Model 771, 75, and 76-RH190 Transits 56
late?Didn’twejustsaythatthereadingonthefarscaleshouldbe2.621”
ifthevialwereadjustedproperly?Yeswedid,butsincewejust
learnedthatthevialisnotadjustedproperly,weknowthatourini‐
tialreadingof4.035”onthenearscalecannotbetrusted;itisnotona
levellinefromthetelescope.Rememberthatwearenolongerhalfway
betweenthescales,wheregeometryallowsustodetermineatrue
levelline,whetherthelineofsightisactuallylevelornot.
Whatwearegoingtodonowisdeterminewhatthefarscale’s
readingshouldbe,ifthelineofsightwereactuallybroughtlevel.
Howdowedothat?Whatweneedhereisagoodruleofthumb:
Let’sworkthroughthisinourexample.Rememberthatourread‐
ingonthefarscalewas1.958”.Our“optimalreading”(theonewe
wouldhavelikedtohaveseen)was2.621”.Sowesubtractthese
twonumbers:
2.621”‐1.958”=0.663”
Nowwedividetheresultby8:
0.663”/8=0.083”
So,weneedtorotatethetelescopeupfromourcurrent1.958”posi‐
tiontothe2.621”position,andthengo0.083”further.Whatthat
meansisthatwewanttosetthetelescopeonthefarscaleatthis
reading:
2.621”+0.083”=2.704”
11.Sonow,let’srotatethetelescopetoseton2.704”atthefarscale,
whichisourdesiredscale
position.There’saneasywaytodothis.
Lookatthelasttwodigitsinthedesiredposition—”04”(forget
aboutthe2.7foramoment.)Lookintothetelescopeandrotatethe
RuleofThumb:Whenthetransitsits1/10oftheyway
fromonescaletoanother,takethefarscalereadingminus
the“optimalreading”anddivideitby8.Then,onthefar
scale,rotatethetelescopetowardandpasttheoptimal
readingbytheresultingamount.
CAUTION:Whendeterminingthedesiredscalepositiononwhichtosetthetelescope
inthestepabove,youwillalwaysmovethetelescopefromitscurrentpositiontoward
the“optimalreading”,andthenbeyond,by1/8ofthedistance.Inourexample,weare
movinginthedirectionofanincreasing
scalereading(goingfrom1.958”to2.704”),
soweaddthe0.083”above.Butifyouneededtomoveinthedirectionofadecreasing
scalereadingyouwouldbesubtractingthenumbers.Remembertheruleofthumb,
notthearithmetic,ofourexample.
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