Model 771, 75, and 76-RH190 Transits 42
Thisisaquickandeasychecktohelpdetermine
whethertherehasbeenanymajordamagetothever‐
ticalspindlesuchasballorracedenting.
1.Mountthetransitonastand.Installthemicrome‐
tertoreadverticaldeviations(i.e.,positionthemi‐
crometerdrumonthesideofthetelescoperather
thanthebottomortop).Setthemicrometertozero.
Remembertousethegoodmicrometermeasuring
practicesdescribedinChapter3ofthismanual—
alwayscometozeroandanydesiredreadingposition
fromthesamedirection.
2.Holdascaleverticallyusingamagnetorother
convenientmount,about17feetaway,andapproxi‐
matelyonalevelsightlinefromthetelescope.
3.Rotatethetransitaboutitsverticalaxisroughly5timesinthe
samedirection.Thisensurestheindividualballsintheverticalaxis
bearingshaveallmadeatleastonecompleterevolution.
4.Usingtheverticaltangentscrew,achieveaccurateregisterbe‐
tweenthetransit’shorizontalreticlelineandanarbitrarytargetpo‐
sitiononthescale(e.g.,thethreeinchtarget).Makesuretheverti‐
caltangentclampissecure.Notethepositionoftheverticalreticle
lineofthetransitaswell,sothatyoucanreturntothisapproximate
azimuthposition.Keepthehorizontaltangentclamploose.
5.Gentlyrotatethetransitaboutitsverticalaxisinthesamedirec‐
tionasitwasrotatedbefore,andre‐pointbackatthescaletargetto
observeanyerror.Ifyouovershootthetarget,goaroundagain.Be
suretorepositiontheverticalreticlelineatitsoriginalposition.
Usethemicrometertoreadanyerror.Repeatthisprocess4or5
times,rotatingthetransitinthesamedi‐
rectioneachtime,andreadinganyerror
after
eachrevolution.
Ifanerrorgreaterthan0.002”isobserved,
rerunthetesttoensurethattheobserved
errorisactuallybearingrunoutandnot
movementoftheentireinstrument.Ifit
isdeterminedthattheerrorisbearing
runout,thetransitshouldbereturnedto
themanufacturerforrepair.
Description:Initialindicationof
whethertheinstrumentrotatesaccu‐
ratelyaboutitsownverticalspindle.
Tolerance:±2arcseconds(0.002”in
thistestprocedure.)
Affectedparameters:Alladjust‐
ments.
Requiredequipment:Opticalmi‐
crometer,instrumentstand,anda
scaleplaced17feetawayinvertical
orientation.
VerticalSpindleRunout
Wechoosetoputthescale
17feetawaybecauseatthis
distance,0.001”equals1
arcsecond.Ifyou’reusing
ametricscaleandametric
micrometer,placeyour
scale4.125metersaway.
Atthisdistance,0.02mm
yields1arcsecond.
NOTE:Aswiththehorizontalbearingrunout
test,itmustbesaidthatthistestmayindicatea
spindlerunoutproblem,butdoesnotnecessarily
provethattheinstrumenthasagoodvertical
bearing.Verticalspindlerunoutissomewhat
sneaky,andmayshowupinaninabilitytoper‐
formother
calibrations,suchastheplumbline
check.Oneoftheplacesthatverticalspindle
runoutismostlikelytoshowupisintheprecision
plumbingprocess(seeChapter2).Ifthetransit
maybemadepreciselylevelinseveralazimuthori‐
entationsspanning270°,butotherpositionsare
notlevel,theverticalspindlemaybesuspect.
To Next Page