Brunson 771 - Plumb Line Check Procedure

Model 771, 75, and 76-RH190 Transits 46

Forthistest,wewanttoputacoupleofscalesinfront
ofthetransit,oneuphighandonedownonthefloor 
orclosetoit(Figure4.9).Thehighscaleshouldbe
aboutthesamedistanceabovethetransitasthelow
scaleisbelowthetransit.Thelowscalemaybelaid
flatonthefloor,basicallysquaretothelineofsight.
Thesightingdistanceshouldbeabout4‐5feetfrom
thetransittoeachscale.Optimallywewouldliketo
haveabout90°oftelescopemovement(differencein
sightingangle)betweenthetopscaleandthebottom
one.

1.Roughlevelthetransit.Focusontheupperscale.
Makesurethemicrometerissettozeroandthatthe
verticaltangentclampisloose.Usingthehorizontal
tangentadjustmentscrew,settosomeeasilyrepeatabletargeton
thescale(e.g.thethreeinchpairedlinetarget).

2.Withoutlooseningthehorizontaltangentclamp,rotatethetele‐
scopedownwardtopointatthelowerscale.Takeareadingonthis
scale.

3.Nowloosenthehorizontaltangent
clampandrotatethetransit180°about
itsverticalaxis.Plungethetelescope
180°aboutitshorizontalaxisaswell.
Pointbackattheupperscaleandsecure
thehorizontaltangentclamp.Setback
onthesamescalelocationthatwasused
instep1.

4.Onceagain,rotatedownwardtothe
lowerscaleandtakeareading.Com‐
parethisreadingwiththeonetakenin
step3.Ifthedifferenceislessthanor
equalto0.002”,thetransitiswithintol‐
erance.Ifitisn’t,proceedtothenext
step.

5.Notewhetherthereadingonthe
lowerscaletakeninstep4istotheright
ortotheleftoftheprior
readingonthe
samescale.Ifthereadingistotheright,
thecross‐axis(alsocalledthetrunnion
axis)istiltedupontheright.Likewise,
ifthereadingwastotheleftoftheorigi‐
nalreading,thetrunnionaxisishighon
theleft.

Usingthisinformation,
wemakeanad‐
Description:Determineiftheverti‐
calspindleisperpendiculartothe
horizontalaxis.

Tolerance:±1arcsecond.

Affectedparameters:None.

Requiredequipment:
Opticalmicrometer,instrument
standandtwoscales.
PlumbLineCheck
Figure4.9