Model 771, 75, and 76-RH190 Transits 22
Forsomeapplications,itisnecessaryto
autocollimatethecross‐axistelescope
(“crossscope”)toamirror.
Whenweautocollimatedthemaintele‐
scopetoamirror,weusedautoreflectionas
thefirststep,togetusintheballparkand
makeautocollimationeasier.However,re‐
memberthatthecrossscopeispermanently
focusedatinfinity,soyoucan’tautoreflect
ittoamirror(whichrequiresfocusingal‐
ternatelyfromneartofar).Soyoumust
useaspecialproceduretoprepareforauto‐
collimationwhenusingthecrossscope.
1. Installtelescopeandcross‐axistele‐
scopeillumination.Installalightingat‐
tachmentonboththemaintelescopeand
thecross‐axistelescopeasdescribedin
Chapter2.(Itisnotnecessarytoactually
havealightsourceinbothatthesame
time.)
2. Makethemaintelescopesquaretothe
verticalspindle.Wemustbringthemain
telescopeaxis“square”totheverticalspin‐
dle(SeeFigure3.6).Thismaybeaccom‐
plishedbypreciselevelingthetransitas
discussedinChapter2(“PreciseLeveling”).
Preciseleveling(a.k.a.precisionplumbing)
actuallyaccomplishestwothingsatonce.
First,thetransit’sverticalspindleis
broughttoplumb(i.e.,straightupand
down;paralleltothegravityvector).Sec‐
ond,themaintelescopeisbroughttohori‐
zontal,whichofcoursemakesitsquareto
theverticalspindle(Figure3.6).Thislatter
partiswhatwecareabout—wewantthe
maintelescopeperpendiculartothevertical
spindle.Whenthisistrue,thecrossscope
andthemainscopewill“follow”eachother
asthetransitisrotatedintheazimuthdi‐
rection,aboutitsverticalspindle.Thatis,
thelineofsightofboththemaintelescope
andthecross‐scopewillbeinthesame
horizontalplane.Therefore,ifyouturnthe
maintelescopetolookatpoint“A”,and
thenturnthetransit90°,thecross‐scope
willnowpointatthesamepoint“A”.Now
here’stheimportantpart:thisistruewhether
ornotthetransitremainspreciselyplumb.As
longasyoudon’trotatethemain
telescope
upordownwithrespecttothestandards,
Figure3.6
Figure3.7
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