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    Re: Problem with a sextant
    From: Robert Gainer
    Date: 2006 Apr 24, 10:38 -0400

    Is it possible Alex that the engraving for the zero mark is off by 9/10 of a 
    minute? You get consistent errors at other measurements so this might be the 
    only reasonable answer. Do you have enough adjustment in the system to use 
    the one minute mark as the zero mark and see if that makes a difference? 
    Maybe the one minute mark is engraved into the correct spot relative to the 
    other graduations on the limb.
    All the best,
    Robert Gainer
    > From: Alexandre E Eremenko 
    > Date: 2006/04/24 Mon AM 10:17:10 EDT
    > Subject: Problem with a sextant
    > Dear list members,
    > I already addressed this problem in the past,
    > but I am still at a complete loss.
    > Maybe someone can help.
    > I have a misterious systematic error in my SNO-T.
    > The essence is this: no mater how carefully I determine
    > the Index error, I get systematic substantial overshots
    > more or less independent of the angle.
    > Here is a typical result, of yesterday. I can send or post
    > complete data.
    > IC was determined by 14 measurements, before and after
    > observations, using various stars. It is DEFINITELY between
    > -0.2' and -0.5', the average is -0'.3 and sigma is 0.15.
    > All checks (perpendicularity, of both mirrors, collimation etc.
    > were made as carefully as possible before).
    > Then I measured star distances, under ideal conditions,
    > using 9-10 pairs of stars, approximately at distances
    > 10d, 20d, .... ro 100d.
    > Each distance was measured 3-5 times, as carefully as possible.
    > THEY ARE ALL OVERSHOTS by almost 1 minute! (No index correction made).
    > More precisely, the average error is -0.8, with sigma only 0.22.
    > On the previous day there were some clouds and conditions were
    > not perfect. The average error in the same experiment was -0.9
    > and sigma 0.3.
    > What can this be???
    > I tried Bill's Astra (with my scope) and the star distances were OK.
    > Bill tried my SNO with his scope and was -1' off.
    > The only explanation I can imagine is that the arc is deformed near 0.
    > (Worn after so many index checks, as Bill joked:-)
    > Inspection of the arc and teeth with powerful magnifying glass
    > shows no defect.
    > Alex

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