Brunson 771 - Page 56

Model 771, 75, and 76-RH190 Transits 52
Herearethestepswithwhichyoucanamazeand
mystifyyourcolleagues:

2.Setuptheinstrumentonanimaginarylinebe‐
tweenthescales,atthehalfwaypointbetweenthe
scales(Figure4.13).Agoodwaytojudgewhether
youarehalfwaybetweenthescalesistofocusonone
ofthescales,thenflipthetelescopeovertolookat
theotherone—ifit’salsoingoodfocus,you’rethe
samedistancefromeachscale.

3.Rotatethetelescopesothatthecoincidencevialis
ontopofthetelescopeandeasilyviewable.Install
themicrometeronthetelescopewiththegraduated
drumtotheside(Figure4.13).Setthemicrometerto
zero.

4.Roughlevelthetransitusingtheplatevial.

Beforewegoon,thereareacoupleofthingstoreal‐
ize.First,therelationshipbetweenthevialandthe
lineofsightisconstant,aslongasthevialisnotad‐
justed.InFigure4.14,thelineofsight(redarrow)willalwayspoint
atthesamedownwardangleifthevialis“incoincidence”.Re‐
memberthatwhenthevialisincoincidence,itindicatesalevelline
(bluearrow).Intheprocedurethatwe’lldoinaminute,weknow
thatifwelookatoneofthescalesandturntheverticaltangentad‐
justmentscrewtolevelourvial(putitintocoincidence),thelineof
sightwillbepointingatsomearbitraryangleupordown.Then,if
weturntolookattheotherscale,andagainlevelthevialusingthe
tangentscrew,thetelescopewillalsobepointingupordownatthe
exactsameangleasbefore.

Sincewehaveplacedthetransitthesamedistancefrom
eachscale,andsinceweknowthe
telescopewillal‐
wayspointdownward(orupward)atexactlythe
sameanglewhenlookingateitherscale,thenwe
knowthatthetelescopewillintersectthetwoscales
atthesamepointasanarbitrarylevelline(Figures
4.15and4.16).Itiseasytoseeinthesketchthat
the
coincidencevialisfarfrombeingproperlyad‐
justed—it’snotatallparalleltothelineofsight.
So,thisisthewayyoucandeterminethevertical
offsetbetweentwoscales,evenwhenyouhaveno
levellineofsighttouse.Solet’scontinueanddo
this:
5.Pointinthedirectionofoneofthescales.Bring
thevialintocoincidenceusingtheverticaltangentadjustment
screw.Readthescalewiththemicrometerandrecordthisnum‐
ber.Inourexample,wereadScaleA(atleftinFigure4.15).Our
readingis2.283”.
Figure4.14
Figure4.13