance from the field center and extinction have a larger effect.

Figure 8-4: Photometry along star trails at 10-minute exposures. During this exposure duration no pre-exposure effect can be detected (Rendtel, 1979).

For much longer exposures the situation may become different: Malmström found a decrease of sensitivity (Malmström, 1985a), expressed in loss of limiting magnitude ∆m. The effect is strongest for high density (i.e. “black” plates).

Figure 8-5: Change in background over the photographed field at the meridian (South is down, North is up). The effects are larger for wide angle lenses.

Generally, any background exposure leads to a decrease in the signal-to-noise ratio of the signal measurements resulting in the loss of often faint meteor trails. Another effect of background illumination (Furenlid et al., 1985) may be ignored in most cases, as it can be expected to be of the order of only 0m · 2. It arises from the different colors of the meteor and the background and the use of one characteristic curve for reduction of photometric measurements. The magnitude of this kind of error is, in practice, unpredictable. Night sky brightness varies with time, and color varies due to a host of factors (twilight, moon, local conditions). For example, the Full Moon has the color of a K0V-star, with U − B = 0.45 and B − V = 0.91.

References and bibliography: Ceplecha Z., Boˇcek J., Jeˇzková M., Porubˇcan V. and Polnitzky G., 1980: Photographic Data on the Zvolen Fireball (EN270579, May 27, 1979) and Suspected Meteorite Fall. Bull. Astron. Inst. Czechosl. 31, 176–182.


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Ceplecha Z., Boˇcek J., Nováková M. and Polnitzky G., 1983: Photographic Data on the Traunstein Fireball (EN290181, Jan 29, 1981) and Suspected Meteorite Fall. Bull. Astron. Inst. Czechosl. 34, 162–167. Ceplecha Z., Jeˇzková M. and Boˇcek J., 1976: Photographic Data on the Leutkirch Fireball (EN300874, Aug. 30, 1974). Bull. Astron. Inst. Czechosl. 27, 18–23. Degewij J. and VanDiggelen J., 1968: A Photometric Investigation of a Bright Geminid. Icarus 8, 404–408. Furenlid I., Westin T. and Westin B., 1985: Some Sky Background Effects in Photography. AAS Photo-Bulletin No. 40, 8–10. Hajduková M., 1974: The Dependence of the Color Index of Meteors on Their Velocity. Bull Astron. Inst. Czechosl. 25, 365–370. Halliday I., 1985: The Grande Prairie Fireball of 1984 February 22. J. Roy. astr. Soc. Can. 79, 197–214. Hawkins G.S., 1957: The Method of Reduction of Short-Trail Meteors. Smithson. Contr. Astrophys. 1 No.2, 207–214. Malmström R., 1985a: Meteor photography limitations and the beginning heights of meteors. In: C.-I. Lagerkvist, B.A. Lindblad, H. Lundstedt and H. Rickman (eds.) Proceedings Asteroids, Comets, Meteors III, pp. 609–614. Malmström R., 1985b: The Pre/Post exposure effect in meteor photometry. AAS Photo Bull. No. 38, 5–8. McCrosky R.E., 1957: A Rapid Graphical Method of Meteor Trail Reduction. Smithson. Contr. Astrophys. 1 No.2, 215–224. McCrosky R.E., Posen A., Schwartz G. and Shao C.-Y.: Lost City Meteorite – Its Recovery and a Comparison with Other Fireballs. J. Geophys. Res.,76, pp. 4090–4108. Millman P.M., Hoffleit D., 1937: A Study of Meteor Photographs Taken Through a Rotating Shutter. Harvard Annals 105, 601–621. Rendtel J., 1979: Einige Ergebnisse der Fotometrie von Meteorspuren. Die Sterne 55, pp. 97–104. (in German) ReVelle D.O. and Rajan R.S., 1979: On the Luminous Efficiency of Meteoritic Fireballs. J. Geophys. Res. 84, 6255–6262. Russell J.A., 1986: The Relative Importance of the Pre/Post Exposure Effect in Meteor Photometry. AAS Photo-Bull. No. 43, 3–4. Sterken C. and Manfroid J., 1992: Astronomical photometry. A guide. (Astrophysics and Space Sciences Library, vol. 175. Kluwer, Dordrecht. Verniani F., 1967: Meteor Masses and Luminosity. Smithson. Contr. Astrophys. 10 No.3, 181–195. Wetherill G.W., ReVelle D.O., 1981: Which Fireballs are Meteorites? A Study of the Prairie Network Photographic Meteor Data. Icarus 48, 308–328. Whipple F.L. and Jacchia L.G., 1957: Reduction Methods for Photographic Meteor Trails. Smithson. Contr. Astrophys. 1 No.2, 183–206.

