Amateur Telescope Making Advanced, Book Two (A Sequel to Amateur Telescope Making, Book One)

AMATEUR TELESCOPE M A K I N G ADVANCED (BOOK T W O )

A SeqfLel t 0 AMATETTR TELESCOYE ?I~AKIXG (BOOK ONE)

A ~ B E RG. T INGALLS' Editor Contributing Editor, Scientific American A eollection of contributionv to cclnateur preciaion opticv b y numerouv authorities

SCIEN'I'IFIC AMERTCAN, INC. 1952

.U7 rigltts ) . ~ s P I < I . P ~ Th? lti-lith o E Tr;iii~ltitioii: i 1 ~ , ri..-ri.\ ~ ( fl n r al1 l d ~ ~ z t i11(,111fIing ~ ~ i g ~ th? ~ ~ Sc;~~~diii:~vian Eiglitli l'rilitiiig 1 9 5 2

PRIXTED

IX

THE

ITNiTED STATPS

RIXGSPOIIT PI<EóS Ih- oii 11akiiig Byepieces

Oculars a t Smnll Cost-Patterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .187 The Refractor-Met;~l Pnrts nnd Mounting-Triylor . . . . . . . . . . . . . . . . . . . . . .192 The Refracting Selescope-Principies of Operation and ConstructionHavilancl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .212 1Iainly on tlie Objeetive Lens

Target Scopes-Kirlcliam

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .262 For thc Rifleman

Testing Convex Splierical Surfaces-King . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Collimatioii and Adjustrrient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 How t o M;il<e a Diagoiial for a xentoninn-Hindle . . . . . . . . . . . . . . . . . . 282 Making Srtting Circlrs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 . Tclescope 1)rives-1, ower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 . Drives for 1.arger Telescopes . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 319 Hand-Wound Spring Drivrs for Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321i She Sprin.gfielc1 Mouriting-Porter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 The Springfield Mounting-Pattern Making-Porter . . . . . . . . . . . . . . . . . . . . 339 With Principies of Yolding and Casting

Molding a n d Castiiig Springfield Mountiiig Parts-Ferson ...............3.1.i Moldirig and Casting a Fork-Mason . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i36l Sidelights on Molding and Casting

v

vi

CONTENTS PlOE

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 Machining the Springfield 3lounting.Piirter Motor Drives, Coiiiiterweighting. Pier-Cpringfirl(1 Moiiiitiiip-Porter . . . . 371 'I'he Riiilding tif a 19-Inch lieflecting 'I'elescopll Theory and Design of Aplanatic Heflectors Eiiiployirip ;i Correctirig 1.eiisWriglit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 Ineluding tlie Se-Dall... . . . . . . . . . . . . . . . . . . . . . . . . . 579 Atmosphere, Te1escol)c arid Obierver-Douglass . . . . . . . . . . . . . . . . . . . . . . . . .565 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606 Heflrctors versus Refractors-Pickering U'oodin Tubes for Reflectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1 6 Dealinp witli Spider Diflraction-Couder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 . . She Itichest-Field Telescope-Walkden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .623 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 ..

H A V I N G TO DO WITH T H E CONSTHUCTTON OF OPTICAL INSTRUMENTS

By A . TI7. EI'LREST Pittsfield, 1\Iüssacliu\etts A

I'II~:UE

or

OI,ASS

He lahorrd late into tlie night, At rarly iiiorii' liis task rehulned, To f;~bhioii thus a < l i h ! i o£ glass Intu 2% sui~tleCUITT.C, not drep, But iiieasiired only by the slindes of light P'roin ;t uirnple yinholp meosition, liowrrir, tlie riirrc :iI.," tliat t h ~prohnhlc e r r o r of observntiori is ~ v c l lvitliiii tlie toler;irice givcii-:ill of wliicli 1iie:iric tliztt, if n mirror ir figureti t o this clegrce of acciirncy, bri1li:iiit ~,erforninnce triny he expected, provicled tlie ri~imcrousotlier conclitions necessary for tliis perforrii?ince ;ire corrcct. 'I'lie otlicr cliiss of errors incl~iclcs close zonnl irregulnrities, dog-hisciiit, lemoii peel, buiiilis, etc. One illustrtitioii witli tlie 10" iiiirror will serve t u

sliow h o a closc.ly tlir thiclinrss of tlicsc ~irotiihc~r;inces m:iy he e.;tim:itecl 1)'. visual irialiectioii, :iltlioiip~iit is irnpossil~lr:ictii;illy ti1 irics:isure it. Figiire 17 sliows tlic al)li?irciit cross-sertion iritli tlic Icriife-edpe a t tlie Iialf-\\-ay 1)o~itioti, hriiigiiig tlie c r e d ut 7U pisrcerit of tlic t1ist:iiice froiri ceiit(ar to riiri. Referring t o 1i;ige 9, it will h r sccm tlint tlie t1evi:rtioii of tlii\ crest f l ~ ) l i i the refcrerice splicrc is oiie qu;irtcr of 1-~/8liZ.o r :ihout 5 millioiitlis of ;in inch. Se\.er;il iliglit firotiihri.:~riccs I i ~ i r r1)ccii l)l;icehcd ir/' ctnd (Ioz~n,,causirig the sliadow t o more vertically betmeen tlie stre;il<s, tirid ni;iking it c:isizr t o rcail. After tlic first stage of fine grinding, the siirface will be srnooth enough t o perniit tlic ¿ictii:il figure oii tlie iiiirror t o be seen, tlie iris of tlic rye arting as a knife-eclpe. lliis cffcct, of course, will show u p unly wheri tlir rye is crt a very critica1 point very nrnr tlie rxact crnter of currature, m;ikirip tlie test :it tliis st;ipr ;~cci~r;ite to a fraction of nn inch. Xeedlrss t o say, tlie spit rnust I)c allowed to drop straight down, and not be ejected iri :i p;ir;ibolic psth. 7'001 ~ , f í , c t : This term refers t o the more rapid grindinp (ir poliihirig which ;il\v:iys occrirs in tlint zone of tlic mirror ~vliich is at tlir edpc of tlie tool n t tlie en(] of tlie stroke, provided t h r edge of the tool is in rontact. Tlir c:iiise is oh\i«ue. A t thc enrl of :i stroke, tlie iinsiipportecl :ire;i of the mirror p;irti;illy coiinterba1:inces tlic o1)posite side, relicving t h r ressi sir re tliere, so tliat tlie grc;itest pressiire is ;it the edge of tlie tool nc.arrst tlie unsupl>«rtc-il inrea. 'ihe net effect is, t o a mirior clegree, tlie s;imc. :is with tlie ovcrli;ing stroke made p;~r;illel t o this edge of tlie tool, excvpt hlendt-d out more iiito tlie nd,i;tcent zories. If tlie edge of tlie tool htis hren proiriid down excessively, or ttie edge of tlie pitcli Inp has been puslie(1 cln\rn. tlie cause in citlier c:i<e linving hren lorig strokes or overliiing, tlie tool effect will, with sliorter strokes, he soine~vliat f:irtlirr in wlirre actual rorit;ict eiiíls. A rcltited t.tfrct is present \vlirn gririding or polishing face u p xvith rirc,itl«.r sub-r1i;inictcr tools, unless tliere is a frrcluent cliange in the chord of the rnirror ovcr \\,liicli tlir stroke is directed. l'Iierv,nl c ~ f e r t :Tliis is distortiori of the niirror's surftice clue t o 1oc;ilized heat or cold, c:iuied by the hetit uf poliihing friction, or tlie tcniprriitiire drop of ev;!l~or:itiori frurii :trr;is of tlie tool zinc1 nijrror exposecl during the .itrolte; :ind since glahs lias a corficient uf rxpansiori runriirig into sigiiific;int figures, tlir Iiot spots \vil1 teiid t o swell oiit ;ind the cliillcd :irr;is slirinlc awny. If tlie bit11 of tlie tliiiiiit) is Iield :ig;iiiist tlie facc of the niirror for a momeiit, arici tlic rriirror is tlirn placrcl on tlie tcctirig stand, a pronounred tliermal hurnp \vil1 he heen. I n fnct, ~ r i t l ia little exlierience nn(1 our linowledge of tlic amouiit of ni:ignification of tlie teít. me muy rstiinate approximately how ninriy rnilliontlis of nn incli tlie hiimp protriides. If polisliiiip is re~iiirir(1before the hump has reccdc.ti, tlie first effect wiIl be a rriore proiioiincrd cweiiing of tIie l~urnp,since it wiil rrrrive more frictli;tri t h r srirroiiii~1irig tire;\. I)iit, clue t o tliis adi1ition:il fricttion, tlie burnp w i ~ ialsu b(: poiisIiing fastcr iip t o the point wliere its excess iieat is ~lissipated ;is fast as ~>rocliiced,whcii thc burnp mil1 sucldenly sliririk baCk

28

MIRROR MAKING

and he replaced by a hole. This iaperiment may be readily performed with ordinary glass, and aeems t o explain tlie cau\e of tlie dog-tiisruit aurface siicli as shown in Figure 24. Tlie protrutling iLre;ii ivill behave in exactlp the ' ~;f t e ri~ little polisliing they will recede and same manner aa the tiiuriib ~ i i ~ i r k the valleys iviil become the burnps. Tfiis would indicate tliat the whole surface of a mirror is in a constant state of slow, irregular oscillation during polishinp, if tlie polisliirip speed exceetlc. a crrt:iin limit. 'rlierefore, there is a defiriite limit to the speed ivith wliich fine optical work may be performed, this lirriit beconiirig a m:itter of iristinct if tlie worker is alert t o what is going 011. l'he greater tlie polisliing drag, the ilower the stroke must be,

so that the result of the two will not exceed the alloivable heat of friction per square inch. Here the hand worlcer has the advantage over tliose working with machines, since he can te11 tlie ainourit of drng mucli hpttrr by the "feel" thaii he can by tlie groans of a macliine. This is probably offset, tliough, by the fact t h a t the macliine worker is generally content t o take several times as long to polish as by the hand mcthod, since he doesn't have to work. Assuming that he keeps out of this dog-biscuit mess, as is quite likely if Pyrex is used, there are still two general thermal effects which must always be considerecl, since they are present in al1 polishing operations, arid particularly witli tlie usual amateur practice of polishing face down ori a full-sized tool. One of them is also present during the grinding operation, if water is used as the ahrasive vehicle. These will be given separatc: hend: ings for easy referrnce later on.

polisliing Inps tire rn:idc. I%ut Iiere \ve sliall íind an offscttirig f:ictor, a s f a r as tlie edpe is coiiceriicd. Friction ~ f f ~ c Froiii l : oiir ronsitlcratioii of tlie tool rticct, it i\ cvidcnt that tlir Iieat geiieratthd by polisliini: friction fr«rri oiir-tliir:I littlu hürcler ;it thut central tLrcti. Perhiips, : i ~ : i i r i , no results are sccri. Iliit tiftcr sevcr:il such attenipts, tlle lniinp will sucldenly stiirt t o shririk bnclí as wr watcli, ~viiidingu p iii :i deciisingly diffictult t o squeeze

out the surplus roupe mixture a f t e r each npplication, and get the mirror in proper contact with the lap. Tlie edge facets ülso iieed consideration. With the regular method of channeling, these a r e al1 undersizecl, nnd siiice sinking speed is a function of facet dinmeter, these margin;il facets will llave less resistance t o pressure than the complete facets farther in, resulting in tiirned-up edge. Trimrning the lap o r rounding the edges of tlie marginal facets will only make matters worse arid tliese dodges should riever be used in the preliminüry polishing, wlien tlie aim should be in the other direction. A soft metal s t r a p or plaster of paris d a m around the edge of the l a p \vil1 limit the flow, 6 u t these are hard t o keep in acljustment just below the leve1 of tlie pitch a s i t sinks. Another clodge is to d r y u p the l a p and paint airplane dope around the edges of the marginal facets, allowing it t o set hard before using the lap. Perliaps tlie simplest is t o fill in the channels between tlie marginal facets with pitch, since this will reduce their sinking speed by limiting the flow to t w o directions, and also help retain the rouge mixture. Poliahizy: Fine optical practice tlrmands slow intermittent work, with temperature ancl liumidity under strict control, using 50 liours or more to poiisii a 10" piirror, and an equal length of tiirie to figure. But who has the patience, or tlie necessary control of temperature and humidity t o reap the benefits of such slom work? F o r the amateur's purpose it is just as well t o polish a s quickly as possible by any method that will preserve a figure of revolution, givirig no serious attention t o the figure until the polisliing is

SO

MIRROR MAKIXG

completed. The mirror will, of course, emerFe from this stage looking undar test like almost anpthing but an optical surface. Rut with a little EXperience in zonal correction this can be rapidly changed to FL splierical snrface with practicnlly no eviderice of zonal hang-overs, and tliese disappear in tlie final figuring. This preliminary correcting not only adds t o tlie fun, but it tunes u p inind and rri~iscle so that in the last touching iip of the paraboloid, the worker, nnd not tlie mirror, is the boss of the situation. The main thing t o watch for is astigmatism, testing for this a t frequent intervals early in tlie polishing. If signs of this are found, work without pressiire; if not, give 'er the works. The reasons for the liard lap have been given, and the strokes should be short enough t o keep the marginal zone polishing somewhere nearly as fast as the center. The HCF lap, described ir1 "A.'I'.M.," provides almost a fool-proof method of bringiiig the mirror throiigli this stage with a brilliant, scratchless and sleek-free r~isual polish. Rut, since the pitch l a p will be needed for the final work, mariy will prefer t o go it on pitch frorn the beginning. And here one must he ever on the :ilert t o prevent scratches, taking a lesson from the busy spectacle maker who works with grit al1 over the place but seldorri scratclies a lens. H e is jiist ha1)itually prit coiiscious and keeps out of it. So rol1 u p tlie sleeves, scrub u p everything that is t o be touched, includirig the testing equipment and under the fingernails, and never touch anythinp elsr. C;irho perms a r e dead ones and won't hop u p on the la11 of their own accord. \Vlieti picking u p the rouge jar, bring i t around the lap, not over it. Etc. Frequent exchange of newspapers on the hench is good insurance, plticing anything t o be handled on clean slieets of white paper, Keep an eye on the visitors. These a r e always leaning on first one thing and then another, and just can't be made t o unclerstand. Most scratclies ocrlir imrnediately after placing the mirror on the lap. So be careful in applying rouge. Stir thorouglily each time it is used, wait about five seconds and then draw from tlie top with a large medicine dropper. Run a narrow line of rouge along the center of each row of facets, and set the mirror down carefully t o spread it out. Use ten seconds' tapered pressure, make a couple of small circular strokes t o distribute the rouge better, and then press ten seconds again before starting t o polish. Not only will tliis push any coarse grains down lrvel witli tlie rest, but al1 rouge granules will have a chance of getting a toe-hold in the pitch rather than being pushed off into the channels. F o r uniform action, the direction of the stroke must be changed often. This can easily be seeti the first time a pitch tool is used, before the surface gets too discolored. Apply thin rouge so that the fhcets can be plainly seen, and keep the strokes in one clirection. The facets will take the roupe charge in streaks ir1 tlie same direction as the strokes. These streaks will throw minute thermal b i i m p ~on the mirror, resulting in a "lcmon perl" surface. Change the direction of the strolíes 90° anil the firqt streaks will gradually be replacecl by others in the new direction. Now go into the clock stroke and tlie facets will soon take o11 an even hue al1 over. Lemon peel is too

