Brunson 771 - Autocollimation for Accuracy

19 Operation and Adjustment Manual
mirror.Mountthetransittothestand.

4. Pointatthemirror.Pointandfocusthetelescopeatthe
centerofthemirror,thenlockthehorizontalandvertical
axeswiththetangentlockscrews.

5. Setonthe“far”target.Focusthetelescopetoviewthe
autoreflectiontarget(ortheendofthetelescopeitself)in
themirror.Thisisthe“fartarget”.Justaswhen“bucking
in”before,makeangularadjustmentsasneededtoalign
thereticlewiththefartarget.

6. Setonthe“near”target.Nowfocusthetelescopeto
viewthemirroritself.Thisisthe“neartarget”.Makelat‐
eral(shifting)adjustmentsleftandright,orupanddown,
asneededtoalignthecrosswiresgenerallywiththecen‐
terofthemirror.Youcanapproximatethisifthemirror
hasnotargetprintedonitssurface.

7. Alternatelyrepeatsteps6and7untilnofurtheradjust‐
mentisneeded.



Ultimateaccuracyinangularcontrolrequirestheuseofautocollima‐
tion.Inautoreflection,thereticleofthetelescopeisalignedwiththe
reflectedimageofatargetwhichismountedontheobjectiveendof
thetelescopeitself.Butinautocollimation,thereticleisaligned
withitsownreflectionfromthemirror,notthatofasecondarytar‐
get.Thismakesautocollimationmuchmoreaccuratethanautore‐
flection.However,inpractice,autoreflectionisperformedfirst,
makingautocollimationeasiertoachieve.

Toautocollimateaninstrument,themaintelescopereticlemustbe
illuminated(discussed
earlier).Ourilluminationadaptersarebuilt
insuchawaythatyourabilitytoviewthereticleisnotimpeded,
eventhoughitisbacklit.Whenthereticleisbacklitandthetelescope
isfocusedatinfinity,parallellightrayswillexittheobjectiveend.If
youthenlookinto
thetelescope,whenpointedatamirror,thepar‐
allellightraysbouncebackfromthemirror,andyouseeareflected
imageofyourowninstrument’sreticle.

Whenthemirror(ortheinstrument)isadjustedsothatthereflec‐
tionofthereticlefallsexactlybackontheoriginalreticleitself,the
telescopehasbeenveryaccuratelypositionedperpendicularlyto
thesurfaceofthemirror.

Thisisanextremelyaccuratewaytoestablisharightanglewithre‐
specttoanopticalreferencelinedefinedbyamirror.
Areticleimagewhichisre‐
flectedfromamirrorisonly
visiblewhenthetele‐
scopeisfocusedatinfin‐
ity.
Autocollimation
Aprecisionlateralslide
allowsshiftingtheinstru‐
mentbackandforth;apre‐
cisionliftallowsmoving
theinstrumentupand
down.