Brunson 771 - Double Center Check Method

Model 771, 75, and 76-RH190 Transits 40
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Thisfieldcheckismadeinordertoeliminatethene‐
cessityofmakingseveralotherfieldchecks.Ifthe
“doublecenter”checkissuccessful,thereisnoneed
toperformtheHorizontalCentering,HorizontalCol‐
limation,Plumbline,andmicrometercenteringfield
checks.

Twohorizontallymountedscalesmustbeused.The
firstisplacedabout30‐35feetawayfromthetransit,
andbetween2‐5feetabovealevelsightlinefrom
thetransit.
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Thesecondscaleisplacedverynearthetransit,on
thefloor.Theideaisthatwewantbothhorizontal
separateandverticalseparationbetweenthetwo
scales.Alsonotethatthetransitneednotbepreci‐
sionplumbedtoperformthistest.Oncethetransitissecuredtoa
standandthescalesareplaced,performthesesteps:
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1.Makesurethemicrometerissettozero.Focusonthefarscale.
Usingthehorizontaltangentadjustmentscrew,setonsomearbi‐
traryscaletargetposition(e.g.,thefiveinchpairedlinetarget).
Makecertainthatthehorizontaltangentclampissecured.
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2.Swingthetelescopedowntothenearscaleandtakeareadingon
thescale.

3.Reverseandplungethetransit(rotatethestandardsby180°,and
alsorotatethetelescopeby180°).Thetelescopeshouldbepointing
inthesamedirectionasitwasbeforethereverseandplunge,but
willnowbeupsidedownrelativetoitspreviousposition.
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4.Focusagainonthefarscale.Besuretore‐zerothemicrometer,
andsetontheoriginalscalepositionthatwasestablishedinstep1
above.
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5.Swingthetelescopedownagaintothenearscale,andtakea
readingonthescale.

Ifthereadingonthenearscaletakeninstep5differsfromtheread‐
ingtaken
instep2by0.002”orless,youmayskiptheHorizontal
Centering,HorizontalCollimation,Plumbline,andmicrometer
centeringfieldchecks.
“DoubleCenter”check
Description:Simultaneouscheckof
severalcalibrationparameters.

Tolerance:±0.002”

Affectedparameters:Notapplicable,
noadjustmentismadeasaresultof
thischeck.

Requiredequipment:Opticalmi‐
crometer,instrumentstand,andtwo
scales.