irregular pieces shoiild be kept floatinp in a pan of soapy water, ready f o r use. The so;cp will riiake the rouge stick. F o r the roupli work the rouge may be niixecl as tliiolisliingtools of only about one-foiirth of the area of the glass were used witli l~led,"he says, a n d lie :idds, "it is difficult to riinke a goo(1 letld lap that.way." Anotlier wiiy that has oftrn been uied is to hold a disk of lend in tlie li;in L " , before iirie griiiding. Four Iiours of finc grindirig witli No. GOO resiiltrd in t l riirr;iture of 70". A t tliis poiiit 1 triccl ~)cilisliirip.1)iit tlie siirface w;is I~;~clly scr;rtclird. Tt i> necessnry to ~vtrhli tlic. No. :M) :irid No. 600 C;lrl)o. l%«tli sizes cont;liii tr few 1:irgc grniris, ~vliicliis tlie eaiiie o f tlie scratclics. \\'lit.ii priiiding gltiis these 1:irgc.r grtiiiis tire broli'ii d o ~ r i iiic:ir tlie rtlgc :iritl tlirrt,by c;iii\c no harrn, hiit ~vitliiiirtzil tlie gr:~insbur) tlirrriselves iii tlic ioft tool (c;ist iroii) ancl ~ r o , j e c tIiigli rrioiipli :ii)orr tlie siirf;ic.e to ciit a. proa\-r in tlie dislc. 'llicy start as scr:itclics ancl rrid with abrupt Iioles. 'I'lie expI:in;ition is obvioiis: the shiiviiip of steel huiltls u]) ir1 size aliecici of tlie gr:tiri. At tliis st:ige 1 encounterect many tritils :ind fnilures. First, 1 triecl H C F witli No. 600 Car1)o. 'l'liis producecl a Iiiiriipy surf:ice. 1 nest riiatle a hard pitcli liip nnd, iiftrr Coiir Iioiirs of grinortnrit it is to proxide rvery kinri of assurance that no trace of it

is discoverable in a mirror wliile tliosr froni ( ' aricl 1 ) ;lre uiisc;itlirti. The a]>l>varaiice then will be like Figiirc 2 tit ('. When inserted in tlie p1:iric of F 2 the knife-etlge will first encouiiter the ray from B, and then, wlien tlie ceriter is reached, it mil1 cut tliow from A and C, giving us :in appenrarice like that of Figure 2 at D. I f the knife-eclpe cuts tlie corie of liglit rniich iiiside of F1 or outside F 2 the sliadow's eclge u'ill be more nearly vertictil, as in Figure 3 :rt E. The sequence of shadow sh:il>es th;it we moiild see, if oiir knife-edge was rnoiirited on an accurate carrizrpe, alinecl with tlie mirror's axis and cut into the cone just htilf w ~ i y ,~vouldhe like those in Figure 2. Tliis, of eoursc, ctlii cover oiily the case of a mirror that is spliericnl cxcept f o r tlie :istigriiatism. Other shapes of surfiices gire other appear:irices. The most conimon of tliese is that exhibited hy an astigmatic paraboloicl. hyperbol«id or ellipsoid-the iiioiittd. Agnin referring t o Figure 1, if we should orient either the mirror or the kiiife-edpe so that, when the latter is cut into the cone a t F1 it \voulcl be parallel to the foca1 line a t that positiori, it ~vouldtheii cut al1 rays roming from the mirror simultaneously and the mirror would be seen to darken evenly al1 over. With the knife-edge still parallel t o F1 let us ciit into the plarie of F2. Tlie ray from B is the first c u t ; then, a s the center is reachrd, the rays from A and C a r e intercepted anrl finally tlie D ray is stopped. The shadow seen is one whose edpe is parallel to A C and progressing from R t o D, opposite in direction to the cutting of the knife-edge. This is a normal indication, in a perfectly spherical mirror, that the knife-edge is cutting the cone outside the O cif C. Thiis, if we shnuld happen to set iip rvrn n severly astigmatic mirror and accidentally have either axis of tlie astigmatisrn parallel t o the knife blade, we would get no indication of this clefect froiii the shndow test. The infctllible test f o r the presence or absence of astigmatism is one performed with the mirror set u p in a perfectly collimated telescope. The stars' images procluced by it a r e examined n i t h as high-powered an ocular as is available. If astigmatism is present, racking the eyepiece slightly in and out of focus will show the tell-tale elongations. I n the shop a tiny pinhole as a source, not too brightly illuminated, and a short-focused eyepiece for examining the image, will suffice t o detect most ordinary cases of astigmatism. The ocular must be corrcctly alined with the mirror, othermise a confusing effect is sometimes seen. The astigmatic elongation of tlie sliglitly out-of-fociis indication as seen in this test-the image-must follow a rotation of the mirrof, otherwise it is a spurious effect due t o the test set-iip. The matter of determining the axes of the astigmatism is quite import a n t to tk worker of speciila, inasmuch as any corrective measures t o he applied will have t o be carefully directecl so that tlie figure be correctecl and not further distorted. Also, tlie discrepancy betu~eenthe F1 and F 2 points is of practica1 importante. There a r e several possible methods for gaining the desired information.

ASTIGMATISM

75

First, the eyepiec? metliod-notinp the two foca1 points F i aricl F2 :ind obserring tlie directiori of e1oiip:ition of tlie irn6ipe :it eitlier fociis. Iteferring to Figure 1 :ig;iiii, Tre c:in see tli:it if we :rre iri focus witli tiie siiorter curr:rture, as a t F,, tlie eloi1g:itiori of tlie im:rge will be iri line ~ r i t l itlie lonp Curvnturr iixi,. If tlir septir;rtiori betweeri F1 ;iiid F2 is not vt.ry g r i a t ("nder it i"Iifiicii1t to deterrnirie tlie ex;ict lociiti»ri of tliese p»irit\ ;ind the me:isurrrnelit is hiibject t o 1:trpe errors. Mirrors 1i:irinp severti1 zones, or in whicli tlie p;~r;ibolic corrections are preseiit, otier :iiiotlier difficulty, in that the ini:ige sern in tlie eyepiece is subject to aberration. The seroiid rnetliocl, a iriotlificttLiori of tlie first, is one ir1 \vliicli :L ni:isk is pli~cedover tlie ftice of tlie mirror wliile it is uncler test. Tliis rn:r?ik l i i i ~ two equal :ipertiires, c1i:rmetric;rlly opposite each otlier, orer tlie m:irgiri:il zoiie of tlie rnirror. With tlie mask in place the eyepiece can be very accur:itely re-focused because, wlien out of focus, the image will iippe:ir doiihlt~, like Figure 2, lower left. Also, if our pinliole is sufficiently small arid orir eyepiece liigli enough in power, interference fringes will be seen when the focus is reircliecl. Tlie fociis of our starting position of tlie m:islí can he taken :ts zero and then, r»t:itiiip tlie mask about tlie niirror's center, tlie foca1 lengtli clieclced at each of severi~lorieiittitions and coriipiirr(1 m-itli that of tlie oripiiial position. We mtiy tcibiilate our resultinp fipiires or ni:ike a graph to represent them. Tlie gr:iph of ;L simple flexured niirror rvill he siriusoiaríifiiie or Cereiene will rrielt, setit tlir glass against tlie pltlte, allowiiia four bl:inlcs t o exterid beyoiid tlie e d ~ ea t four diametrical points for the purpose of miking tlie tliickness duririg tlie grinding, and then allow the whole t o cool. Tlie blockirig plate with its :ittaclieed tire se;rtc(l oii n fleit iron I)lockiiig @ate in ;I geoiiietrict~l patterri with ~iielted~>;ir;ittiiieor liglit oil. 'Tlie oil rnetliod difTers froin tlie par;ifiiie iii tliat no 1ie;iting is necesstiry, the oil giving enougli c«lirsion t o prrinit ti pltister block t o l)t. forriiccl \rritliout shiftirig the blanks frorii tlieir positions. Obviously, the 1)locking plate sliould be optically flat, notwitlistancliiig the fact that the seatecl surf:rces are finally grotind in block form I~efoi-e hring polislied. A f t r r seatirig iintl cooling (if tlie wnx is usecl) a metal band is securecl about the pliite to rrt;iiri plastcr. The l);ind slioiilcl he wicle enough t o cleor the liigliest bltinlí diriirnsioii. P1:ister is niixvd tincl poiired, and a fltrt iron btccking plate is keyed rritli plaster t o the k)loclecause of rliffererit polishing speeds. If a prisrn block is to t ~ eattaclircl t o a riiachinr for polishing, tlie 1):ickiiig plate prrviously (lescribrd will be iiecess;iry. Fine grinding of the ring ;isserribly retliiires tli~it tlie pressure used be fairly equ:il on al1 sides, so t h a t the angle of the prism face will not be changed. Poli.shing: Believing that procedures in macliine polisliing may be of use to those wlio have riot become ncquaiiited with tliern, 1 will set them forth and, following them, gire sorne supgestioiis for their eriiployirient ir1 liarid polishirip. The systern commonly used in romriiercial sliops is what is knorrn as tlie "sniall polisher systern." The lap is smaller in diameter than the surface to be polislied. The surface is polished face up. Figure 4 is an illustration «f the arrangement, thoupli in commercial use the machines a r e not so sil dified. However, the simple arrangement will enahle you to do good wor

Polishino mnchine n t ii,ork o n hiock of ~ j r i i r ~ d m i c i s . P i n on b i f r ~ r c n f e d dricin,q nrrri I w h ~ c h nlnl, 1 1 ~lijtprl like n 1ti11ye) punhes lo??. Cross.nrrn rults betwepn sectorx ( s e ? spction d r . u u > i n y ) nt eitlivr v n d . Holes n ndjust for stroke position, ndjuetnhle-throw crank nt o t h ~ r?nd o f conncr,ting rod f o r stroke tcngtli. Crrink ¡,Y worrn d r i r e n . I I ~ n r n t htahle, ~ ~ L I I P ou n wlorm year is belted l o n jackshaft, this t o s p i n d l e , w h i c h runs nt speeds other thnn lhnt o f opur s h ( i f t . Belt < l r i i e U ~ C L C ~ ~(1e11r P S iiuriri~licit!/. .Lt the l v f t : La),?jliill or do~vriliillgcner:illy recliiirr-.; no m;i.ior ltip miitiliition. Tlie Inp mny I)e scrtitcli(~t1;iroiind tlie eclge for greatrr nctiori iii retlucing convesity, or scratcliecl in tlie crriter to briiig tlic hlñck iil)\c-;ird tow;ird fltitiiess. F o r Ii:in(l ~)olisliiriga dumniy polislier iri:iriisIi,iiricl ~)l;icedtop t h e r aiid pressecl by li;~ii t o it, o r a t 90° t o eacli other.

Figure 8 ~ 1 1 0 solnc ~ 5 of tlic :ingles :ind tiie pthiicr:ii form. I t is n r r r u oltl i r i veiition hy tlir 1t;ili:iri Aiiiici (liHíi-1864) hiit coiiinioiily I)c:iri; tlrc ii:iiiit. of roof pri\iii I)cc:~ii%~, :is witl! otlivr tyl)es of roof 11ri\in, lliv ;i

/

'

!

l'hcse nrc tlir actu;il intrrsections of tlie rnys frorn tlie 7", 5.. nnd 3?? aones refcrrecl to tlie position o f tlie emiilsion inside fociis

FLATS

Flats I t Iias hren said tliat a truc plnric "is tlie riiost clifficult of al1 surfaccs to rn;il<e." 1 'ilrhetlier or not tliis is trur, it is rnost cert;iiiily U f;ict tli;it :i large fl;ct wliicli is xvitliin 0.1 friiige of plaiie, iriclu(1ing tlie edgr. ;ind wliich is corripletely free frorn pits, sleeks ;irir1 scratclies, is a truly difficult object t o coiistriict. Iri comp;rrisori, a high-quality paraboloid of equal size of aperture ratio (i or 8 is easily mtide. I n tlir folluwiiip ptiragrnphs only those methods and materials will he disciissed w7Iiich liare Iit.eri usecl by the writer and nhicli llave proved satisfactory. I t must be empliasized that, in other hands, other procedures llave yielded eqilal or superior results and tliat no one can have a rnonopoly in the matter of tecliriic.

Polishcd plate: Tliis is the usii;il material employed by amateurs. F o r most applications i t i s entirely suitable. In tliickriess greater tlinn liarmful strairis may be encountered. I t is ratlier soft (4.5-5.6 hIohs) and scratclies easily. The high linear expansion coeficient (8.8-9.2 x 10-6 per "C.) makes testing time-consuniiiig and tempcrature effects in use serious. P y r e r : ? F o r applications in wliich liglit does iiot trarerse the flat this material is superior t o polished plate. Scratches are less easily formed, due to greater h a ~ d n e s s(6.2 Molis). Disks of al1 sizes are uniformly free from large strains in the type designated a s "Telescope Blanks." Sight glasses and slieet Pyrex a r e not so satisfactory in tliis respect. Tlie low expansion coefficient (3.2 x 1 0 - 9 helps to mnke testing ancl use easier and more reliable than with plate. Bubbles, s t r i e and stones must be expected. Fused quartz:VTliis rather expensive medium is excellent for the construction of 10" and smaller test pl;ines and standard flats. I t is hard (7.0 hlolis) arid changes but little with rarying temperature. (Exp. 0.54 x

x",

10-6).

"Rlack" glass:4 Many types of opaque glasses a r e sold for building decoration, counter tops, etc. They resemble plate in hardnesi: and in expansion but some are not annealed and a r e useless for optical work. As test planes on whicli t o examine other flats, for moon diagonals, polarizing mirrors and Colzi sun oculars, the deep color forrns a background against which fringes appear witli great brilliance and which reflects only a small amount of the incident light.

-

R. \V. Porter, "A.T.31.," page 16. '1 Corniiig Cilass Works. Corning, N. Y. General Electric Cu. Echeneetady, N. Y.

T h e r ~ n a lSyiidicate, f.td., Brooklyn, N. Y.

4 Wells Glass C o . Kokoiiio Ind.

Pittshurgh Plat;

Glass

c;.,

Pittsburgh, Pa.

FLATS

114

Filler gla.~srx: These are colored gl:isses whicli trnnsmit restricted portions of tlie spectrum. They vary ~re:itly in working (lu:ilities. Siicli glasscs can be obtained from Corning (;las.; Works, Cliance Bros., aiicl Scliott & Ccnossen. They a r e obtainable both "r;lw" and polislied. Tlie polislietl grade usually has a surftice figure similar t o plate. Sperinl ? ~ l n t e r i n l s : l'liere sire rri:iny optical and otlier instriiments ~vliich utilize flat surfaces oii inirrors, Icnscs, prisrns and grntiiigs of otlier mnterials, sucli tic (Iozens of types of optical glass, various rnettils and such rninerals as fluorite, sylritr, rocksalt, calcitc, crysttil quartz, etc. These materi:ils rccluire such speciol metliocis of working that tliey cannot be considercd here. Grincling nbrnsizt~s: F o r surfacing, edging and perforating flats, Cnrboruiirlurri gr:iiris aiid powclers are quite suitable. She writer uses Nos. 70, 150, 280, ,100 :ind 600. Altlioiigh more rnpid and liarcler ahrasires, crushed steel niid carbicle.; of tuiipstcri and t)oron,j are :ir:iil;ihle, tliey need not be coiisidere i Amnteurdom" i t ~ m h a s b e ~ nconstructed: .llr. Srlbij is chief chrmirit f o r n, Cnlifornia foud yroducta ylant. D E h a ' al80 been n t r u c k driver, a ~ i d a d i s h u ' a s l r ~ rin n lumber c'amp. H i s avocation is r n n t h ~ m a t i c n lopties and he statrs that he dislikes t h e u'ork o j lnaking ol~tiral syslems, nrirl t h a t tlte only pleastcrcs n r ? to br f o u n d i n conlputation aiid use. He h n s c ~ » ~ p i ~and t ~ (conutricctecl l for his o w n ?tsp n n npochro, matic triplet tulrscope o l > j ~ ~ ' t i v rn, nlrmber o f astronomical oeulnrs, t w o anastigmatic photogrnphic objectives n?id sucli viirroacope yarts u s o B ~ r t r a n d l e n n . a wido npertclre achromntic condpnsvr and (1 1 2 ?nm. achromatic obiuctive. H P hns mnde a big paraboloid, t w o f / l ~ p h r r e snnd aboztt 50 pats. H e dislikes to w r i t e Z e t t ~ r s . - E d .

