11 Operation and Adjustment Manual
ThetelescopeofeachOpticalToolinginstrumentcontainsareticle–
orcrosshair–whichdefinesthecenterofthelineofsight.When
youlookintoatelescope,youseethereticleʺsuperimposedʺupon
theimageofwhateveryouhavethe
telescopefocusedon.Thereticleis
splitintofourquadrants;twohavea
singleʺwireʺ(filar),theothertwohave
adoubleʺwireʺ(bi‐filar).Thiscon‐
figurationwasoriginallyadoptedbe‐
causethehumaneyeisextremely
goodatdetectingtinydifferencesin
alignmentbetweencloselyspacedob‐
jects.Therefore,thispatternmakesit
mucheasiertovisuallyalignthereti‐
clewithotherimages(ex.,opticaltar‐
gets,scales,andreticleimagesre‐
flectedfrommirrors).
Focusing sounds easy, but it’s surprising how many people don’t
understand the effectsof something called parallax.Here’s how to
properlyfocusyourinstrument.First,pointthetelescopeatasheet
of white paper or other light colored background.Don’t worry
about focusing the telescope using the focus knob, the image can
remainblurry.Instead,usetheknurledringontheeyepiecetofo‐
custhereticlesothatitissharpandblack.
Nowpointthetelescopeatatargetofinterestandfocustheimage
usingthefocusknobonthetelescope.Slightlyshiftyourheadleft
andrightwhilelookingatthereticleasitis“superimposed”over
theimageofthetarget.Ifthereticleappearstofloatslightlytothe
leftandrightoverthetarget,
parallaxispresentinthesystem.This
occurswheneitherthetelescope’smainopticalsystemortheocular
(eyepiece)system,orboth,arenotfocusedpreciselyontheplaneof
thereticle.Parallaxcanleadtoerroneousmeasurementsifnotre‐
moved.Toremovetheparallax,repeatthetwosteps
again,being
carefultoachievecriticalfocusineachstep.
Therearemanyoccasionswhenitwillbeimportanttoputyour
telescopeatinfinityfocus(e.g.,autocollimatingyourtelescopeona
mirror).Infinityisimportantbecause,fromanopticalstandpoint,
lightrayscomingfroman“infinitedistance”areallparalleltoeach
TheReticle(crosshairs)
Howtoproperlyfocus
Thefilar/bi‐filarreticle
imageasseenwhen
lookingintothetele‐
scope.
Figure2.6
Findinginfinityfocus
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