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    Problem with a sextant
    From: Alexandre Eremenko
    Date: 2006 Apr 24, 10:17 -0400

    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.

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