FEATS

131

Acccaaci For mnny npplicntions, t h r "quartrr-wave" lirnit of accuracy is sufficient. This applies to pliotograpliic filters, diagontils, prism faces and the plane surfaces of objcctivrs. For interferometer p1;ine.s 2 . 0 5 wave is none too flat21 and surf;iccs slioiild be p;ir:illel 2 . 1 mnve per incli. For coelostiit mirrors and for tlie outhide reflecting planes of telescopcs, huch as tlic Gcrrish, tlie acceptccl lirnit of error is e.1 w;ive. Freedoni from zori;il and periodic errors is assiimed. Pitine mirrors used iri the trsting of parnboloitls antl of completed telesco~~es a t tiirir foci must I>e of liigli quality a s regírrtls srnootliiiess o f figure (c0.1 w;ire rn;~rimurn),biit iiriiform sphericity of the order of c 1 to 10 waves can be tolerated, del~enclirigupoii the applicirtion. The supgestions offered in the abore short article a r e admittedly sketcliy and perhaps incornplete ancl the methods offered will be considered incorrect by some. T1iei;e criticisms are t o be expected, for the subject of flats can be thorouglilp treated only in a large volume writtrn by a triie master of the art, and tlie writer could by no means arrogate t o himself this title. 21 IIiclielson-"Studies i n Optica," 1927. 22 >I~liiistroiii, K1.i.. 81.i.I ~ i a t s . , \-. 7 . So. 5 ( 1 9 3 6 ) , PP. 216-7. Describes ver9 einctlg tlie riietkiod of sCpnratiun of exteriially aiid internally reflected images.

siic.

132

ASPHERIC SURFACES

N o t e s on the Optical T e s t i n g of Aspheric Surfaces The amateur telescope maker has at his command methods of testing spheres, paraboloids, hyperboloids, ellipsoids :~nd planes which are amply precise and thoroughly satisfactory. Many sucli metliods, however, depend on visual acuity, ocular contrast sensibility and personal judgment of geometric forms. These apparently new test methods a r e different, in t h a t they give results in terms of measurement and are, therefore, in some cases preferable to Hb

other methods. No crying need for these methods exists-they a r e not necessary f o r most work. Tliey are published here as possible aids t o the advanced amateur in solving specialized problems, and as "different" methods by which otlier methods iriny be cliecl<ed by the hobbyist wliose eartlily joys a r e rnathematics and optical esperimeiitation. The ellipsoid: The ellipsoid is, by dcfinition, the conic of revolution which will bring al1 rays cmanating from :I point to a focus a t aiiotlier point. The secondary mirror of a Gregori:in sliould h a r e an ellipsoidal figure. The common methods of trstirig ellipsoidal surfaces (Kirkham's, Hindle's and Foucault's) yield results of higli precision and are satisfactory. The criterion of perfection is the degree t o wliicli the surface approaches apparent flatness under the knife-edge, or the degree to whicli the bands approach straightness and parallelism if the tests of Jentscli or Ronchi are used.

133

ASPHERIC SURFACES

I t seems, however, that mnny experimenters find the judgment of flatness and straightness difficult and prefer a method of measurement. Also, instnnces may he found in which a definite amourit of under- or over-correction is desired. Such a metliod is outlined below, with the usual matliernatical developmerit. The only variables considered a r e tlie easily-measured distances, P, P', rll and D, tlie distance of the source. I n Figure 1, HI>SoH'I>is a n ellipse symmetrically described about o, the interiection of the x asis with tlie !/ axis. and I.', are the foci. V1lTltis the tnngent a t 11%. H I L S , bis the rne~witli tlieir own Iionds. 1 llave for years urged mirror ~ n ~ k e tros t r y their hand a t small lens making, telling thein that the difficulties a r e no greater, and t h a t the fun is just a s great. 1 have picked

u p a few more ideas on the subject since y r i t i n g the cliapter on eyepieces (page 66, "Amateur Telescope Making,") and am here offering them t o the "fraternity," along with a description of the spindle 1 have been using here a t Pasadena the past few years. Figure 1 shows the general set-up, the spindle and motor bolted t o a cast iron slab. The motor is 1 h.p. (1726 r.p.m.) giving arnple power for the larger work running iip to 21,"" dismeter. TIie spindle is 6" long, X6'' in diametei and runs in baii bearings. The dish ,4 is removable, and a horizontal stud at R allows turning the spindle easily into a horizontal position f o r centering work. The di\h can be claniped, C, in any azimuth so as t o bring the lerer a r m D t o the most comfortable pnsition. The lerer a r m is provided with a universal joint E, and the rod itself can be slid endwise and rocked, so as t o bring the pin P, carrying the lens, t o just the right place over the lap. Two pulleys on tlie motor and two on the spindle, of 5/aW, l", ll/L>",and 2" diameter, allow a v i d e range of speeds for tlie different sized lenses.

1 experienceI.") tire tlie \vriiikles and metliods worked out by niyself witliout beiiig prt.jiidice(l by a knowledge of professional practice. Undoubtedly sorne of tliern will appetrr crucle and amiising to the professional. For exnmple, it rii:iy he tliat a better way of making t~ lens run t r u e o11 a spindle is t o use a fork ( ( 1 , Figure 6 ) and, as explained on ptiges 69-70, "A.T.M.," 1 llave usecl therri hotli, but 1 rernemher a h:i(l scratch that developed when 1 used the forlc. Tliere m:iy be other simplrr tina more ortliodox ways, hut it's heen lots of fun ~vorkingout one's own technic. To me a lens is a \von tiie ruiirler, place a drop of oil in tlie hole in the back and, with tlie pin bar horizontal and the pin vertical, üllow the runner t o spiri for a few secoiids on the lap. Do not allow it t o p m s over

3 % " mountrd r u n n r r .

the center or edge of the lap. This spinning witli 220 is not essential-it is sirnply done to insure a leve1 sdrface t o s t a r t with. Wash u p thoroughly, place clean papers, and give a short wet with F Carborundum. Thirty to sixty seconds stiould serve to take out your 220 pits. Examine the edges-ttie center will take care of itself. When no more pits can be seen, wash u p carefully, charige papers on your beiich, aiid proceed with 600. Do not spin longer than necessary, a s your l a p is getting out of truth a11 the time and thc less grinding you do on it the better. UTatch the edge of your runner for pits-when al1 a r e gane, wash u p again, place fresh papers on the bench, and cliange over t o GF emery. Run three or four wets of this, nllou,ing tiie lap t o become fairly dry between each wet, in this way tlie individual particlis of emery a r e broken down tlioroughly and a better surface will result thnn if the emery is kept too wet. I t is interesting to gnge your runner betwcen each grade of abrasive aiid see exactly how much is taken off by each.

Y a k i n g the Flnt l'itch L n p : Wash u p your cast iron lap and warm it thoroughly. Melt and strain about one half of your remaining pitch and pour i t into a clean tin piin large enough in diameter t o take your lap. H e a t u p your pitch thoroughly a n d a t once d i p tlie hot lap, flat side down, into the pitch. Do tliis several times, endeavoring t o g r t a thin, bubble-free coating on the face. Hemove from the pan and, holding it vertical and rotatirig it slowly, play the open flame of the bunsen on the pitch surface. Most of Llie pitcli will run off, and al1 the bubble Iioles should clase. The ideal lap is perhaps i/iU" tliick. Placc your Iap on the beiich t o cool, making sure that i t is level. If the surface has waves, pass your flaine over it a few times and tliey will soon

FIGCRE 21 8" d i a m e t ~ rpat pitch lop sume detall as In Ebgure 1 7 .

T h e part i n d ~ e a t e d b y bottont arrow calls for t h e

leve1 out. Allow it t o cool slightly and, while waiting, clean u p a piece of plate glass (old windshield) about a foot square and paint it over with rouge and water. Invert your l a p on this rouged surface, work it baclr and forth, iising plenty of elbow grease and lreeping the lap revolving. If your pitch is fairly warm, oiie such working should serve t o give you a good pitch surface. Small air bubbles l/s" or so in diameter a r e rather desirable than otherwise, provided their distribution is fairly even over the whole siirface. Figure 23. Cool your l a p thoroughly, place i t in your spindle and, with your pocket knife, cut out a '/2" hole in the center. Also cut channels, clear down t o the iron, about S" apart over the wholc surfacr. Wash in cold water, scrubbing the surface lightly to remove any particles of loose pitch. Rtglaee the l a p in your spindle, coat thoroughly with rouge, place your runner in position with the pin in the oiled Iiole aiid then s t a r t your motor. Probably your runner will chatter on the rough l a p for tlie first few revolutions, but

172

SMALL LENSES

Iiold it down firmly and it will soon quiet down. It is well t o have a cup of water handy to your rouge pot while poliqhir~g. Keep your Iap thoroughly wet with rouge and water. Use only iiioderate pressure. Dori't allow the runner t o get set ovrr tlie center of the lap-it mny stick if it does. Your polish should s t a r t in the center and work evenly t o the edge, exactly a s was the case on yoiir ronvex surfaces. Polish clear out t o the edge of the runner, wash u p and examine with your lens. Al1 pits, no matter how minute, should have disappeared. Remore the lenses from the runner, by heating until tlie cement i5 perfectly liquid. Pick them off-don't slide theiii to the edge, or scratclies may result. Do not atteinpt t o wipe or scr:ipe tlie cement off, but allow it t o cuol aiid place the lense5 in turpentine a ~ o u p l cof hours or, better, over night, t o soak. Thii will looíen al1 cement and grit. Wash witli warm water and soap, rinsc thoroughly, and wipe d r y with soft tissue paper.

Brass centering t u b e and centering fork.

C ~ n t e r i n y :When the mechanical and optical axes of a lens coincide with each othcr, the lens is said t o be centcred. If these axcs a r e not coincident the lens must be thicker a t one point of the edge than i t is a t another. I n other words your lens, taken as a whole, is prism-sh,cpecl-light rays will be resolved iiito a spectrum and very unpleasant effects will re5ult when any bright body is viewed through sucli a n uncentered lens. See Figure 57, a, page 69, "A.T.M." Chuck a 2" length of %" brass tubing in your lathe, center it a s accurately as possible and, with a pointed tool, chamfer the end both inside and out. The resultant edge should be quite tlii11 a n d niust run perfectly true. Figure 24, left. With an alcohol lamp heat the end of the tubing and apply a thin coat of cement t o the extreme edge. W a r m u p one of your blanks and apply cement to the convex side. Next, reheat the tubing and stick the cemented convex side as centrally as possible on t o it. Then, with the lathe running a t its slowest speed and with the alcohol lamp in one hancl, warm the tubing. A t the same time, and with a forked shaped piece of soft wood (Figure 24, right) in tlie other hand, gently move your blank on the tubing so that a reflection from its fiat facr will stay absolutely still. This job takes quite a bit of patience. Do not press so hard that you will stop the glass from revolving-to

SMALL LENSES

173

do this will invite a scured ring on the convex side. Take luts of time. When perfectly centered, allow t o cool. of grinding the edge of the lens conThe n i x t step iii ceiitering-that centric with the axes-rriny be performed by using yoiir tool holder and cross

Left: 10" z

4" r

FIGURE2 5 '/4" t r a y . R i g l i t : Gtrut f o r c e n t ~ r i n gd i n g b a t

feed saddle. However, it is rather messy, as Carborundum and water usually get over the lathe bed, etc. A better scheme is t o cut a piece of %" board 2" wider than tlie lathe bed and 12" or 15" long, and place this on tlie bed before you start centering your leiis, cover it with paper and, in this way, protect tl>e lathe. Also, shoirld your lens slip, it is not so likely t o get chipped.

FIGURE2 6 C e n t e r i n g gadget

Make u p a shallow tin dish 4" wide, 10" lorig, and 1/4" high. Figure 25, left. Take two pieces of wood 9" long, 1%'' wide and S/s" tliick, and hinge them together a t one end. Procure a strip of mild steel 1" wide and 7" long. Bend and dril1 it as shown in the same figure, a t right. A j/," bolt with 3" of thread and two ten-cent iron clamps complete the outfit. Set up as shown in Figure 26, using one clamp to hold the outfit securely

SMALL LENSES t o your lathe bcd. The otlier clariip Iiolds ti small strip of 16- or 18-gage sheet brass, in place on the upper member. Adjust so that the strip is just in contact with the edge of your Iens. Do not press hard. Apply 220 Carborundum (if your lens is away off center, use 80) and, revolving the lathe a t its Iiighest speed, proceed t o grind the edge of the lens t o your finished size. Stop grinding when the lens is about 0.01" larger than desired. Remove al1 of yoiir rigging exccpt the bottom '/," hoard, wash iip and finísh with a Carborundum slip. P u t a slight bevel on each sidc, wash again, wipe d r y and, with your alcohol lamp, warm the tubing. Remove the lens ancl cool. Place iii turpeiitine t o soak, wash iii würrri water and soap, d r y on soft tissue paper-and yoiir lrns is completed. Spinning: I n most writings on lens making, the spinning method is given preference over the handled method outlined above. It is by f a r the best method for lenses of longer radius (either convex or concave), but with are several inherent difficulties short radius convex lenses-1" or under-tliere which cannot be surmounted. Chief of these is your inability t o make a small radius convex lens travel away from the center of your lap-either the lens clings tightly o r else lets go suddenly and flies off its runner. Concave lenses, no matter how short the radius, can be spun successfully and, if properly managed, as Porter says, "will leave the l a p a s a perfect surface of revolution." T u r n u p two small runners 1" in diameter and l/s" thick and cement t o each of them a glass blank (same size a s before-1%" square). Grind these roughly on your w r t Carhorundum stone-one concave t o your convcx tcmplate and the other convex t o your concave template. Place your convex l a p in your spindle and, a t 300 t o 500 r.p.m., s t a r t and rough out your concare curve. I t will save the curve of your l a p if you make u p a special rough lap and use i t to take out the major portion of t h r plass. Go throiigh the same steps with your 1" radius concave lap, and 1 think you will demonstrate t o yourself how hard i t is t o make your convex blank beliave itself. Your concave blank will go along fine-watch the curve closely, it is very easy t o spin away from the template. Keep the pin pointing a t a slight angle t o the toward thc cclge of the lap deepens surface and keep it moving-spinning the center, while spinning with the glass nearer the center wears down its edges. Wheri workirig a double coiicave or a meniscus, make your runner t o fit the curve t o which i t has t o be cemented. P o r a conrex surface your ruiiiier sliould have a slightly deeper curve than the glass-this gives a iittle more cement in the center. F o r a concave surface make the runner curve slightly shallower. This puts a thicker cushion of cement in the center. In finishing a n achromatic pair whose second and third surfaces are of the same curve, i t is well t o cement the first surface (work i t first) t o a runner, the back of whicli has been finislied exactly tlie sanie as a lapconical hole and screw complete. You can tlien spin your third surface on top. Your curves, with proper manipulation, will then run together. Acliromatic combinations a r e given in the latter p a r t of these notes.