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PART 9: SHORT GUIDE FOR ADVANCED USERS

Summarized information Here you will find a summary of all the information concerning the different fields treated in this Handbook. This part may be helpful for advanced readers or to use as a checklist just before you actually start your observations.

FAINT METEORS Appropriate equipment should include: (1) A camera allowing long exposures which is easy to handle in the dark and which is not influenced by dew and cold (2) An appropriate fast lens (for example f = 50mm, f /1.4) (3) A warming device for the lens, preferably with a transformer to use a low voltage outside (4) A rotating shutter with a synchronous motor (about 15 breaks per second; where shower meteors are expected have a high atmospheric entry velocity, about 25 breaks per second are better) During the observation you should note the following information for each exposure: (1) Precise time of the start and end of the exposure (use UTC only to avoid confusion) (2) If possible, the exact time of the meteors which are bright enough to be photographed (3) Notes about other moving objects in the camera field (4) Some notes about the sky conditions, especially about clouds at the start or end of the exposure (note at least 10 stars which are not covered by clouds at these times to allow positional measurements) (5) In case of a meteor storm, the beginning and end of the exposure must be noted with an accuracy of ±1s. Check your watch after the event. Please also follow the hints for the camera field direction given in detail in Part 1 (Faint Meteors).

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91

FIREBALL PATROL Appropriate equipment: (1) all sky mirror + camera OR fish eye lens + camera with sufficient film size OR wide angle lens + camera, ideally with enlarged film format (2) A suitable mounting of the camera which should resist even strong winds and allow a repeatable orientation of the camera (3) Warming system for lens and film (4) Rotating shutter, preferably with synchronous motor (about 15 breaks per second) (5) Device for fireball timing (6) Cloud detector, plus twilight dimmer switch (7) Timer for start and end of exposures (5), (6), (7) are helpful, but not essential During the observation you should note the following information for each exposure: (1) Precise time of start and end (use UTC only to avoid confusion); if you use an electronic timer, check its accuray! (2) Sky conditions, especially clouds at the start or end of the exposure, because they may affect the measurement of positions (note about 10 stars which are visible at the appropriate time).

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METEOR SPECTRA The appropriate equipment should consist of: (1) A camera with a normal lens or slightly longer focal length (preferably f = 50 mm . . . 140 mm) (2) A prism OR a transparent diffraction grating which is well mounted in front of the lens and oriented such that the dispersion of the expected shower meteors is optimal (3) A heating system for the whole optics (4) A rotating shutter in front of the prism/grating The data to be noted before or during the exposures are: (1) Focal length f of the lens (2) Data for the prism (deflecting angle) or the grating (lines per millimeter), respectively (3) Start and end of each exposure (use only UTC to avoid confusion) (4) Region of the sky photographed (5) Orientation of the prism / grating (6) Type of film used (7) Time and magnitude of meteors brighter than 1m if observed visually

Part 9: Short Guide

93

METEOR TRAINS The equipment and practicalities of photographing trains is different for the different types of trains, and ranges from the area of “ordinary” daylight photography to the need for very fast films and fast lenses for the nighttime appearances of transient train phenomena.

