Model 771, 75, and 76-RH190 Transits 28
transitcanalsobeputinreticleprojectionmode.Thisallowsyouto
projectanimageofthereticleontosomesurfaceatwhichthetele‐
scopeispointed.Todothis,youmustfocusthetelescopeonthe
surfaceonwhichyouwantthereticleimagetoappear(ratherthan
atinfinity).Lowambientlightlevelsarerequiredtoseethepro‐
jectedimage.Thisissometimesusefulwhenyouhaveestablished
areferenceline,andinturnneedtoprojectanimageofthecross‐
hairsonasurfacetogiveaclearvisualindicatorofthereference
line.Thisisusefulforbuilding,componentpositioning,etc.
Todothis,youmustuseaneyepieceadaptercapableofreticlepro‐
jection(model196‐1).Thisunithassettingsforstraight‐through
viewingaswellasreticleprojection.Withoneoftheseinstalled,
lookthroughthetelescopeatthesurfaceonwhichyouwishtopro‐
jectthereticle.Focusthetelescopeonthatsurface.Turnontheil‐
lumination,andswitchtheeyepiecetoprojectionmode.Thiswill
projectthereticleimageontothesurface.Again,rememberthat
thesurroundinglightingconditionsmustbeverysubduedinorder
toseetheprojectedimage.
Sofarwehavediscussedanumberoftechniqueswhichallowyou
tosetyourtelescopetoreferencelinesestablishedbytargetpoints,
othertelescopes,ormirrors.Oncethosereferencelinesandplanes
areestablished,measurementsofphysicalobjects
relativetothosereferencelinesmaybemadeusing
opticalmicrometers.
Opticalmicrometersaredesignedtooffsetthelineof
sightbyamountswhichmaybereadonagraduated
drum.Inasense,they“pickupthelineofsightand
setitover”usingaveryflatblockofglasswhichis
manuallyrotated.Thistechniqueshiftsthelineof
sightlaterallyratherthanchangingtheangleatwhich
it“enters”thetelescope.Theamountofthelateral
shiftisthenreadonthemicrometeritself.Sincethe
shiftedlineofsightremainsparalleltotheoriginal
line,themeasuredoffsetdeviationisthesamefor
alldis‐
tances.(Thereisoneexceptiontothisrule—
micrometersdonotfunctionatinfinityfocus.Atin‐
finityfocus,whenlightraysareparallel,lateraldevia‐
tionsproducedbyamicrometerarenotvisible.)
Let’stakealookatwhatyouseewhenyoulookthroughatele‐
scope,
andmeasurewithamicrometer.Saythatyouhavepointed
thetelescopedirectlyatatargetsothatyouseetheimageinFigure
3.12.Sayalsothatthemicrometeronthetelescopeissettozero.
Now,forsomereason,thetargetmovestotheright.Youseethe
imageshowninFigure3.13.Tomeasurethedistancethatthetarget
moved,youcanturnthemicrometerdrumandshiftthereticleback
MeasuringwithMicrometers
Thegraduateddrumon
anopticalmicrometer
readsthelateralshiftof
alineofsight.
Figure3.11
Projectingareticleimage
ontoasurfaceissome‐
timesusedin“build”or
“set‐out”procedures.The
projectedreticlelinesgive
real‐timefeedbackastothe
positionofsomecompo‐
nentofinterest.
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