SMALL LENSES

175

Several Icnses of rather flat curvc may be run tngether as one uriit porter has explained and illustrated the making of these, in "A.T.M.," in a very lucid manner. 1 linve devised several jigs and fixtures for rnaking eyepiece mountings. The time spent o11 tliese will be repaid several tinies over if you contemplate making even one full set of eyepieces.

Take a piece of steel 4%" long and 1%" in diameter and turn u p a mandrel to fit yoiir latlie spindle (Figure 27). Make the threaded p a r t exactly 1.157" in diameter, and cut 40 threads per inch on it. This thread will fit the interna1 thread of your eyepiece tubing. The threads shown on

Stpel ring.

pieces in Figures 27, 28, 29, 30 will be standards for al1 your future mountings, so take pains with them. Turu u p a steel ring, as showii in Figure 28-making your finishing face cuts with the ring screwed on your mandrel, thus insuring these faces being absolutely square.

Next, rriak~ tlie piece sliown in Figure 29. The interna1 thread of this should accurately fit the m:indrel, Figure 27. I t will be best t o face u p this piece, bore and thread it, tind then finish the externa1 thread with the piece mounted on tbe Figure 27 mandrel. The externa1 thread is the same as

the outside threads on your eyepiece tube, and forms the gage to which to make the inside threads of your caps. I t s diarneter before threading should be 1.25". The mounting proper consists of four parts-the tubr, the two lens rings, and tlie eye cap. Al1 threads are 40 per inch. The tube is started as follows: Cut off 1%' of your 1%" O.D. brass tube. Center this piece accurately

FIGCRE30 Thread gage.

in your latlie chuck, and face one end square. Cut 1%" of thread inside, and 1/4" on the outside. Make sure that these threads are an easy fit with jigs 27 and 30 (Figure 31). To make your lens rings, chuck a piece of 1%" O.D. brass, several inches long, center dril1 the tail end, and run u p your tail stock. Turii tlie outside to 13;16" ( a s these rings will be standard aricl riiade to fit any ordinary lens, you may as well make up 10 or 12 of thern at one setting). With your cutoff tool, cut into the bar to a depth of 3/,", leaving %21' bctween cuts. (See Figure 32). This will leave your rings %2'' wide and eonnected together in the center by a b a r 1/2", more or less, in diameter. Next, turn off each ring t o

SMALL LENSES

177

1.157" for a distanre of '/x", leaving Vj2"t o the full diameter of 13/lfif'. P u t a f i n e knurl on this narrow flange and thread tbe 1.167" p a r t ( t o fit your gage No. 28). Figure 32 shows four of these rings. The 1.157" is arrived a t as follows: 1.12.5" is the internal diameter of your tube. 0.0'32474 is the double depth of a 40 thread. Therefore, 1.125 0.032474

+

FIGCRE31 E y e p i e c e tube.

= the bottom diameter of your internal thread. Leavc off the last three decirnals and your thread will run nicely. Eye-lans Caps: Chuck a piece of 11/'" round brass (Figure 33). If over 6" long, use a steady rest. Face off the end. Bore t o a depth of 1/4" and face the bottoni of the bore. Cut a 40 thread inside t o fit your standard on No. 29. Turn the outside tu lgG"for a length of ?/4", partly cut off t o a depth of 5/," a t 5/16'' from the face, put a fine knurl on the 4/1,3" wide face, a n d finish cutting off. Details are given in Figure 33.

FIGCRE32 Rrass lpns rings.

E'inishing and Fitting a Lens Ring t o a Lens. Place mandrel No. 27 in your lathe spindle, screw jig No. 28 o11 the end, and into jig No. 28 screw one of your roiighed brass rings. Your lens may be of any diameter u p t o about 1". Dril1 and bore your ring t o about .006" smaller than the O.D. of the lens (this gives a shoiilder .OO.?'' a11 arouricl for the lens t o rcst against). Counterbore to within ?&'' of the back of the riiig .002" larger than t h ~ 0.D. of t h e lens-.001" al1 around is about right. Next, with a sharp tool, t u r n out the face around tlie Sore ro a dapth a little lower ($&'') than the upper tace of the lens will be when resting against its should--, leaving a

wall as thin as you possihly can (.002" or less) st:~ndingu p :iroiind tli? hore. Use n magnifying glass for this operation, ;ind keep yoiir ton1 shn.rp. Finish u p with fine steel wool. Figure 34 sliows: a 1i:ilf-section iind side elevation of a finished ring a n d lens for ;i Ramsdeii ryepiecc. Reversirig tlie lens in its s e t t i ~ i gaiid bevelling tlie shoulder will rnzike tliis settiiig suit;rble for a Huygens field lens. Blackeii and polisli as detailed in :i later parzrgrapli.

16 FIGURE3 3

Brass eue c a p .

F a s t e n i n g the Len* in I t s R i n g . Procure a piece of S" round hrasi welding rod ahout 6" long, file u p the ends t o a blunt point of about 342'' radius. Smooth u p the ends very carefully. Run u p your steady rest and adjust i t so t h a t it is just helow tlie center of tlie leris ririg. Place the cleaned lens in its settinp anrl, holding it in place -71th tlie t i p of one finger of the left hand, qtart tlie I:rtlie. Run 4ow and, with t h point ~ nf the brass rod wliich is held in the right hand, resting on the ste:idy rest, very carefully b u r r over the thin wall of brass onto the lens. Do not press too hard. Take plenty of tirrie, and gradually work tlie brass into shnpe. T r y speeding u p a little-

F I G D I ~34 E Lens ring.

sometimes the brass will turn orer easier a t speed than when running slower. Do not omit the final stage in blackening your mount; that is, heating as hot ns pos5ible and coolinp under tlie tap. Rrass is somewhat annealed by this treatment, whereas steel would be hardened. The brass wall or collar itsclf should be of siich height that when burred over i t just covers the bevel of the lens. A very hard, smooth, steel point is recommended for this burring operation, also a tool with a small roller a t the rnd. 1 have tried both, but have found t h a t soft brats such as welding rods are made of, is less likely t o tear the delicate brass wall.

179

SMALL I.ENSES

Tlie wholc appearanm of a n otherwise perfect mount can be utterly spoilerl by a poor joh of burring, so take time and work carefiilly and slowly. Finisliing the l'ubr: First determine tlie separation requircd. This is given in tlie table (Figure 35, left) a s 0.77". Tlie field lens is 0.20" thick and the eye lens is 0.10" thick, or a total of 1.07" from plano- t u plano-surface of the two len\es. Both riiigs stiould be borecl and shoulrlered ho that the plano-surface of the lens will he 7/&" (0.03125") below thc outer flat surface of tlie ring. I n other words, the total separation will equal the distance required betweeii ririg flanges. Place mandrel No. 27 in your lathe and screw your lI/?"long threaded brass tube on i t ; bore out t o a diameter of lig;;," for a distance of 0.43". Tliis will leave exactly 1.07" of thread in the tube.

teft

F I C U R35 ~ R a m s d ~ n (Haotingn), e.f.1. 1.015". R i g h t .

Finished pye cap.

Make the inne; shoulder square, so that the ring of tlie field lens will fit properly. Remove from mandrel and screw in your rings and their lenses and look a t a diffused light. .If you can see any marks (dust, etc.) on thc field lens j'uur separation is too long, so replace the tuhe on tlie rnandrel and turn out a one or two tlireads. This allows the lenses t o close u p slightly-finally point will be found when dust on the field lens cannot he seen. This is the best practica1 separation, altliough not correct theoretically. Finishing the Eyepiece C a p ; Screw jig No. 29 on your mandrel, and a roughed cap on the jig. Face iip smooth. Dril1 a 3/16'' hole through the center. Chanifer the edges of this hole t o 2O0, clean u p the eye hole so that i t is cxactly round and srnooth, then t u r n a depression deep in the face, starting '/X" from t h r outer edge and t o within %6" of the rhamfer aruund the central hole. Finish al1 over with fine steel wool. Remove any slight burr on the inside edge of the eye hole, and your mount is complete. A finished eye cap is shown in Figure 35, right. Black oxidized finish is applied as follows: Tc~ke your smooth finished pieces (lens rings must of course be blackened prior t o the insertion of the lenses) one a t a time and dip them in a solution of 76 percent nitric acid and 25 percent water in whicli has heen dissolved (with heat) as mucli copper wire as tlie acid will take up. Cool aiid decant into a plass jar and cover. Keep a few pieces of copper wire in your solution al1 the time. Dip tlie piece

SMALL LENSES

180

in your solution, and then hold it in the hunsen flanie (turn on al1 the air, in order t o keep the flame oxidizing). The piece will dry up, turn green, and finally black. Immerse it a second tíme a t once, while still very hot, in your copper nitrate solution. Heat up again. Repeat a t least four tirries to get a good coat. After the last heating cool under tlie t a p and wipe dry. Procure sorne powdered graphite (soft pencil or an old soft carbon brush). Mix with a little machine oil and, with a rag, rub some of the mixture al1 over the piece-especially the threads. Screw on the proper jigs and, with the lathe running fast, polish lightly with a dry, soft rag. The eye caps are finished black al1 over; then, holding a picce of fine emery paper on a flat wood block against the face, polish a ring a t the outer edge 1/H" wide and another ?/i6''wide around the eye hole bevel. Only the outside of the tube is polished bright-al1 other parts should be thoroiighly blacked. Class: Finding suitable glass for siiiall eyepiece lenses has always been quite a bugaboo tu the amateur. Rollrd, annealcd plate glass, as made by modern metliods, is entircly suitable. I t s index is th:rt of ordinary crown. The Yittsburgh Plate Glass Company make a beautiful colorless glass which they have appropriately named "Crystallex." 1 have used it in making orer 75 Rarnsden and Huygen's eyepieces and have yet to find a flaw o r striae of ilie rninutest kind. Furthermore, the surface of this glass is so perfect that pieces 2" square will show practically perfect interference fringes when tested with a n optical flat. Not al1 of this glass, uf course, has such a perfect surface, but very little selectiori is iieeded t o find pieces entirely suitable for diagonals and otlier small work. EYEPIECESPECIFICATIOSS:The following prescriptions for eyepieces have been collected from a number of sources-Bell, Porter, Orford, Conrady, Hastings, etc. 1 have made al1 of them a t various times. Come perforrri rather better than others, but in general it may be said that for ordinary amateur use the Ramsden type is prcferable t o some of the more complicated ones. Hastings' solid oculars, especially his Type D ("A.T.M.," page 178, 3rd edition, 4th edition), give a small, clear field. However, they are very sensitive t o slight variatioris from the set formulae, and are especially hard to center properly. The stated radii and thicknesses read frorri the front of the field lens to the hack of thc ege lens in each case. The figures given are for an e.f.1. of l", and should be divided or multiplied in order t o increase or d e c r e a ~ e tlie foca1 length. ("A.T.M.," page 71, 3rd and 4th editions). The diameter is drpendent on the radius ünd the center thickness, and can be made anythiiig desired, as can your edge thickncss. The crown and flint glass used in these eyepieces should, unless specifically noted, have the following approximate constants.

Crown Flin;

V

D

63.6 3 6 .'7

1.5137 1.6164

C 1.5113 1.6120

F 1.5194 1.6288

The former can he procured u p to .G25" thick from the Pittsburgli Plate Glass Compaiiy, Pittsburgh, Pa., and the latter from Mr. L. D. Keller, 2344 N. 19th Strret, Philadelplii;~,P;t. However, as mentioned before, ordinary rolled, annealed pldte will prove very satisfactory for any of the ingthe rnUirwineter focus control. The wide knurled band is the rotor, which i s 2" i n diameter. The two knurled rings to the left are atufing buz rings. The ample-sized rotor is easily ~ e i z e d with the bare or gloued fingern of either hand, from any position,, thus rnaking focusin.g both simple and convenient.

tube, one not easily damaged or thrown out of line by accidental kiiocks from different sources. An added cost of two or three dollars is little o r nothing on a fine instrument.' A heavy tube, when properly balanced, has a smooth and ponderous action, not easily affected by every little breeze that blows; also of notable importanee, a l/a" wall thickness permits reducing the diameter on each end of the tube sufficiently t o form a good shoulder and cut a No. 28 thread for the control end asserribly without weakeniiig the tube. This assembly will weigh 6 to 8 pounds and will continually take the brunt of manipulation when in service. Amateurs' telescopes are generallyyes, generally-built too light; seldom if ever too heavy. Machining the M a i n Tube: The plan of threading the main tube l/z" back, to a shoulder on each end, has been adopted as offering the surest means of gaining and maintaining a mechanical axis with a high degree of truth. This plan also offers ease of assembly and dis-assembly, clean sightly design eliminating small screws, and requires about the same time and skill to produce as other methods.

-METAL

PARTS

197

Without further discussion on this point l r t 11s get down t o husiness. Observe, pleasc, tlie secluence of preserit:ttioii, or what to do iiext, :ir; it is importtiiit Iiere and through the remaindcr of this chapter. If carefully followed you will he saved the emharrassment of doing your work over. I n a screw ciitting latlie, owned, borrowed, or rented, and of sufficient length to take the rriain tube between centers, chuck a billet of well seasoned hard wood (maple) about 12" to 14" in length, with sufficient stock to finish, when turnecl, to a diameter slightly larger than the inside dinmcter of tlie main tube. Support the loose end with the dead or tail center. With a wood turning gouge in the tool post of the lathe (do not t r y t o turn by hand), machine the billet of wood to a diameter, say, ?&''larger tlian the inside diameter of the main tube. L e t us cal1 this diameter "No. 1." Reduce this diameter 3 o r 4 thoiisandths of a n inch over most of the length, leaving 1" approximately of No. 1 diameter on each end. This reduced diameter is No. 2 diameter. Again reduce the diameter, as before, leavirig 1" of No. 2 diameter on each erid. Continue reducing the diameter, as above, uritil you are sure the diameter of the wood is smaller than the inside diameter of the tube. With a hand saw cut the wooden cylinder transversely into two equal lengths, which will give two tapered plugs. Do not remove the plug from the chuck, but move the tailstock back. Into one end of tlie main tube drop the loose plug. Over the plug in the chuck, assemble the otlier end of the main tube. Move u p the tail center t o its depression in the tail plug. I n the drive end half inch (chuck end) of the main tube, drill tlie brass and bore t h e wooden plug for a wood screw. Drive the screw, and with a hacksaw cut off the screw head close to the brass tube. The screw keeps the tube from turning on the plug. If tlie main tube is to be worked with a file and sandpaper (preferable for brass) t o a finishcd surface, this is the time t o do it. The tail end of the brass tube is now macliined forward 1" to a shoulder, reducing the diameter i/16" or more. The chuck end of the tube is riext machined hack 1y2", lncluding the screw head, to a shoulder, reducing the diameter to corresponcl with the reduccd diameter of the tail end. A standard, No. 28 thread is cut on the reduced diameter of each end. With a partirig tool set to leaie 1/1" of the thread from the shoulder, cut rings from each end of the tube. Be sure to do the tail end first. The chuck end ring will include the screw. These rings of sample or key thread should be promptly witness-marked with the end from which each camr. While i t is in the lathe, wrap the main tube smoothly witli one or two thicknesses of firm wrapping paper, sticking down the edge and leaving the thread and shoulder on each end exposed. This covering protects the tubc until the refractor is finishrd. Remove al1 from the lathe. Prickpunch t h t end of the scrrw and drill i t out to free the ring on the chuck end plug. Even if the main tube, when mounted in the lathe, stiould look and revolve a t the start like a plorified frankfurter-to exaggerate-it will make no difference. The machining and threading of each cnd, as described above,

x"

THE REPItliCTOR will make the two ends concentric and pnrallcl, witli a common mechanical axis. T'Re ('el¡: Tlir cell assernbly for aii «b,jective is import:rnt; it sliould be carefully rnade. There is u wide vziriety iii ccll design (See "A.T.M.," also Bell, "Tlie Telesco~>c"). Section:il drawiiigs and description of only two forms of cell will be included in tliis cliapter. The cell assembly is wiseiy rnade in two cections, one section being the

