Brunson 771 - Page 24

Model 771, 75, and 76-RH190 Transits 20
Oneofthemostpowerfulcapabilitiesofopticaltoolingisitsability
totransferopticalreferencelinesfromoneplacetoanother,with
extremelyaccurateresults.Transferoflinesismostaccuratelyac‐
complishedusingautocollimation(whichusesamirror)orcolli‐
mation(whichusesasecondinstrument).Autocollimationisex‐
tremelyusefulwhensettingyourinstrumenttomirrorswhichmay
bemountedondriveshafts,spindles,machinetools,otheroptical
toolinginstruments,laboratoryequipment,etc.

Intheprevioussection,wediscussedautoreflection,whichcanbe
usedtomakethetransit’slineofsightperpendiculartothesurface
ofamirror.Autocollimationisarefinementofthisprocess,squeez‐
ingoutmostoftheremainingangularerror.Youshouldbeableto
autocollimatetoamirrorsothatthelineofsightisperpendicularto
thatmirrorwithinabout±1arcsecond.

Theautocollimationprocessisabletoaccomplishtheseaccuracies
becausethefocallengthsinvolvedaremuchlonger.Thisgivesyou
farmoreleverageandangularcontrol.We’lldiscussautocollima‐
tionusingfirstthemaintelescopeofyourtransit,thenusingthe
cross‐axistelescope.


Autocollimationusingthemaintelescope

Theautocollimationprocedureissimplyanothertypeofbuck‐in
process.Therefore,thenearandfartargetsmustbedefined.The
neartargetisalwaysthemirroritself,whilethefartargetisthere‐
turned(reflected)reticleimage.

Followthesestepsforautocollimation:

1. Installtransitillumination.Installoneofourlighting
accessoriesintothebarrelofyourtelescope(describedin
Chapter2).Turnonthelightsource.

2. Autoreflection.Setyourtransitperpendiculartothe
mirrorusingthetechniqueofautoreflection(discussedin
theprevioussection).

3. Focusatinfinity.Onceyouhaveautoreflectedthetran‐
sit,youmaybetemptedtoturnonthetransit’slight
sourceandtrytofocusonthereticleimagethatisre‐
flectedfromthemirror.Youcantrythis,butyoumay
havetroublefindingtheimageduetothefactthat
autoreflectiondoesnotalwayssetthetransit
perpendicu‐
lartothemirrorwithintheminimalrangerequiredfor
autocollimation.Ifyoucan’tfindthereflectedreticleim‐
age,focustheinstrumentatinfinityusingtheprocedure
discussedinChapter2.Rememberthatatinfinity,even
thetiniestangulardifferencecanmakeverylargeshifts
intheapparent
positionofthereflectedreticle.This
Above:thereflectedreticle
isseenoff‐center.Afterthe
transitisadjusted,there‐
flectedimageisbrought
directlyintoregisterwith
thetransit’sactualreticle
(below).
Figure3.5