REGULAR TRAINS AT NIGHT (1) Camera with a very fast film (ISO 800/30◦ or higher) and fast lens (f /1.8 or faster) (2) Shutter already opened, but covered e.g. by a dense piece of cloth (3) At the moment of the appearance of a bright meteor leaving a persistent train you must immediately remove the cover and expose the film depending on the brightness of the train (at least 20 . . . 40 seconds) (3) If the train lasts for more than one such exposure, continue with exposures of about the same duration

DUST TRAINS BY DAY (1) Conditions are comparable to “normal” daylight photography; a slight underexposure is recommended (2) Try to make a whole series of photographs and note the time(s) of the appearance of the fireball (if seen) as well as the time(s) of the train exposures, or the time lapse between the meteor’s appearance and the exposures

DUST TRAINS DURING TWILIGHT (1) Camera with a fast lens (f /1.8 or less) and fast film (ISO 400/27◦ or higher) recommended (2) Try to make several exposures of different durations, depending on the film and lens at hand (try at least 10 . . . 20 seconds) (3) Note the time of the fireball’s appearance (if seen) and the time(s) of the exposures taken.

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PART 10: EXAMPLES AND APPENDIX 1. Meteor showers of particular interest for photographic work There is a working list of meteor showers established by the IMO’s Visual Commission. It gives activity and radiant data for meteor showers throughout the year. The list includes both well known showers and minor and suspected showers. All these are of interest for photographic work. During a major shower we may obtain data about radiant details or, in the case of meteor storms, about number densities as described in section 8 of Part 1 (pp. 22–23). On the other hand, the minor shower radiants are unknown to a certain extent. The determination of shower radiants requires precise postional data of individual meteor trails. Here the photography clearly allows more reliable investigation than visual material. The procedure worked out for visual data (Arlt, 1992) can be applied to photographs as well. Visual studies such as IMO’s Aquarid project (Koschack and Rendtel, 1991; Arlt et al.,1992), allow only restricted conclusions because of the limited plotting accuracy of visual observers (Koschack, IMC Proc. 1991). Hence video and photographic work may contribute very much to our knowledge about radiants of minor showers. More important, double station observations allow orbit determination of these meteoroids. In the case of low activity the number of photographically observable meteors requires data collection over many years until the existence and position of a radiant can be confirmed or excluded. Therefore, Your contributions are welcome. The list given at the next page (Table 10-1) contains both data about major showers and minor showers that require confirmation. For radiant determination a field in about 40◦ to 60◦ distance from the radiant is recommended. In this case the trail length is not too short, and the angular velocity is still relatively small. All observations should be done with rotating shutters in order to allow a shower association. Double-station work does give the most valuable data about the radiant as well as the atmospheric trajectory, and together with the shutter breaks, also the meteoroid’s initial orbit. The given radiant list is meant for a choice of target for different kinds of observations as described in the Handbook, like radiant determination, activity analysis, photometric studies, train photography, etc. As it was the case with the visual Aquarid project, there will be also projects for photographic work initiated by the IMO’s Photographic Commission for other than the radiants listed in Table 10-1. Such projects will be published in the Journal of the IMO, WGN. Of course, there are no periods to be emphasized for fireball patrols, as the bright, meteorite-like events are not related to known showers. Fireball patrols need to be run throughout the whole year.

Part 10: Examples & Appendix

95

Table 10-1: List of meteor showers selected from the IMO working list. It includes the major showers as well as some minor showers that are known for bright meteors (marked with an asterisk ∗), or for which radiant and orbital data are urgently needed (marked with a dot •). Meteors of the showers marked with a dagger † are known for displaying trains.