A n o t h e r close u p o f a v a r t o f W q u r ~2 , .vhouing tlte refractor mountin,q, al80 t h r short focus reflector. T h e azzmulli bearang zs a 6" tapered cone 2'h long. Coosunecks support 2%" diamrter h u b s wltteh are t h r brnrtngs in a l t ~ t u d e . T h e , ~h u b r nre tntegrai z~itli Irenry a i d ~p l n t e ~ Z L I ~ L (tn ' I I t u r n are f n v t ~ w d t o a h e a ~ ysupporttng alreve. Phe rnntn tubu slhdps through tkla clobe fitting Y L P P V P to tlie iequlred balance, and ~ , , h e l d from drtfttrty by rrwaria uf lryht preaaure f r o m t h r t=o t h u m b s e r r w s o n 1 curued, cork fueed, bravv ~ L Y ~ Y . T h e d e v ~ c ean the h u b o f the b e a n n g an altitude regulates a n znternul expanding brcike T h u mirror end of the reflector, with t h e mtrror holdlng a.s-sembly, acrews o n the ~nrrintube For a v i e w o f the ~ n s t d e sur , Figure 8 Tltr dicigonal suppolt zs integral wlth t h adnpter ~ tirbe, the whole a-vsembl~ng f r o m inside the m a t n tube into a chirck u'hirh hnlds tlt!.q n s u ~ m b l g firrrily 7 1 1 nrLli~rtirrent T ~ clanzpiny P rzng o f the chuck c a n be aeen l u s t beyond the face of the eyeptece T h e threeíled s h a f t ~ 1 2 t hthumbacreu headv on eac71 e n d , moues the u ~ h o l eeyepzece uxaembly, zncluding the dzagonai, longztzcdinall~i zn and out of focas Phe reflector m o u n t l n g has a semi unzvrrsal joint zn f r o n t , w i t h a push pul1 s y s t e n ~a t the mlrror e n 4 f o r adlzcsttng a n d lochinq Ijy looszng four t h u m b screws, t h e reflector w i t h complete m o u n t i n g can be t a h e n o f the refractor, and ay e a v i l ~ r ~ p l a c r d , wzthout Sisturbing a n y ad?uetments prevrouslv madr. Whzle thzs reflector vnay be u ~ e das a jinder, it uau not rnounted u n the refractor primartly for thnt purpnsr but maznly because the sante m o u n t i n g sprups both telescopes [ I t comes pretty close t o b e ~ n ga rtchest field telescope-aue chapter 0% thal aicb~ect-Ed ] Al1 brtght s u r f a c e s , w ~ t hthe exception o f the refractor mazn t u b e , are finished in baked.on l a c q u ~ r .

-METAL

PARTS

199

outer cell, whicli screlvs to tlie rntiin tiihc, tlie other section or inner cell, wliicti Iiolds the objective and screws iiito tlie outcr cell. See Figure 5 , Ieft and riglit. S h e t w o forms here presentecl are ;i departure from standard desigri. Tlie first form or design is adjustable and is m:rchined frorn one casting. Tliis requires making n pattcrn. TIic sicond form or design is not adjustable, and is made frorn lieary-\~allbrass tubing, tlius climinating the necessity of rnaking a patterii aiid :I c2istirig. Some professional makers prefer üdjustablc cells, ispecially for ob,jecti\res of 3" and uver. O t h i r

All d r a w t n g s b) the author

FIGCRIí

l r f l l i a n d rlrnwiny i v a n ndlttitcrbl~ oiitrr rpll, ~ 1 1 t h t h r ~pul1 ~ YC~PU~L'C l Z O T h a p a r t , t h u r qivirig [zmztud ndltirtrntnt hi, rolllng thp haines o f the cell o n t h e douhli enti rone. I n thz. rlvutgn thr < i o u i i artd prnt of thr nbjectiar ar? niounted darpct cn th? objrct rnd Iialf of t h r o r ~ t e r rrll T h t tnnrr crll ronstrurtznn siiorr~n at t h ~ i g h tzs also suttablu and drstrable t o tcse zn the othrr deazgn .VI/ addznq tliree purh s c i e w s t o t h p t h r ~ rpnll irreuvr t h r re11 1.071 be made atrnlght puslipull zn operatzon, thici niakzng t h e dotcble und con? u n necesvarv T h e doublv end corte c o n s t r u i l i o n , u~itli three piill scrptur, zn the bptter d ~ s i g n ,r h i r f l y beeauor, u z t h the doicblr r n d cone, thr loint z * rnade p r a c t t c a l l ~ ~dihst proof, thus cuttzng d o w n o n t h e nnzount of cleanzng neoessarv t u k e e p t h r objertzve zn good conditton Th6 ~ i g h hand t drawzng shou)s a n znnPr cell constructzon %n n n o n a d l u s t able outpr crll castzng. A n y combination of t h r t w o drawzngs rnay be made t o s u i t t h 119~7'7 ~ rrquirements I t tuill add tu thp appearance of the fin~shed job if the r i m of the protrcting enp zs hnurlud, also the odge o f t h e sleeve o f t h e znner cell a n d the o b ~ e c t end edge of the inner cell P

expert makers will tell you t h a t all work of centering-i.o., coordinating the optical axis of the objective with the mechanical axis of ttie tube-should he so perfectly done as t o make an adjustable cell unnecessary If cell adjustments a r e necessary when your refractor is completed, i t will be highljgratifqing if the cell is adjustable. The right hand stctional drawing shows how the cell is designed when an iiiner cell is t o be used; the other shows how the objective can be successfully mounted in the end half of thc adjustable nr outer cell, thereby

200

TIIE REFRACTOR

eliminating t h r making of aii irinrr crll. Wliile tliis latter plan shortt-ns tlie work, arid would prove satisfactory iri srrvice, i t lacks tlic coiiveiiirnceeasy aricl safe rerrioval of t h r objective for cleaning, etc.- protection, and coinpleteness of tlie first plan, whicli calls for an inner cell. IIere follow some prrictical details for making the complete adjustable cell assembly shown in botli drawings. For clarity, tlie teriiis outer arid inner cell will be used exclusively. The inner cell sliould be made first, in case the inner cell type, nt the right, is chosen. I n the lathe, chuck a piece of brass tubing of sufiicient diameter, wall thickness, and length. (Length includes desired lengtli of inner cell, length of threaded sleeve, backing harid, lcngth of tubing held in jaws of cliuck, plus, say, 3/4" for facing and four cut-offs). Use your owii design for the inner cell, o r follow the design shown. First, face the end of the tube. Increase tlie inside diameter of tlie tube t o the actual o r planned diameter of the objective (the objective may be made t o thc cell, or the reverse) to forrn one side of a collar or flange. This collar provides the front seüt for the objective. Cut a thread on the inside of this new diameter, for the sieeve. Reduce the outside diameter of the tubing, as indicated in the drawing, in order t o form the bkirt of the inner cell. Ciit the outside thread on the inner cell t o the seat previously machined; this thread, of course, screws into the outer cell. With a parting tool cut off the cell thus f a r constructed, according t o your requirements for length. Lay this cut off portion aside until the sleeve and backing band have been made from stock ivhich remains in tlie chuck. The sleeve sliould he made first. Reface the tubing a t the cut-off and knurl the edge of the end before reducing the diameter and threading. The machining of the band is obvious. The fit of the band in the cell should be the same a s for the objective, Le., so t h a t i t will just drop in without forcing-loose but not "sloppy." After removing tlie tubiiig from the chuck, chuck a piece of cast iron or brass. This is t o be mactiiiied and threaded as a mandrel on wl~ich the inner thread of the inner cell di11 screw. Screw the inner cell on the rnandrel a n d finish the front or objective end of the inner cell. The Outer C e l l : I t is assumed t h a t the casting for the outer cell is now on hand, having been made froni a pattern, a s shown in Figures 2, 5, 6 a t No. 1, and 8 a t No. 2. The one-piece casting for a two-piece outer cell is firmly chucked and the rough surfaces "hogged" t o approximate dimensions. Bore t o requirement the inside diameter of the casting and cut the thread for the outside thread of the inner cell. Next, increase the inside diameter on the outside end of the casting, forward a s required (see drawing) t o a seat, for the seat ori tlie outside diameter of the inner cell. This new inside diameter on tlie casting should be '/y'' larger than the largest outside diameter of tlie inner cell. The seat should be deep enough, or f a r enough from the end, t o permit the inner cell t o be buried and leave a t least y!" on the inside end of the casting, on which t o cut a thread for a protecting cap o r

-METAL

PARTS

201

cover. Machine the outside of the casting t o the required dimensions and firiish it. Drilling holes in the flangr nf the casting for the adjusting screws is wisely the next step. Do this drilling on the dril1 press, removing from the lathe the chuck holding the casting. On the finished front face of the flange prickpunch the lncation of the drilling centers for the acljusting screws. A t this point you can choose between the oiiter cell design shown in Figu r e 5, and a push-pul1 design. The push-pul1 will require 6 screws, lorated in pairs--one push, one pull-around the flange, 120" apart, and will not require the separating band with cone end bearings (Figure 5, a t left).

FIGURE6 T h e v n r i o u ~ patterna u s e d i n m a k i n g t h e r e f r a c t o r s h o w n i n Figure 1 . F r o m left to n g l r t . h o . 1 ad]ustable ceU. Thta p a t t e r n separates between the u p p e r a n d mtddle r m g for'moldtno. T h e cyllnder nboar tile t o p rtnn zs a core p r ~ n t Thc bottolh cylinder i s cnit solid, f u r holdlny an the lnthr chuck. Nos 2 a n d 3 are for the mountrng h a 4 in a m n < i d ] t ~ r t a b erll l ~ w i t h core prznt o n t o p and solid c y l ~ n d e ra t bottom, f o r holda~ry 2n thr chuclc N o 5 zs fov the control end asmemblg. T h e core should r u n throunh t h e c a s t m g . N o . 6 ts a dtxk f o r the protecting cap

If the adjustable cell design is adopted, only three pull screws need be used. They should be located 1200 apart on the face of the flange. No. 8 round-head brass screws are a good s i ~ eto use. Remember to witness-mark the flange of each half of the outer cell; also that the tapped holes for the push screws should go through the front half but not into the second half of the cell; that, after threading the holes for the pull screws in the casting, the latter is divided into two parts; and t h a t the pull holes with a thread, in the front half of the cell, should be enlarged in order t o allow the screw t o slip through the hole. Replace tlie chuck, with the casting, in the lathe. With a parting tool dividc the casting into two, parts, by cutting transversely through the flange from the circumference toward the center, leaving %" of flange thickness or leugth for each part. Face the half o€ the casting which reinains in the chuck, and bore and thread it t o take the thread on the No. 1 end of the

202

THE REFRACTOR

main tube, using the No. 1 ring cut from the No. 1 end of the main tube as a saniple or tritil threacl. Macliine tlie outsitle entl of inside di:irrieter with a 4.50 taper, as per Figure 5, at left. Cut off from stock in cliuck. From a suitable piece of stock put in the chuck, a mandrel is made, on which to screw the front end of tlie front half of the cell. After the front half of the cell is mounted on tlie ~riaridrel, face the flange aiicl cut tlie 45" taper for tlie double-end cone. Machining the short band or doiihle end cone with 45. taper on each end, from a piece of brass tubing, completes the niiter cell. Cell A s . ~ s m b l yiifade f r o m Henvy-wdl Tubing: The procedure in making a double cell (double: meaning outer and inner cell-altliough in this example the inner cell is not completely buried or encased in tlie outer cell) is practically the same as for the adjustable cell already described, except t h a t thrre are no patterns to make, no castings to get, no flanges to divide, no adjusting screws t o locate. After the inner cell is complete and cut loose from the tubing in the chuck, as described above, and the threaded sleeve and backing band or ring made, i t will become obvious that the tubing in the chuck is to be threaded t o take the inner cell. When this thread is complete, screw the inner cell into place and finish the field end, and machine the outside surface of both cells. A t the joint of the two cells i t will be desirable t o cut a shallow V, as a finishing touch, also t o indicate tlie line of separation. To cut tlie thread on the outer cell which screws on the main tube, the outer cell is mounted on the mandrel described under "adjustable cell." The objective, when not in use, should have some form of protective cap or cover. A brass disk with a knurled edge, whicli screws into tlie field end of the ccll, is most complete and impressive and therefore tlie most desirabre. To make this cap is not, in any sense a problem, except in securely holding the stock in the lathe for machining. Here is a wap to use the chuck fnr holding, which will probably prove successful if you do not attempt to take heavy cuts-in other words, if you will nurse the work along. Chuck a llh iron " pipe coupling, face the end and remove it from the chuck. Chuck a disk of brass of suitable size, either cast or Iieavy plate, face a n area of a size niore than equal t o the end of the pipe and remove i t from the lathe. Solder the faced end of the pipe to the machined surface of the disk, centering them as best you can. Return the iron coupling t o the chuck and machine a collar of suitable diameter on the brass disk. This collar should be not more thari 1/4" long, with a wall tliickness of I/s". 0 1 1 tlie outside of this collar cut a thread which will screw into the thread in the field end of the outer cell. Knurl the rim of thc brass disk. Machine the chuck side or face of the brass disk, leaving the knurled rim %" long or thick (however you view it) and cut the disk from the coupling. Support the disk in the chuck by expanding tlie jaws of the chuck agaiiist the inside wall u i tlie collar. Daub a little pitch paste (pitch cut in acetone) on the jaws of the chuck, in order to kecp thc disk from slipping. Face what becomes thr outside surface of the disk or cap, to a nice finish.

