Model 771, 75, and 76-RH190 Transits 54
6.NowturntheinstrumenttowardScaleBandre‐
levelthecoincidencevialusingtheverticaltangent
adjustmentscrew.TakeareadingonScaleBandre‐
cordthisnumber.Inourexample,wereadScaleBas
3.697”.Aswehavelearned,wecannowcalculatethe
verticaloffsetofthesetwoscales:
3.967”‐2.283”=1.414”.
7.Now,movetheinstrumentclosetooneofthescalesandonthe
linebetweenthem(wemovedclosetoScale“B”inFigure4.17).
Preferably,theinstrumentshouldbelocatedabout1/10ofthedis‐
tancebetweenthetwoscales.Inourexample,wewouldtrytoset
theinstrumentabout3feetawayfromoneofthescales,sincethe
totaldistancebetweenthemis30feet.Again,roughlevelthein‐
strumentusingtheplatevialandthelevelingscrews.
8.Pointatthenearscaleandpreciselylevelthecoincidencevial
usingtheverticaltangentadjustmentscrew.Takeareadingonthe
scale.(Ifyouhaveaprecisionlift,youcanmakeyourlifeeasieratthis
pointbysettingatsomeniceevennumberonthescale.)Inourexample,
sincewedon’thaveaprecisionlift,weread4.035”.
9.Turntheinstrumenttowardthefarscale,andre‐levelthecoinci‐
dencevialusingtheverticaltangentadjustmentscrew.Now,be‐
forewetakeareading,wecancalculatewhatwewanttosee.We
knowthatthefarscaleinourexampleis1.414”higherthanthenear
scale.Sowecancalculatewhatnumberwewouldhopefullyseeon
thefarscaleifthevialisinfactadjustedcorrectly.Dothisbytaking
thereadingonthenearscaleandadding(orsubtracting)theverti‐
caloffsetbetweenthescales.Inourexample,wesubtractthenear
scale’sreadingfromtheverticaloffsetdistance,sincewehaveal‐
readydeterminedthatthefarscaleishigherthanthenearone:
4.035”‐1.414”=2.621”
Soifthevialisperfectlyadjusted,weshouldsee2.621”onthefar
scale.Now,sincethesuspense
iskillingus,weactuallyreadthefar
scalewiththemicrometer.InFigure4.18,thereadingonthefar
scaleisdeterminedtobe1.958”.
Now,noticethat1.958”islessthantheoptimal2.621”readingthat
wouldhaveindicatedalevelline.Thistellsusthatthelineofsight
ispointingdownward.Ifwehadtakenareadingonthefarscale
whichwasgreaterthan2.621”,wewouldknowthatthelineof
sightispointingupwardwithrespecttoalevelline.
10.Tocorrectthiserror,wewillcalculatewhatthefarscaleshould
bereadingiftheinstrumentwereincalibration,re‐pointthetele ‐
scopeatthatproperreading,andadjustthecoincidencevialback
intocoincidence.Nowholdonhere,whydoweneedtorecalcu‐
Note:Ifyouwanttomakethe
arithmeticmucheasier,useapre‐
cisionlifttobringthetransitupor
downtoanevennumberonScale
A,settingonsomethinglike
2.000”or2.500”,withthemi‐
crometersetonzero.
Thisprocedureisone
ofthosethingswhich
becomesmucheasier
afteryou’vegone
throughitonce,and
understandwhat’s
goingon!
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