Shower Quadrantids Lyrids η-Aquarids α-Capricornids Aquarid complex Perseids κ-Cygnids α-Auriguds δ-Aurigids S. Taurids N. Taurids Orionids Leonids Geminids Coma Berenicids Ursids

Activity period (visual) Jan 01–05 Apr 16–25 Apr 19–May 28 Jul 03–Aug 19 Jul 15–Aug 28 Jul 17–Aug 24 Aug 03–31 Aug 24–Sep 05 Sep 05–Oct 10 Sep 15–Nov 25 Sep 15–Nov 25 Oct 01–Nov 07 Nov 14–21 Dec 07–17 Dec 12–Jan 23 Dec 17–26

Peak Jan 03 Apr 22 May 04 Jul 29 Jul 28 Aug 12 Aug 18 Sep 01 Sep 09 Nov 03 Nov 13 Oct 21 Nov 17 Dec 14 Dec 19 Dec 22

Radiant α δ 230 +49 271 +34 336 –02 307 –10 339 –16 46 +58 286 +59 84 +42 60 +47 50 +14 60 +23 95 +16 152 +22 112 +33 175 +25 217 +75

Diurnal ∆α +0.4 +1.1 +0.9 +0.9 +0.7 +1.3 +0.3 +1.1 +1.0 +0.8 +0.9 +0.7 +0.7 +1.0 +0.8 0

Drift ∆δ –0.2 0.0 +0.4 +0.3 +0.2 +0.1 +0.1 +0.0 +0.1 +0.2 +0.2 +0.1 –0.4 –0.1 –0.2 0

v∞ km/s 41 49 66 23 (1 ) 59 25 66 64 27 29 66 71 35 65 33

Remark

† † ∗ • (1 ) † ∗ •† ∗ ∗ † † ∗ †

(1 ) There is an activity from the Aqr-region between beginning of July until mid-September. Usually this is described to consist of four radiants being the Southern and Northern δ-Aquarids as well as the Northern and Southern ι-Aquarids. Their activity periods and radiants are discussed in connection with the IMO’s Aquarid project (Koschack, 1991; Koschack et al., 1992). The respective meteoroids do belong to different streams. Their atmospheric entry velocities vary from v∞ = 31km/s (N. ι-Aqr) to v∞ = 42km/s (N. δ-Aqr). Together with the close position of the radiants, meteors of the four showers are not distinguishable by visual observations. Here, photographic work clearly can help to answer open questions.

References and bibliography: Koschack R., Rendtel J., 1991: Aquarid project 1989. WGN 17, 90–92. Koschack R., 1992: An analysis of visual plotting accuracy and sporadic pollution, and consequences for shower association. In: J. Rendtel and R. Arlt (eds.), Proceedings IMC 1991, Potsdam. Arlt R., Koschack R. and Rendtel J., 1992: Results of the IMO Aquarid project. WGN 20, 114–135. The IMO Shower Calendar. Edited annually by A. McBeath, IMO INFO 2.

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2. Photograph section There are many effects described in the text which may occur on meteor photographs. In order to give you more than just a short written description, we think some real examples may be helpful for identifying meteors as weIl as artefacts or trails with other origins on your images. You will easily note that the long-exposed fireball patrol photographs show the most unusual features. Also it is often unknown at what time these phenomena appeared since no visual observer was outside at the same time, while photographs dedicated to faint shower meteors are mostly carried out if a visual observer is also active. Such unusual features may be caused by different sources of light, for example: • satellites • airplanes • lightning • diffuse phenomena: noctilucent clouds (NLC), aurorae, other questionable spots (incidentallycaught persistent train of a fireball appearing before the start of observations), haloes (lunar light pillars, parhelia) • artificial lights: reflections from street lamps, reflections inside the optics (preferably wide angle or fish eye lenses) with often curious shapes, flash lights or car lights, and even cigarettes. To each example we also give some background information as well as data of the event and the used equipment, if known. In most cases we mention the local zonal time instead of UT and the latitude of the site. This allows to easily get an impression about the conditions, for example the depression of the Sun or the status of twilight.