204

THE REFRACTOR

refractor. It may riot be an exiigperation to add t h a t the new design simplifies makirig a rack and pinion control, while offering a superior device. Let us s t a r t a practica1 consideration of tliis new control by giving each major p a r t a name, with n rough pictiire of how the parts assemble. See Figure 7. The shield is fastened to the main tube by means of four screws with nuts. The barrel extends throrigh a hole in the shield, and is fastened to the shíeld, preferably by "sweating." The sleeve which carries the Iialf-riioon-shaped piece which supports the rack, slides into the barrel, and is locked in place witli a set screw in the wall of the barrel. The cylinder carries the eccentric pinion shaft, piniori, and finger wheel, and slides into the sleeve, being lockecl in place with a set screw in the wall SlDE

INSIDE

'STUFFING BOX

FIG~RE

7

Detaib uf thr: focus control.

of the barrel. This screw passes through a n enlarged hole (for adjusting the sleeve with the rack) in the sleeve t o hite on the cylinder. A 2" length of rack is fastened t o the focusing tube (not the draw tube, which slides through the focusing tube) preferably with a neat job in "sweating." Brass rod, round stock of suitable diameter-1%'' or more-should be used from which to make the barrel, sleeve, cylinder, and finger wheel. Study the sectional drawing in Figure 7 until you have a clear mental picture of just what t o do, and how to proceed. Make additional drawings, with dimensions. First, machine the barrel. Bore and niachine the sleeve (note the flange on the sleeve, the rim of which should be knurled). Machine the outside

-METAL

PARTS

205

diameter of the sleeve from right tn left, up to tlie flange. F i t tlie sleeve to the barre1 by trying the harrcl on the sleeve. The cylinder is rnachined in tlie s:iirie rn:rnner as tlie sleeve, except for boring. The flange on the cylinder shoiild be the same size as the flange on the sleeve, and ttie rim sliould he knurled. F i t tlie cyliricler to tlie sleeve by tryiiig the sleeve nn tlie cylinder. Later, wlicn tlie chuck is free (nothing in i t ) , carefully chuck the cylinder and finisli wliat hecomcs its outside face or end. Drilling the eccentric Iiole ir1 tlie cylindcr is preferably done ir1 tlie lathe, by chucking tlie cylinder in the four-jaw independent chuck. Machine the finger wheel to any drsired design, and knurl the rim. T o locnte the rack on the tube for sweating, scratch a line on the tube, squaring i t witli the end of the tube (use a t r y square), or put the tubc in the lattie and scratch a line on i t with a pointed tool in the ton1 post, by sliding the eompound slide rest from right to left. Carefully clamp the section of rack to the tube a t this scratched line, and "sweat," Arst tiiining the surfaces. (Note: Supporting tubes or tubing in a lathe may be greatly sirnplified by using a revolving cone tail center. The cone should have the standard 60" taper, and be a t least 3" in diameter a t the base of the cone. This size gives a wide range in usefulness. Such a revolving tail center is a n important lathe accessory and should be made ball-bearing.) W e will again take u p the sleeve, in order to complete it. The sleeve carries a half-moon-shaped section of a brass disk (see Jl, Figure 7 ) on whieh the rack rides, made as fnllows. Machine a hrass disk to the outside diameter of the sleeve (slightly smaller), with ;L width or length equal t o the width of the rack, plus lh2". Machirie a sliould~ron the disk, letting the disk fit into the sleeve 1/32" (tlie two pieees will later be joined a t this shoulder by "sweatjng"). With a hacksaw cut the desired sectioh from the disk, finishing this cut on the section, in the lathe, by facing in a twojaw chuck. On this finished face, each side of center, (length) prickpunch for two 3/ltit' holes. Dril1 and t a p them. Turn two small corks (bottle stoppers) into cylinders whicli will screw into the X6'' holes. To cut the revolving cork use sandpaper. Fastcn a small cork bottle stopper t o a piece of j/," round brass rod, with hot pitch or seatling wax, and put the brass rod with the cork attached into the lathe chuck for turning down the cork. "Sweat" thc half-moon-shaped segment or section to the sleeve. Screw the cork cylinders into the holes and, with a razor blade, cut off the corks to within 1/S2" of the face of the segment, according to your requirernent-in other words, so that the rack will ride flat and square on the corks. The shield, when finished and mounted, should be about 2" square-curved, of course, to fit the main tube. Make it of annealed sheet brass 1/1,3'' thick, accurately formed to the main tube, o r froni a section of brass tube of suitable dimensions. This should be machined on the inside for fit with the main tube, and finished on the outside to harmonize with the remainder of the job. This latter riiethod is a definite procedure and takes, probably, no more time to make than bending sheet brass into a clase fitting shield, though

it iiisures it snperior r(-si~lt. Siip1)ort t l ~ ihIii(-l(l in tlit* t\vo-.j;t\v cliiick, in or(1t.r t o boi-c. tlie Iiolc fur ;i ~ u fitg mitli tlie btirrcl. Prick1)iiricli tlii four coriicrs of tlir sliiel:lrtitioii oii tlie celt.sti:il splierc is ciie secoiitl of ;I rc. I t 1ii;i)' tic iiirritioiie(I ~):irciitlirtic.aIly tli;~t ; i i i iiri:igcs of a fiiiite otjjec.1 siil~trriclstlie s;iiiic ;ingle ; ~ tlic t fociis ;is tlie ol>.iect. I'or r~iiiii1)le,tlic. iiiooii subteiids aii :iiiglr of ii~)l)roxiriiotc~Ip 30'. l'lie size, tlierefi)rr, o f tlir irii;ige V)

W

r

tlORIZONTAL

Ctt

KJ -I-I-

-J

a ORH

Al1 drnwings a i i d yhotogrul>lih 11). tlie ;iutliol.

Fic;ri;i
/ii i i n i n ydialnrter.? of din),lrr,lp)i! s f o l i . jocll.silig fui>?, n n < í , h . ~ znf ~ j ~ r i ~ r n . I n t l i a / P .tha 4 " I ~ I I P 1.1')1r?~rnt.sf h r o h j ~ ~ l i ~I I . I rI ~ ..id flrp i ~ t i n g u s i z ~ . n!, r>~cn.suririp n l o n g t h a hortznritnl n.ris l o f h r lot'ations of t h r i S n r i o ! i s i t c m s , r l i n n i c t r ~ . ~rin!j ~ hu s p l p ~ t p d tvkirIi i,,ill ) I ~ S Sn / / t l , ~light jrrirrr t h r o I > j ~ < . t i !i,hrlz r~ JO~LIR OIP I a ~) % object t k e S I Z C o f t l i ~rnoo~z. S P C tr.rt jor d e t ~ r r n i ~ i ~i r~r (i«yy ~dizr.

of the moon is determined hy tlie foca1 Iength of tlie telescope, and is independent of the ilinrrieter of the oI>jecti\e. The s i ~ eof tlie iiiiage is important in reflector.; wheri selectina a pri51n « f proper size, and in refractors when determiiiinp tlie bize of holeb in cliaplirapms arid eye tuhes. S o take a concrete example, consider a 4" telescope of 60" focal lenptli. The tanpent of 30 minutes is approximately .009, so that .O09 x 6 0 " ~ . 5 4 " , the diameter of the imape. If a ~ c a l clayout of the qyqtem be drawn, with the lens diameter and imape size full scale and tlie focal Iriipth 1/12 scalr, and lines a r e drawn connerting ttie extremilies of tlir lines representing the lens and iriiagr, as sliown i i i Figure 1, t h - riiinimiirn iliametrr of iliaphr,igms and

tiiI)~-\1ri:iy 1 ) ~grapliic;illy (1t.tcrtiiiiii~il. .\ir c~>-t~l)it~c(~ ttil~c,prihrii 11iagori;iI. ;ind ~li:il~lir;igni;iriiii ;ciid the 10" refr;ictor a t tlie Fr:inklin Institiitr a t Pliiladrl~iliia. Tlie seeiiig \vas poor, whicli seriously handic:ipl>ecl tlie larger nperture but, crrii ;illowing for tliis, the perforrniince of the refractor sliowed a marked sup F ( n f - n c ) is used. Also tlie foca1 lerigth Eor tlie iiidiridual lensrí .ire not determinrd-ttioiigh they may be, if desirid. using the usual formulae. F o r tlie w k e of cornp:~risoii tlie saine exnmple appears below, worked by ;he conventioridl method.

Check by reciprocals. Arid, by tlie way, dori't neglect giving your figures a check. It i\ moch easier to cliange a few figiires th:in to regrind ;L finished

It.iis wliilr :ittciitioii is ~i;iidto tlie rri:itter, il is r:i\icr to roiiplr oiit tlie 1:ij). rougli grin(1 tlic leiis, ;iii(l I~ririgtlie l:ip h:~ck to :iccur:itc r:itliiis oii the Iaitlir for tlir fiiica griiidiiig o~ii~r:ttiiiiis.'I'liis h;is thc nd:will of riecessity be separatrly crnterc(l. Iri eitlier cverit tlie process is t h e sanie. Tlie lathe

(loes iiot 1ciicI itseif t o cciitcriii~. ; i \ too iiiiich ski11 is rrqiiiretl to 1n:iint:iin tlit. liitc-li :ct tlic. prol>rr t~~iiilic~r:itiirt~ ti) pcrriiit tlic. le115 to I)c sliiftr(l oii its sI>iricllc : i i i t I yet iiot 1t.t it slitl(x tlo~viirvlit.ri relr:ise >/i2'', wliicli riiakes hiplier Iiowrrs obtuiriable. 'i'lic ;ipf);irc.nt fielcl of ric\br is ;tboiit ec1ii:il t o tlie stop di;irrieter S- seen

x-

:it tlir (.y(. tlistiiiict.. Tlie tlinriirter of tlie cycpic.cr .;lioulci he eqiial t o tlie stop (liarrietrr ~ i l u stlie exit pupil diameter. Tlie err.ctor lenses sliould be f:\/f, tirries tlir cxit 11iipil di:trnrter, :ifter ;illo\ring a rirn for rnoiiriting. J f tliey :are sep:ir:rted somcwhat, make them a trifle larger anrl put a stop of t1i:it c1i:iriietc.r I)ctwcxeri. ~ h e ' d i : i n i c t c r of tlie stop SI is riow f 2 / f 3 time8 tliat of S2. The objectire diaineter i\ giren by

The first part of the cxpressioii gires the "effective diameter" mentioned ahove, wliile tlie sc.cond p s r t is thnt wliich is ;iddcd for the disp1:icrnient of the cones of r:iys frorii ohjects :it tlic edge of the fieki, :is sliown in Figure 2. Tlie widtli of trny fieltl w, :it a dist:ince D ( i . c . , t o :i tnrget), is w= ( S 2 / F ) D , to a close approxirri:itiori. Ciirratiire of ficld dcpends directly on the focal powcrs of al1 the Ienses :tdded togetlier, hence very 5hort focus lerises are t o he rigorously aroided in any desipn. Figure 2 indic:ites Iiow the lensrs should be turnecl, and also sho~vsthe ~ x i t h sof liglit rays from an ob,ject a t thc edge of the field. Figure 3 gires the coristruction of e;~cli lvns. Tlie liatclied component represents ordinary Aint glnss of rcfrnctire iridex 1.615, antl dispersion 36.6. The conrex lenses may he rn:i(le frorri horosilicate crown pl:iss of refrnctive index 1.917 íind dispersion G4.5, or of b:irium crown harinp an index of refr:iction 1.976 and dispersion 57.3. TIie r:i
is n f n r inorc iniport:int coiisi(1cr:itioii tlian tlii. first. Since tlir second piirt of tlir forriiiila giren for tlir ob,jcctire (1i;rrneter ,j~ct SL'CIIIS i i hit ~rristyt o t l i ~riflriiiriii, prrli:ilis tliis iric.:ins inrrvly t1i:it \ \ e hlii~iiltl turii t o Cli:ipter S I , 1';irt 1, "h.'S.hl.," ori "1M:iking Eycl>ic.jt.ct-i~trtl I)y St;,iieo nistril)iitors, Tlo~\-t.\t.r,Sii,jol, ; i i i "iriterri:il Iirliric:int," I C . 1 i r v . S Y , , S. . 1)riiig \v:iter \vliite ;iii(l r;isily ot)t;tiri:rl,le Iojectiori tlitit tliv clispersioii of tlic liquids mny not be cclii~il t o tli;it of tlie vrowii ,gl;iss. Of ctoiir\c, tcsstiiig iri soject suggest checking the collimation by racliing out tkie cyepiecr and observing whetlier or not the out-of-fociis images a r e circul:rr. If this is done, ti moderately Iiigti-powrr cyepiece slioulort:irit to pirforriiiiiice, Iio\vevcr, arid p:iiiis will be fiilly rel~tiicl ir1 l>(.tti.r (lefiriitioii. Tlie follo~ririg rnetlioossihIc : ~ tlie t :ingle slio\i,n, and is 1)laced tliere only to facilitate tlie driiwing and permit visualiziiig t h e ronstruction triangle J ? ( ' S .A t tlie point where this relationsliip is ap~>licnt>le, iiariiely, wliere the decliriation axis is nearly Iiorizontal, tliis tri:inglr nenrly vanishes. Strictly, i t cari haie no arca except whcn tlie declination nxis is sliglitly :trv:iy from the n tlie two from horizontal nnd the opticnl axis vertical due to any d c r i ~ ~ l i oof pcrpcndirularity, arid this is ~v1i:lt we wisli to investigate. Draw tlie liorizontal P E to lie in :I vertical pl:iiie tliroiigh P,4, thus malring tlie aiiple Ef'A tlic angle of elev:rtion of tlie polar :rxis. Tliis plane will also ciit tlie circle clrscribed by tlie declinntion axis :it come di:iriieter J~.11. Tlie linc Alr(l. Nrit1it.r a Iiigli sl)e(xd fol. tlie liitlie nor tlir prindirig \vlirtal is dcsirnhle; 20 r.l).ni. for tlic lt~tlie:iii r o g r e i son tlir second circle t o be pr:tdii:itecl. TIiis tirne. Iiowever, tlir,)irgIi 1;ick of coiicentr:ition. nii extra in;irk >vas insrrtecl wlierc it n:ii iiot rccluirerl, ;irid the error wns not disc«vrrr*d iiritil tlie groc1u:ition w:is :ilmost cornplcte. Then to our clingriii, tve foiiiise di,sr.ril>e~l7,!/ I'orter, irt ".I.T.M.," c h a ( ~ t v ro11 " D c , s i y r ~o j . l I o ~ ~ ~ z t i z ~ yich~zre ~ , ' ' l«rgr cross-s,>rtion iri tlip ~r.i'i.ss h n jts i s ?cryrtl.

tion of speeds. A train of penrs can b r rnsily computed, mhicli \vil1 turn the polar nsis orie revolution p r r sidereal d:iy. \Vorrn ge:irs m-ere used iii tliis drire, because of tlieir siiprrior siiiui>tliiicss. Tlie ly. Iii tlivs(. o~uil;irity, nn old l)lioii«gr:il)li iiiiitor iri;iy 1)e 1i;td ;~lriiostfor tlie tisking, :irid its niarvcloiis smootliness c~ritlhilvnt op-riitioii tire ti \voriilerful s:itisf;ictioii, riot tu rnention the fun of ;idal>ting it to your iristriiiiieiit.- 1.708 I'cirX-c.r .lz.cjri trr. E . 11. . I l c ( ' ( ~ r t r t ~ ~~ ! /~, l i ~X.~ J..: ~ o l'rhe ~ ~"11e111~~ste:tc~ , Ilyclr~irit" iiioi~iiting

