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    Re: Index checks with laser and without
    From: Alexandre Eremenko
    Date: 2007 Jan 3, 21:37 -0500

    > Have you tried introducing a bit of side error?
    I always have a little bit, not because I introduce it
    but because my SNO just does not hold this adjustment.
    (And I've heard on this list that this is a problem
    with other SNO's as well). It always looks less than
    1 minute;
    when it is more I adjust it, say every other month.
    But it does not help me much with stars index checks.
    The stars I see are not exactly points, and brighter
    ones look bigger. This could be a telescope problem
    but more likely my vision problem.
    Anyway, I am quite confident with Sun checks by now,
    the results I cited in my previous message are well
    reproducible and typical. And unlike the side error,
    the sextant seems to hold the index error unchanged for
    years. Now I double checked it with lasers.
    Speaking of my main business with this sextant,
    the Lunars, I am still at loss why my results are
    sometimes very good and sometimes very bad (for the Lunars).
    I am pretty sure that this is a sextant problem
    (not my vision) but I still cannot find where exactly
    it is. Frank had my sextant for checking for few weeks
    but he reported only one observation (which was good),
    so his checks did not give much.
    He gave me one advise though which seemed to help a lot
    first (to relax one little screw to ease the movement of
    the worm) but then, with more tests, I found that the
    problem did not disappear.
    Very many observations show that the sextant can be in
    two states: sometimes it measures a Lunar perfectly
    (to 0.1' or 0.2' and sigma is between 0.1 or 0.2
    in a series of 10-12), and sometimes I have errors
    of 0.4 and even 0.8, always overshots, no matter what
    I measure, a near limb or a far limb.
    And the errors almost do not correlate with measured
    angles. So this is not a result of an arc defect.
    This looks like some non-rigidity, some part of
    the sextant which should be rigid actually moves.
    And what part, I don't
    understand. The only thing I can feel is a very slight
    lateral motion of the sextant arm. But they say that this is
    unavoidable, even in the best sextants. This does not
    seem to have any effect: see my checks of the IC
    hand up and hand down.
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