97

Figure 10-1: Satellite trail near α UMi in Cam. This satellite reaches a peak brightness of about -2m and regularly appears in the shape shown here for observers in mid-northern latitudes: two magnitude maxima, divided by a somewhat fainter section. In this case a fish-eye lens f/3.5, f = 30 mm was used and the entire trail is visible, but if the field is smaller and no shutter allows a check of the angular velocity, the photographer might suspect a meteor had been photographed. (1993 January 03, 05h 28m 40s –06h 45m 10s Local Time at φ = 52.4◦ N; J. Rendtel, Potsdam, Germany.)

Figure 10-2: Airplanes flying at rather low elevations when approaching airports. The different lights may cause a broad variety of trails on fireball survey photographs. Often the planes fly with intense lights as in the case shown here. Again, a fish-eye lens f/3.5, f = 30 mm was used. A shutter may clarify most situations, but planes also often appear as double or multiple tracks. (Photo taken in Potsdam near Berlin with its three airports; J. Rendtel, Potsdam, Germany.)

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Figure 10-3: If broken or diffuse clouds are present, the illumination effects can vary more. This example apparently shows a trail which seems to be of a slow moving fireball (a shutter with 12.5 breaks per second was in operation during the exposure). But in fact it is the trail of the planet Venus being accidentally interrupted almost regularly by the fast moving clouds! The other “trail” is caused by an airplane just illuminating low clouds with its landing spotlights. The diffuse clouds present also cause the lack of star trails on this image. (1993 January 25, 17h 46m 18s –18h 21m 27s Local Time at φ = 52.4◦ N; J. Rendtel, Potsdam, Germany.)

Figure 10-4: Lightning on meteor photographs will certainly be a rare phenomenon since you are hardly likely to be hunting meteors during thunderstorms! However, in the course of a night, a camera from a fireball survey network may be surprised by lightning. In nearly all cases this should not cause identification problems because of of the irregular shape of the “trail”. If a smaller field is covered only, lightning near the edges may lead to difficulties. A shutter, of course, clarifies the situation. (1990 August 29–30, 20h 37m 55s –03h 30m 30s Local Time at φ = 52.4◦ N; I. Rendtel, Potsdam, Germany.)


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Figure 10-5: Reflections in the optics of an objective lens, especially common in wide angle or fish-eye lenses, may lead to “mysterious” features if bright light sources are to be seen. These light sources need not necessarily be in the field of view of the camera. In the example shown here, the vaning moon in the east caused a series of linear features and other figures through a fish-eye lens f/3.5, f = 30mm. The shape of such features depends on the construction of the lens. (1988 April 23, 21h O3m –21h 56m Local Time at φ = 52.4◦ N; J. Rendtel, Potsdam, Germany.)

Figure 10-6: Diffuse phenomena surely will not be mixed up with meteors you might think. Nevertheless, the photographer might wish to know what caused certain features. Here we show an aurora which occurred on 21st October 1989 during an Orionid watch. (1989 October 21, 20h 30m –20h 37m Local Time at φ = 52.4◦ N, using a fish-eye f/3.5, f = 30mm and ISO 400/27◦ film); J. Rendtel, Potsdam, Germany).

100


Figure 10-7: Another phenomenon covering large portions of the sky may occur in the summer nights between φ = 50 . . . 60◦ in both hemispheres: noctilucent clouds. They occur at ≈ 83 km elevation and are lit by the Sun during twilight. Their periods of visibility are shown in Figure 4-1. Dust or smoke trains may be expected to appear bright against the sky background during roughly the same periods because they occur at comparable elevations. (1988 July 03, around 22h 30m Local Time at φ = 52.4◦ N, using a fish-eye f/3.5, f = 30 mm and ISO 100/21◦ color slide film which was exposed for 12 seconds; J. Rendtel, Potsdam, Germany)

Figure 10-8: Example of an attractive fireball taken by an all sky-camera of the European Network. Note that the zenith is covered by the camera and also the camera holder obstructs large parts of the sky. The photo was exposed at the station #46 Glash¨ utten (φ = 49.9◦ N) of the German part of the European Network on h m 1974 August 30, between 0l 25 and 03h 44m Local Time. (Photo kindly provided by Dieter Heinlein.)