STOP ANO STA

sliolvn in Figure :í 1i:is a phonopr;ipIi iiiotor clrive, with tlie motor turned iip ori its side ir1 orcler t o e x p o e tlie geors nncl to t~riiigtlie poveriior hhaft vertic;tl. Iri tliis positinn tlie niotor is alrno\t ex:ictly :in astronomical clriving clocly acldirip ;I sliut-off lever :iiid adjiistirig x r e m tlie speed c;in be very closely repiil;itcd. Rliiie runs for 50 minutes ;it 40 perccnt of its n»rm;rl speed, wliicli g i r t thc drive shaft 2 r.p.m. First reduction worm nnd gear a r e 14 t o 1 and polar nxis worin gear lins 208 teeth with single thrend worrii. 'i'liis ])ro(liices une revoliition of the pol:~r nxis in 1135 minutes, ur n ininute off per siclereal rlay, No vibration is ay11):ireriL fruiri tlie rnotor, ereri when using magnificntion of 42.5 tiriies, olthoupli tlie motor is bolted t o the base and no dampirig pntls are used. Xlso, the rnotor can be wound without disturhing the field of view of a ?$" eyepiece. 1 have not tried pliotography I>ut for visual work 1 can't see where a mucli better outfit can be liad a t any reaso~inbleprice. 1 paid :i dul1:ir for the motor and a dollar for the differ-

ential carrier ari ~ u t r ~ ~ s c o~ j. i p t tliu c1irilitui.v 1111 P o r f e r wrrs Iurrt / o I'ii-son nurr,rn! 7no111lis 7n nrir.ali~.p o ) 1iub1ir.n( i o n . D t < v i n g t h c ? ~ t i , l r l l?11orrf11~ ~ o f 1 9 3 6 F ~ ~ S O1111ilt I I l l ~ r) l f f t t ? r ~ r sl, n r r d ~t h e rrrhtzngs, ~riol o r l e t ~ ( , f , b r / u r c ) ~ u l ) l i i ~ n l i r i ri ~~ i « r . ~ i D ! rrll r , r r l o r s IIirrt 1rii!lb1 bu hiilrl1, X r . P o r t ~ ri n Iir!ji!st 1 9 3 6 , ?r'h~rrW P i n s l ~ ~ r ~ l t 't rhle fi~ir.\liid >rioi~ntirigu t B i l o r i . "diys ni111 .~r>irrlil~rcrts,fittirrg nll t o . q f , f h ~ nnd r puttirig t h tele.sc othrr applicstions of it whicli the aiiiateur c a n and iio douht a i l l use in ionnrction wilh telrs of ~miiit (Figiirr l ) , so tlitlt tlie core will e:isily firiC. Slicc oii' ;I l)it.cr, I;, Figiire 1, o11 tlie lower edge of pririt B. 'i'hi, must be ~ l u l ~ i i c : i ~iri < i ltlic corr bnx, :tiid is dorir in ortler to irisurc tliat tlie core

will hc placed i i i the molcl in orie position :il»iie. Be ciirefiil ~vlirn placing tlie tlowels l.', 1,'. One w:iy to locate tliern is t o drive a brad tlie siee of a pin wliere yoii w:iiit tlie doxvel t o be, and ciit off its Iira(l so tli:it tlie hrad protriidrs I/icf'.l'lien squccfic the two hnlres of the pattern togetlier. This \vil1 lriivc aii impre5sion on tlie Ii:ilf opposite the brad. Tlieri rerriore the br;id irnd the indent:rtioris will indicate irhere tlic dowel lioles are t o be bored. Gire the wood two coats of sliellnc-bl:ick (add lamp black) for the surfaccs representirig the castihg and either red (add red ocher) or yellow for the core prints. In the same way tliat i t pays tlie mirror rnaker t o givc good ineasurr t o hiq fine grinding, so is i t worth wliile t o give your pattern as smooth a finíshed surface as pow'ble. How many times h:ive 1 watchetI :ir, oId nioltler, w h ~ ngiven a new pattcrri with deep undercuts, run his finger dowii iiito the hule tu see mliellier it i5 siiior>tli or lihely to tear the sand on drawing. A good final check to the patterri i i to lay each half on the work bencli (Figure

A ) nnil go nll :iroiiiid it witli

:i try-sqi~are,t o rn:tkc siirc t h i r c i rverywhere sufficiciit (lrtift f o r clc;iii ~1r:iwirig. nllt oiic core box is slio~vnIirre-tlint for the core tlint goes witli priiit n, Figiirc 1. Tlie box, (Figure B ) , iiiiist be so constructed, a s alre:il;iincably be ruinecl. Skctcli -4, Figure 2, illustrates the best tool knomn to me for lifting the criicik~lefrum tlie furntice. I t ~ri;iy\)e 1iecess;iry to liavr it m:rdc :it the blaclísmitli's sliop. Slietcli 11 is a bail-like tool wliich rntiy be uscd instriiíl. It is oper:rted hy nie;in\ of a ring ~vliich Iro!ds tlie ends togetlier biit xvhich, wheri slicl upm:ir~ii with a large nail in position to pusli in ;rt ;L 1il;irc wlirre sand is likely t o fall off. So~netimesnails rnay tw put ir1 wlien putting iii tlie s:incl, biit tlirre is tlien a chance of hitting thrni mitli tlic r:ini, ~vliicli\vciulcl tencl t o loosen tlie sand or cause ;i crncl< in it. W'licn re:idy tu put tlie cope back on tlie dr:ig, use tlie same or niore care a s iii lifting it off. I t will he well Lo have :i man watch the under sicle of tlie cope as it is being lowered into tlie drap, as long as it can be seen, in order t o see wliether ;iriy s;ind ftills off. Tliere is no use of pouring a jub if t h r niolcl is no good. l3r si1i.r tlie aluniinum is hot. I t should be fairly hright red wlien sometliinp is Iield over tlie pot to shacle it if outsidc in dapliglit. Poiir withoiit ariy licsitatioii iintil metal riins oiit of tlie risers [comrnent by Ferson: "Riplit-if yo11 stop, tlicre is Hell t o pap"]. Wearing poggles is a piiod idea. 1.e;ive ;i c;isting of tliii hize--5O pourids-in the sarid a t least lialf

aii hour, \vtiile :iii Iioirr \vil1 11c Iitattcr. I.c.;iviiig tlie ositlc tli:it forrii. on tlir ;iliiriiiriiiiri ~iiitil rt.:i(ly l o 11iiiir rii:iy 1)t. wt.11, :is :il~iir~irit~ii~ o~i o ~ itwo t ~vcel<s;iflcr tlic. rrci.11liori

of tlie p1;iis.; it x ; i \ liroiiglit to s l i : ~ ~:I) ~c-oiiil):trati\.i~ly , slii~rtl i i i i c * ioinli:irctl t o tlic trdioii:, biit iritc.rchtirip iiioiitiis trf I;ilior it tcirih t o iigiiie tlir hiirf:ice. SIir fiii~ilhiirfacr of a rcflrctiiig trlciscopr riiirror is i i i tlic f'oriri of :i pttriiholil, t o ~vliicli tlir rcluiition iii C';irtesiiiri gccirrietr)- ih, ,112 = 27t.t.. Tlie foca1 leiigtli of the rriirror is I?/2,>. Iri Fipurc 2, !I iis tlir <list:iiice of :triy poiiit, sucli as I', froiri tlie nxis 211); aiid .r, tlie delitli of tlie mirror below the cliorcl ,joinirig t ~ v o~>«iiits011 llie surface, r:irli : ~ t:I dist:ince !/ from tlic ;tsis. 111tlir I ~ r t ~ s r ncase, t tlie focnl lrngtli ~v:isto l;e 125" t1i:it tlir rq1i:itioii t o tlir final parahola ~vriulcl he y' = ;500.)*. Tlie depth of tlie c u r r e a t various (libtünces frorn the ceriter is rcadily coniputed t o I I thot ~ sliown iii Toble 1.

TABLE 1 Distance from axis 1" 2" 3" 4" 5"

Depth

.002" .008" .018" .032" .050"

Distanee from axis 6" 7" 8"

9" 9.5"

Depth

.072" .098" ,128" .162" .180"

'I'li(. 1)egiiirier m:iy iiot re:ilizr Irow 11c;irly tlie siirf;rcc ;i~i~irorirri:itest o a sl)lirrt. or, \vli:it :iriioiirits t o tli,i Ihr ( . P J I I P ~o j f h (~l i s k . 1 , l i o l i , ~ h i r l ~jro>ti / I l i ~C C I I ~ , .11, ~. /I(I/~ ishirig jrorr~tliu c t l q ~ . I I I , ) ~ o l i x I i i ~ isqo n i p frorii t l i ~c?>itr,rrrtitl .sorrz~fiorri f h i J ~(igu. progresses \re test the r:idiiis o f c i ~ r r n t u r efrequrntly for t h e rarious zones. I t is neccssary t o be uble to interpret these metisures in terrns of t h e hhape of t h e mirror. One c6in rea0rtioii;illy t o tlie cli\t;incc of tlie st:ir frorii tlie criitcr, :irid :ilso t o tlie square of llic r:rti« of tlic. t1i:iriietcr « f tlrc riiirror t o its focrsidr. C,rliforlrin, « > i d nttrichprl tn tlip t u b e o i his 15" Canr,,yrni~iiari. I t hns a 9" wiirrór of 19" f o < ~ i Ipngtli, l alid n 9'' eorreeting plnte s t o p p ~ ecificatioris for tliis c;iriirr:i were tli;rt tlie iiiirror was t o be of Pyrex, 122 riirn cle;ir >il>ertiire;ind 180 r i i i i i focrriition, tliie to tlie corrccting leris, wliicli vzirirs witli tlie uteil frorii -

wliere k is tlic r:itio of Ieni :ilicrtiiri t o F. IIo~vevcr. it is rit.:irIy :tlw:iys closr eriougli tri ri~iiot!l)ic >.?(Ir U . T ~ P Iiollsilig. ? ~ ~ i l l i . . r ? i dctit s ol? < < I ; < I Ii~ni.!j s b r ~ f t i i t g br.ri>iicti's,

camern

top t o hottciiii o f tlie Icris, vithoiit Iinviiig t o use too long a slit. A slit 3/," ir1 lerigtli slioiilr:1ss tlii siirric sizc s c r ~ ~ v e irito < l it witli e o ~ i ~ i t c r s ~ i ~ i k S C ~ P W S , tlie holrs in tlic br;rss being "p:iss size" ;ind the tapped lioles heing in tlir Bakelite. The screu7s rnust not wach tlirough ;ind toricli the :irrri itself, for tlie porpose of Iliis construction is to insulate the hr:iss strip. A Iiole is tlieii drilled :tnd t;ippt.d iicar ttie iippcr end of tlie strip, for ;i screw arid \v;islicr tlint ~ r i l ls e r r r ;is a bincling post for one of the electrir wirrs. A t tlie lower eiicl o f tlie s;ime strip, drill :r liole ;ind rivet into it i i p1;itinum or silver contnct pcint. Tlie latter ~netzilis tlie cheal~ernnd, since its oxide is :r con(1uctor of electricity, it serves s:~tisfactorily. I t s dianieter slioiild be a1>out Yq". Tlie steel roller is turned from a piece nf %" round cold-rolled steel rod, to the (limensions sliowii iii Figure 7. The surfnce of the rollcr should he highlj- polislied with crocus clotli and oil t o minimize friction. T ~ Cozrnt P T l i h ~ a4.sspmbT!/: ~l The niouiiting r i f the criunt .»,heel :issembly is probably the iiiost difficult step in thc wliolc. construction, because two pairs of b ~ i i r i n gholes must be locoted witli cxtreine :iccur:icy. First, clrill :inri t a p ;I hole in tlie ccriter of tlie tallest boss, for an 8-33 screw. Tlien dril1 ir liolc for tlie screw, pass size, in the V-sh:iped benring plate. Notv 1oc:ite the be:iring lioles with a p;iir »f diviclers, 2jV1C>'':ipnrt on tlie plate, and drill tlieni t o size. Fasten tlie bearing pl;itr in position nncl place the b:ick-plate oii tlie rlrill prrss tahlc, witli one be;iririg liole esnctly iint1í.r the center of tlir spindlc. Tliis can be deterrnined by placing the drill in tbe rliiick nnd Ioweriiig tlie spindle ~vitlitlir fred arrn, iis rspl;iiiied prcvioiisly. I t is also necess:iry t o see tli;it tlie lxick-1)l:ite is clamprrl level on tlie tilble, so t h a t the beariiig plate mil1 be drillecl squarely with it. Tliis can be irisurecl by testing tlie position of the b:ick-platc by means of a level :ind tlien cbecking i t by placing thc leve1 on tlie bearing plate. If a aensitive rnarliinist's leve1 is used for this test tlie bearing will be p:irnllel ~vitli tlie base-plate. If the boss has been machined with a planer or shaper, the test for parallelisni is uiiiiicrssary, of course, biit tlie lati ti must nerertlieless he level. With tlie plate securely clamped in position and a rather stiff drill in the chuck, tlie position of the liole for thc bearing in the base-plate can be found. Tlien drill and t n p it for an adjiistahle bearing made from a 6-32 screw. Tlie otlier bearing, which will hold the pivot for the gravity arm catch, should be loc;rted, drilled and tapped by a similar procedurc. The count whcel and its bearing should next be assembled. Turn the

spindle t o the size given iri Figure 8, a t the lower right, and cut it to length. l'heri tiirn u p turo srn:ill hr:lss husliiiigs, :~ccorec:iuse orie cannot calculate the orhit of n doilble star reliably until i t ha\ 1)eeri ohserved over tlie greater part of n revolutitrn. UTitli t i ccntury of obstrration arail:ible for tlre easier pairs, and less th:in hnlf :is much f o r the more difficult objccti;, only pairs witli periocls Irss t1i;iri about 200 years in the first case, and 80 or 100 in the seroiid, are 4-et available. Now the stars are inucli 1110re alike in mass tliari in anything else-which means tliat :t period of XO or 100 years corresponds t o a distance comparable witli t h a t of S e p t u n e frorri tlie sun. To be resolved telescopically sucli a pair must be witliin three or four Iiundred light years-whicli is rnucli nearer tliaii the niajority of the stars of the eiglith and nintli magnitudes. Our list of orbits is tliercfore a selected lis1 of stars nearer than the average. The fclv systems ~vitlilarge cipparent orbits and e:isily observable with srriall telescopes have, without exception, large parallaxes and are nmong the nearest stars. Suppose, then, tliat a. modern observer starts out to Iiiint for double stars whicli a r e likely to be in rapid rnotion, and add t o our lists of reliable orbits and u.ell determined masses, during tlie lifetime of tlie younger a t least of present-day worlíers. Wtiat slioiilition t o introdiice a serious error. But when objects a r e rtiosen nezir the equator, wliere thry a r e nioving rapidly in right ascension and prohahly not a t the sarne rate a s the driving clock, other rnetliods are required. This can be orercome by the addition of another movable crosshair on n plate of its own, actuated by a. screw a t the other end of the box, a t L. The procedure will now be as follows: Thc t w o crosshairs are brouglit t o bear on tkie two objects in the field of view (small drawing n, a t left) with Jl srtting on t h r left-hand htar and Il o11 the right. When tlie observer is s;itisfiel;itcsso tliat tliey will iiot interfere witli e:icli otlier as the one 11l:lte is rri«vc.otli be mate, yet tliry rriiist I I ~so netirly iii tlie salrrie ~ i l t ~ r itli:tt in sliarp focus in tlie ryrpiccr. 'I'lie eyvpircr is first focusecl on tlie crossliairs, :incl tlien tlir eritire inicroirirtrr is forll-jnr~ i ' i t l i cl~nnin!] elrctrridu rit t o l i . Dir~ctlycrhrire >S n cnst ~rlirrriin;c~ricrllo~/ bull-jrrr, nfifl i n s i d e t h i s i s rr chirrk o l i l l i mr r 7 1 1 1 1r i . i Iie siinu>npi~ a 1rn11jor tcnter rnpor, mnde of glnss ((nd coole11 u'ith !¡quid nir. Rclow t h e blise plnte is n switclrbunrd t n furilitnte c o ~ ~ n r c t i of n~ ~ thrs trrirufoirner Bboi.~the trnnsfurwzer iu a lnmw nnd l l i ~sc~ilu of t l i ~K:irid\u~i ~nriirirri nlntlrtvietrr. A 6" mirror lli~zfhas just h r ~ nnliiminizpd i n t h e glriss j r i r , i s slinwn flt t l i ~ rig11t oi tlt P v,,lvr. Thi.7 )ita r e cast \>y tliis object o11 the elsewhere aluminizeci g1:iss surface. Or, if one interposes bet~vceritlie mirror and coi1 :t baffle which is prorirled with a small liole, one gets on the mirror opposite tlie hole a "piii-hole imagc," in condensed aluminuiii, of tlie source, in form sliaped like the profilevf the evaporntion coil. The imnge exliihits a distribution in it of aluminum filrii density proportional t o the ricliiiess of corresponding parts of the coil as a vapor source. It has been shown tliat tlie ricliest source of vapor is not the drop of aluminuni itself but the tunpsteii \vire next tc the drop. Rel:ctively little clcctricnl liower js dissipatecl \c,ithiii tlie d r o p itself; therefore it is not a copious source of aluminiirn, since the drop is rnitinly hcated hy the tungsten wire on hoth sides. Apparcntly, liquid aluniirium flows out from the drop over the surface of tlie hotter tungsten wire aiid is evaporated there. Inadequate vacuuni during evaporation is frequently the cause of failure to obtain high quality first surface niirrors. Leaks which allow air to enter iiito the evacuated space and wliich a r e so siiiall tliat they a r e difficult t o find are the usual reason for fnilure t o ohtain adeqiiate vacuum. Mirrors obtained wlien such leaks esist m;iy have nn inferior reflcctivity from the beginnirig; or, a mirror mhich appenrs t o be perfect a t first mil1 cleteriorate after a time. The attainmcnt of the necessary vacuum and its maintenance during evaporation is a phase of tlie process u.liich u,ill give trouble t o beginners. The attainment of v:icuum requires tlint tlie system he tight. The maintenance of it during evaporation requires a vacuum system ~vitlihigh piimping speed. For esample: Suppose a volurne of air :it atmospheric prcssure, equivalrnt to 1/100 of the volume of a drop of water, le;il<s into a racuum in an interval of time, s:iy one second. If ahsoliite pressiire in the vacuum is 10-4 mm of mercury this gas esp:rnds about 10 million timcs t o yield a volume of approxi-

