Brunson 771 - Peg Test for Vial Alignment

51 Operation and Adjustment Manual
InChapter2ofthismanual,wediscussedpreciselev‐
eling(precisionplumbing)ofthetransit.Withalittle
thought,itbecomesobviousthattheprecision
plumbingproceduredoesnotdependonthefactthat
thecoincidencevialisinadjustmentornot.Thatis,
youcanpreciselyplumbthetransit
(bringthevertical
spindletrulytovertical)withthecoincidencevialin
anyoldpositionrelativetothetelescope,aslongas
theadjustmentofthevialdoesnotchangeduringthe
process.However,ifyouwantthetransit’stelescope
tobetrulylevelafterthatprocess,orifyou
simply
wanttobringthetelescopetoalevellineatanypoint,
itisnecessarytoadjustthevialsothatitsbubbleim‐
ageis“incoincidence”whenthetelescope’slineof
sightispreciselylevel.

1.Setuptwoscalesinaverticalorientation,about30
feetapart,atinstrumentheight,andonanominally
levelsightlinefromoneanother.Trytobringthe
scalesasclosetoverticalaspossiblewithoutmakinga
careeroutofit.(Amodel563scalelevelworksgreat
forthis.)

Tosuccessfullyadjustthevial,wemustfirstdetermine
theverticalseparationofthetworandomlyplaced
scales.Inotherwords,ifyoudrewalevellinebetween
thescales,thelinewouldnotintersectthescalesatthe
samepointunlessyouwereincredibly,amazingly
lucky.Forexample,alevellinemighthitonescale(call
itScaleA)
at2.283”andtheotherscale(ScaleB)at
3.697”.ThisisillustratedinFigure4.12.Ifweknew
theseintersectionpoints,wecouldcalculatethevertical
separationbetweenthescales:

3.697”‐2.283”=1.414”

WewouldthenknowthatScaleAis1.414”higherthan
ScaleB.Knowingthe
verticalseparationbetweenthe
twoscalesiscriticaltoourvialcalibrationprocess.But
howcanwedeterminewherealevelsightlinewould
intersectthetwoscales,whenalevelsightlineisex‐
actlywhatwedon’thave?Wecandoitwithaknowledgeofgeometry.
PegTest
Description:Determineifthecoinci‐
dencevialindicatesalinewhichis
paralleltothetelescope’slineof
sight.

Tolerance:±1arcsecond.

Affectedparameters:None.

Requiredequipment:
Opticalmicrometer,instrument
standandtwoscales.Aprecision
liftmakesthistesteasierbutisnot
mandatory.
Thistestisnothardtoperform,norisitcom‐
plicated—butitdoesrequiremorewordsto
describethatsomeoftheothertests!Once
we’refinishedwithourexplanation,wepro‐
videa“quickguide”attheendofthissection.
Figure4.12
ScaleA
ScaleB