Figure 10-9: Another example of an attractive fireball, taken through a fish-eye lens f/3.5, f = 30 mm in Lardiers, Southern France, during an Orionid campaign. Exposed on 1990 October 21, 19h 49m 02s –21h 03m 50s Local Time at φ = 42◦ N using an ISO 400/27◦ -film and a rotating shutter which produced 12.5 breaks per second. However, the shutter blades of only 60◦ width caused too short breaks, and the breaks were exposed by the enormous overexposure due to the slow-moving -10m fireball which terminated at the horizon. The fireball appeared at 20h 52m 14s Local Time. Figure 10-10 and 10-11 (next page): This is one of the world’s best resolved meteor spectra obtained and kindly provided from the Ondˇrejov Observatory, Czech Republic by Jiˇri Boroviˇcka. ˇ The fireball “Cechtice” was photographed on 1968 October 15, 19h 53m UT, at the Czech station Ondˇrejov of the EN. The sporadic fireball entered the Earth’s atmosphere at 19 km/s. The recorded beginning height of the luminous trail was 72 km, the luminous end height was 30 km. The fireball’s maximum absolute magnitude was -9m . The spectrum was obtained with a f/4.5, f = 360 mm lens, used with an objective (transmission) grating with 600 grooves/mm on an ISO 400/27◦ plate of 18cm x 24cm size. A rotating shutter caused 15 breaks/second. Line identifications and wavelengths are given in the detail image (Fig. 10-10). The resolution is as high as 45˚ A/mm, and the covered ˚ ˚ spectral region ranges from 3600A to 6600A. The wide-field copy (Fig. 10-11) shows a part of the zeroth order (top left) , the first order (at heights 53-34 km) and a part of the second order (right). The fireball flew from the top to the bottom. Note the meteoroid’s splitting: at the bottom.

101

102

Figure 10-11: see caption on page 101.



103

Figure 10-12: Persistent trains may be expected after the appearance of very bright fireballs. The photos shown here are part of a series taken by Karl Simmons at Jacksonville, Florida, after a Leonid fireball of -6m on 1966 November 17, 05h 05m Local Time. A fast lens with f/1.5 was used to expose an ISO 400/27◦ film for 10 to 25 seconds.

Figure 10-13: Smoke train illuminated by the Sun during twilight observed at the Amur River (Khabarovsk Region, Siberia, φ ≈ 50◦ N) on 1982 October 7. (Photo kindly provided by Alexandra Terentjeva)

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Acknowledgements There were some people substantially supporting the work on this handbook. I am indebted to Zdeˇ nek Ceplecha and Jiˇri Boroviˇcka of the Ondˇrejov Observatory, Czech Republic, for valuable hints. Paul Roggemans of Belgium helped a lot with translations from an earlier Dutch Photographic Handbook of Tonny Vanmunster. The Part 6 (Double Station Work) bases on this Handbook. Dieter Heinlein (Germany) and Casper ter Kuile (the Netherlands) assisted in the initial phase of the work. Ralf Koschack of Germany and José Trigo of Spain developed the application of photography for meteor Storms. Dr. Robert Hawkes (Canada) provided the text about video meteor observations. I also wish to thank the following persons for providing very instructive photographs: • Dr. Jiˇri Boroviˇcka of the Czech Republic (the excellent fireball spectrum) • Dieter Heinlein of Germany (the fireball photographed by an all sky-mirror camera) • Karl Simmons of the USA (photo series of fireball trains) • Alexandra Terentjeva of Russia (the twilight smoke train) Last but not least, I thank Alastair McBeath of England very much indeed for proof-reading the text not only once and who also gave a lot of valuable hints. The present text was achieved with the assistance of Peter Brown of Canada through intense discussions in the final phase. Same more people also read parts of the manuscript and contributed to the final text. From this you see, that the Photographic Handbook may be regarded as a result of IMO members from many countries and we hope it is valuable for your practical work. Potsdam, June 1993.

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