Brunson 771 - Page 22

Model 771, 75, and 76-RH190 Transits 18
willnotbeatrightangles(orthogonal)tothemirror.However,if
youtrainyoureyeonthereflectedimageofyourowneyeinthe
mirror,thenyourlineofvisionwillbeorthogonaltothemirror.This
ideaisillustratedinFigure3.3.Knowingthis,itiseasytofigure
outhowtolookatamirrorsothatyourlineofvisionisatrightan‐
glestothatmirror—youjustfindtheimageofyourowneye.

Autoreflectionisthetechniqueofper‐
formingthisprocesswithyourtransit.
Ifyoulookintoatelescopewhichis
pointeddirectlyatamirror,andthe
mirrorisperpendicular(90°)tothe
lineofsight,youcouldfocusonan
imageoftheobjectiveendofyour
owntelescope,justlikeyoudidwith
yourowneye.Ifyouputatarget
(availableasanaccessory)ontheend
ofthetelescopeitself,youwouldbe
abletopositionyourowninstru‐
ment’sinternalreticleovertheimage
ofthattargetthatyouseeinthemir‐
ror.Thisconceptofautoreflectionisusedtoputaninstrument’s
lineofsight(LOS)closetoperpendiculartoamirroredsurface.Itis
importanttonotethatautoreflectionisnotasaccurateasautocolli‐
mation(discussedlater),andismostoftenusedasthefirststepto
autocollimation.
Insomeways,theautoreflectionprocedureissimilartothe
buck‐inprocessdiscussedpreviously.However,thistime,
the“neartarget”istheactualsurfaceofthemirroritself,
whilethefartargetisthereflectionofthetelescopeface(ora
targetmountedonthetelescope).Thereforeweknowthat
thedistancetothefartargetisalwaystwicethedistanceto
theneartarget.

Herearethebasicstepsforautoreflection:

1. Definethe“working”mirror.We’llassumethatyou
haveamirrorsituatedattheendofashaftormountedin
amachinetoolchuck,orinsomeotherrelevantlocation.

2. Installproperautoreflectiontarget.Mountanautore‐
flectiontarget(exampleshowninFigure3.4)ontheobjec‐
tiveendofthetelescope.Wehavetwodifferentbasic
types.ThetypeinFigure3.4isusedforlongerdistances.
Theothertypehasasmallertargetprintedonglass,is
usedforcloserdistances,andoftenrequiresinternal
lightingofthetransit.

3. Roughlypositionthetransit.Positiontheinstrument
standinfrontofthemirrorbyfindingthereflectionof
yourowneyeinthemirror,andthenputtingthestand’s
instrumentmountonthelinebetweenyoureyeandthe
Figure3.3
Anautoreflectiontargetis
mountedontheobjective
endofatelescopetohelp
centerareflectedlineof
sightonthetelescopeit‐
self.Notethatifyouhave
lotsofexperienceanda
convenientsetup,using
anautoreflectiontargetis
notmandatory.
Figure3.4