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    Re: Index checks with laser and without
    From: Fred Hebard
    Date: 2007 Jan 3, 22:30 -0500

    For index error checks, I was thinking that one would more often use
    the horizon at sea rather than a star.
    One thing I noticed previously is that the star pair has to be in the
    center of the telescope.  It might be more difficult to use the cross
    hairs for this at night.  Last year, in part because the I.C. of my C
    +P wanders around a bit, all I did was index checks on stars.  I got
    better.  Previously, my star I.C. checks were worse than sun checks.
    Not sure now.
    Do you notice any correlation of good/bad lunars with the orientation
    of the sextant?  That would suggest something is loose.  That also is
    suggested by the side error wandering around.
    On Jan 3, 2007, at 9:37 PM, Alexandre E Eremenko wrote:
    > Fred,
    >> 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.
    > Alex.
    > >
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