iiiiilrly oiir g:tlloii. '1'0 iii:iiiit;riri siicli :i v:it~iiiiiii:ig:iiiist tliii Io:ili rtlqiiiris a. ~iiiiiil>irig spct*cl of oiie galloii pcr ~i~ls ?s. If proper t>:~ffles:(re not proridecl t o prevent oil v:ipor frorn diffusing k>a~ekinto tlie ev:ipor:ition c1iarnl)er a film of oil may he prodiiced on clenn niirror surf:~ccshefore thcy nrc conted. Sucli filins a r e t o be avoided. Also, if n mirror is :tllowettMed purnpiiig system, it m:ip t)ecoiiie cont:iminaterl. I t is good practicc t o eraporate aluniinurn witliiri ü few rriinutes after the glom disch:irge ceases. 111 spite of al1 thesc prccnutions films mny he ohtainrd occasionally mhicli mil1 he stripped a i t h Scotcli t:ipr. Aiid filriis of :iliiniiiiurn wliich appear perfert \rlieri freslily tn;tde niay cievelop :L multitiide of tiny hlisters :ifter standing for a frm Iioiirs or c r r n da)-s. Tlirse hlisters are prcsumat~lyproduced by gas generatrc' wlien thc aluminiirri film re:tcts 1%-ithcoritariiin;ition oii the glass. In tlie rerri:rcul:~r the dereloprnent of siicli blisters is called "rneasles." In either c:ise re-co:itiiig is iriliysic'isi s l ) e c t r u i ~\vliose i iiiiiiitc 111t~cIi:iiii~:11 iiisLiiict~(lrive liiui iiito overiills iiii~clIIIIISSCS. .Joliii Stroiig is iiot ( t l i t ~ tis, riot yet-ll).bl) ; i i i iiiii;itciir tclcic»l)c. iii;iker tlioiiC'li lic11>(*s ;irt. per'riiii;illy eiitcrti~iric~il l)y tlii. prrsciit xvriter. IIt, 11;is. IIOTV=.vrr, l)tv.ii s o cloie t o Iiiisst~llI'oi,ter for so iiiiiiiy yc;irs iit "Coltecli" tli:~ttlie rliii;cteur's oiitlook 1i;is riil)l) I I I I I I > . '1'11~1rt.liiiiiri:iry ~ N I I I I ~~Iv o r k soii :i c*oiiventioii:il iiicc1inriic;il ~ ) r i r i c i l ~ laiitl c piills tlie :tir I)rt.ssiirport ti. coliiiiiri of iiinhly v;irit.niit 11/," in diamrter, so pl:iced as t o flood tlie colliriiatirig ltans of ;I siriall sliectrogr:tl)li, tlie colliriiiition altrnys bcing c:irefully checked. The rriirror wliose reflecting power TT-ast o be mrnsiired coultl be iiitrocluced betiiren tlie diffusing scrreri :ind tlie slit of tlie spectropr;ipli. iincl alternate exl,cisiirrs giren t o ;i pliotog-ritl~liicplate for clirect :in(l rcflected liglit. As ni:rnv ;is 15 to 20 rsposiires coiild he iri;iile o11 one plate. Tlie densities of tlie irnnges were rne:isiireosuresto dirrct liglit. I t w:is coriipar;iti\c.ly e:is>- to firiow-er. Tlie nietliocl m:is rniicli more coiiiplici~tcd tli:iii it iriiplit liare been liad :i tliernio-coiiple been av:iil:~ble, biit it served ver. well nnd u-e tliink tlie final r:ilues, n-liicli depenrl on the riieclns of rnany cxposures, should 1)e accurate t o one percerit or so. Column 6 of tlie t:tble s h o ~ r stlie measurecl reflecting poxrer of mirror No. 1 after :rb«iit 40 enraiicc. The following notes were niade prior t o inakiiig tlie deterrninations in columns 4 and 6 . Wlien the mirror was fresh, tlie filament of an electric light hulb could be faintly seen tlirough a 5iiiple cont. The iilamerit, whicii li;icl :t reddish tinge without ttie rnirror, upon beiiig interposecl, looked hluidi wliite tlirough the mirror. Reflected skylight looked a deep blue. The sun could he seen

tliroiipli two coats of tlie silver witliout ;iiiy irr:idi:ition nncl lookrrl (1istin:tly l>liie. Direct siiiilight tlirr)ugli ;L single coat g;ive :L hrrinll :iinouiit of irrudiatiori. I3rfore tlie metisiires iri coluriin ti w i r e intide tlie iiiirror lookctl sliglitly vellow, iincl hky Iiglit, reflecte(1 a n d direct, coilld be distingiiislietl. Tliere were one o r t w o little spots «n tlie silver surface. I t noulercciit. Coluriin 7 gives tlie nie;rsurc I O L 0~1 ~ 1 ~O ~ 1I t i ' ~stiii.c« ~t oti tr o r ~ l . IÍl i i < i i . r , ~ / o ~1,i t./,< 1 , ? s r , i . l i u i i .

sesc.rrtrt«r!y: triless your surroundings a r e such as t o make i t an irripossibility-in \vliicli case you liad better look for a more favorable location-don't build an observatory less t h a n t e n f e e t square. M:ikr it even largcr if possible; nnj-one who 1i;is constriicte(1 an obserratory of this size or sm;illcr will he t h r first to {idvise yoii ag~iinstcramping yourself for spnce. I t rcquires no niore effort to biiilcl orie of proper size tlian to make one ton srnall; it m;ikes little clifference, when you a r e on tlie nortli cncl of a scutli-hourid saw, wliether tlie p1:ink you are cutting is ten feet, or ten miles,

long-tlie work is tlie s:imr. 'I'tic sliglit ;itlditional cost of rniiterials will be rnori tlic~ri paid for by ttie extra hpace iiricl coiivriiirrice. Avoid hiiilding tlie observatory ori top of fin existiiig building, es1)eci:rlly if the lowrr one is of frariie construction. If it is nf brick or 'irinsoiiry, however, it offers a good base for the observntory, I)iit presents tlie prohlem of inst:rlling a siiit:it>lr pier. I f a t :111 possiblt. tlii «I>servatory buildiiig shoulcl he sepnrate froni al1 otliers, antl as f a r renioved as ronditio~isprrmit. Tlie base of tlie oi)scr\.;itory (nhich is t1i;it piirt friiiii tlie grouiid up to

I R 3 T h e I(-riglit O l i s c , r i ~ n l o r ~ /B. c r k ~ l u y ,Calif. T y p e B.. Lift-off s h i ~ t t r r ,'zuith h i n g ~ dz f ~ n i t bItntch. S a ~ r i e domr is shoti'n w i t h pict~rG o f Mr. W r i g h t , at t k p e ~ l do / Iiis chrcplrr 011- the Schmidt t e l c x c u p c .

the track or rollers) slioul~lpreferably he rertiingii1;ir in oiitline; this shape is easier Lo construct nnd its corners offer nmple space for desk, chairs, cliart cases, etc. Clirnatic conditioris often play nn irnportnnt p:irt in determining the shape of this base; if it is likely t u be piled higli with snow for sereral rnonths of tlic ycar, sufñrient slope must be given the projecting areas, or tliey should be eliminated cntirely, a s ir1 t h r case of a circular donie. I.1-e~therproofing:Oiie question t h a t arises in the mind as soon as an observntory is rnentionecl ic;, "\Vil1 it be wrathertight?" This shauld give little trouble. After investigating tliis pointt in connection with a t least R a m e n aniateur observatories, only otie rase was fr>iiiid in which the huilder clairried tliat water entered during rains. Purther correspondence revenled t h a t tbe slielter iii qiiestion \vas not a permanent structure and admittedly was a makeshift.

Iiollrrs: S o matter what type of ohservatory is finallp decidecl upon, use :is few ruller5 a\ po\sit)lr Lo :~cliie\etlie iiiotiori desired, whetlier it be upon ;I 5trnight or :L curvec iihout 4" iii diameter ancl not more tlian forir t o six in nurnber, for a revolvirig dome. A roll-off rciof nould requirr four t o eiglit, deperi co t of t l i i 5 i:l>\ricltn a s this. Tlie gen11 of tlie Roriclii t r \ t is suppested iii tlie following, by F»uc.iiilt: "If we wish to inspect the rnirror as :t ~rliole,by a blow of tlie 'ye, it is iiecessnry t o take for tlie ob,ject ;i piece of squarc-meshecl netting ~rhost.irnage hecomes very sensitire t o «rhrstcr, N C Z C1'01.k. ~ IZic/itlit!l iri .Ilorciclici!~s: 'I'liis w;is stroiiply iirgi-(1 l)j- Porter, i i i "l\.'r.M.,>' wlierr tlic triitlciiry ti) 1)roviiIc too 5in;ill :i scc~tiiiii ;tt tlic "l)ottleiirck"-tlie poiiit a t ~vliicli tlic eiitl of tlii. c1t~cliii:itioiiaxis joiiis tlie tube-was stressed.

FI(:I.I:E 5 Pi>lrC I I < / o/ l l i ~))011o.I I . T ~ S lflid thi, / r 1 < 0 :it>lr o f rrrrioviiig tlie ~.iitire\-isihle liglit rnlissiori sl>e(.tr~iin of thc si111 (~xc(~l)tiiig :I rliirrow IJ:ISS h:iri(I of red liglit coiiieiirigslroiiis' efTcctivr mi~mrtir 'Shr problrrri of ohtitiiiiny liipli quality cluiirtz can h e an mnoyirig one. It may best he lilccned t o tlie liorse triicling nf rarlier years, rvlierr tlir qii:ility of tlie rnerclian(1ise is rnost likely to he gorerncd hy the sarne "riiles," niiich N. Y . ; Americiiri (;e111 and Pearl Co., 6 West 48 St., S e l v York 19, N. V. Optical qiiartz is sold only in certain cryst:il size r;iiigrs hy weight in graiiis (45:I.íi grarns/lb.). SIie price incre:ises r;ipitlly mith wcight. 'I'hree ranges will be of iriterest: :UO-500 pms., 500-700 griis., iiiid 700-1000 giiis. Wliile sonie 1" squares can he scliieezed fr~irntlie first group, t h e .i00-700 yiii. rarige seerns tlie best choice. F o r llh" scliitire ~)l:ites;irid tlie 2" Iniig 1il;itex tlie 700-1000 p. crystals will he l)rst. P r i r r s per pourid viiry pre;itly ($25-$50, 1952). Ask for G r a d e 1 opticrtl quartz cryst;tls (sometimes called stones) having a usable proportion o f íiO t o 1 0 0 lirrceiit. By usahle portiuii is iiie:int a single large a r e a free frnm iill defects clete bripht covergiiip light c«ne and polarized liplit. S o u will hiire to determine usablrIiess yoursrlf. See Figure 3, left, sliowirip rarv :ind semifinished quartz. I n tlie iowrr left corner of the troy aiid ori the orieiiter plste a r e srnall a n d lnrge end

C:IIISt ~ frivt.r I I I I : I ~ ~roiiiitlf ~)l;it slioul~lhe iriourited separ;itrly, o r thern1:illy iiisu1:itcd frorri tlir testiiip unit t o ovoid anrioyirig ternperature cli:inpes iri tliis iinit. F o r tliis reason, it woiild be bctter t o rrioiint tlic light source abo\-c. aiicl hriiig tlic light dirrctly tlox\-ti tliroupli thc. test iiiiit :iiirrtroscol>e. h very small , 4 5 O prism o r mirror arranged t o covcr helf of the slit series t o fccd ti11 I I a coiii~):trison s1)ectrutii froiii a Iiytlrogrri diseh;trgc tu1)c (No. HT':3.5, Ceiitriil Scicritific Co.) irito tlie saine firld of view. 'I'he use of tliis i a ~iliiiost rsseiiti;il. Slie s;itisfactory poiver soiirre recoriiriiiii11t'k('o,i.l,f1.I C-cl:iiiil~ witli ;I %'> 11y l . ? ' ' jlrct ~ ~ l i i tf;istciic(l i. to tlic, I~ottoiiif;rc(., tlic siii;ill iilrlier fi)ot ~ r r c \ ~ i i oii ig ;i \iiiiil;ir (lislc). ('1c:iii tlic. cliiairtz ;iiitl pI:iss ~~l:itt>s t o l)c lc Iqor tlir pl:~trs dcscril)ed 1 xvoiild rccoriiiiiriid tli;it r a r h 1il;ite tliickrirss he Iicld t o :it Ir:i\t pliis o r miniis :i ten tlioiis;iiiiltli (0.0001") of tlir 1)l;ite tliiclf tlit, l,:" 1)l;itr ( l i l i ~ ~ t i ~ g ~1i )t )~ > tlir nb.sr~rlitir~ii 11:1ridsof tlic 14,;" pl;it~.(1~liotrigri11)li 11). X o t r t1i;it tlicsc ;ilisorl)tiori Ii:iii

:ivitl W'nrsli:i\v, wlio coiitril)iitc.tl to st~vc~r:il ~ili;ise\of tlir t c s t oiitliiictl. r)ririiig tcstirig \r;itcli oiit for tcrrilier;itiirc ctTcct\ ctiiiscd liy Iiiiiitllirip, particiilttrly witli tlilotlc~l" lioicsing ;ind its ol)tic:rl ilriiieiits. l'atteriis for thc Iiouhinp may be seeri iri tlie foreproiind of Fipiirr 6. N'liile t ~ l ~ i\l2l t11C fi V : I ~ I I ~ ' S pi~C'11. ~ . < ' I I S!/ 1 I I ; l ~l)t3 ~0111i t c l s i i ~ l c c t - i g r o r i i i ~ ~ l ~'l'licst. s. It.iiscs iii;iy ;i11 I)r siiiplr lilanoconvex lrnsrs fiicing :is iiidicatetl; Iiowcvrr, ;icliroiri:its iiiaiy wcll I>c ~iil)stitiiti.d iit c ; ~ i i d 1.r.rigtli c~iiiltlli T h r m c r o v g i o r t h ~ncliing r r > n i < f < , r c r n s t r « ~ ~ o ~ ~ i ri ,,l~>os pl r ~ < , rsrlr~ctrr7 , from t h ~R « r r i i t < l O h s o , . ~ n t o r !"Piririforp": ~ D~irijr ri ),iiiol>e;triiig iii Lliis geiierul are:i :!re likely to be e e n hy two «r more of t h r coiiprr;iting observirs. Tlie stirric clirectioris :is under "Plotting of Mctcors" are t o be followrd, using tlie stimc rrittps :ind d : ~ t : stieet. ~ The tirne of occurrence is Iiere of pritiie irril)ort;iricc, for it is tlie priricip;il nietiris o f idcntifying tliose meteors seen sirriulti~itrouslp frorn the several xvi»\ure 1i:rs 1)c.rri rii;i