Model 771, 75, and 76-RH190 Transits 24
neartheeyepiece(i.e.,backlightthereticle),youcanshinelightrays
outtheobjectiveend.Thismeansthatifyouilluminatethereticle
ofaninstrumentwhichisfocusedatinfinity,itprojectscollimated
(parallel)lightraysoutthefront.
Pointingyourtransit’stele‐
scopedirectly“downthe
barrel”ofaninfinity‐focused
instrument(andtherebyto‐
wardcollimatedlightrays)
willallowyoutoseeanim‐
ageoftheilluminatedreticle
oftheinstrumentintowhich
youarelooking.Bothinstru‐
ments’telescopesmustbefo‐
cusedatinfinityforthistobetrue.Ifyourtransitisbuckedintothe
othertelescope’slineofsight,youcanpreciselysuperimposeyour
transit’sreticleimageoverthereticleoftheothertelescope,andthe
linesofsightofthetwotelescopeswillbeestablishedasexactly
parallel.
Thisisanextremelyaccuratewaytotransferareferencelineestab‐
lishedbyasecondtransitorotherinstrument.
Rememberthatifthelinesofsightareparallel,itdoesn’tmeanthat
theynecessarilytraveldowntheexactsamelineinspace—ifthey
did,theywouldalsobedefinedas“collinear”.
Let’stakeacloserlookatthis.Think
aboutwalkingdownaroadona
moonlitnight.Asyouwalk,youpassa
lightedstreetlightontheroad.Asyou
passthisstreetlight,youcastashadow.
Initially,theshadowisbehindyou,but
asyoupassthestreetlight,itmoves
aroundinfrontofyou.Thelightrays
fromthestreetlightarenotallparal‐
lel—theyextendoutfromthesource
(thelampinthestreetlight)inalldirec‐
tions.Asyouwalk,you“pass
through”lightrayswhicharegoingin
differentcompassdirections,andyour
shadowmovesaroundaccordingly
(Figure3.9).
Now,afteryouarefurtherdownthe
roadawayfromthestreetlight,
younoticethatyouarecastingashadowbythemoonlight,offto
yourleft‐handside.However,itdoesnʹtmovearoundyoulikethe
shadowfromthestreetlightdid.Theshadowcastbythe
moonlightalwaysappearstobeinthesameplacewith
respectto
yourposition,regardlessofhowfaryouwalkdowntheroad.This
isbecausethemoonissofaraway,lightraysthatreachtheEarth
Anytelescopewhichis
focusedatinfinity
maybereferredtoasa
“collimator”.Anytwo
telescopeswhicharefo‐
cusedatinfinity,pointed
ateachother,andwhose
reticlesarebroughtinto
registrationaresaidtobe
“collimated”.
Figure3.8
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