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    Sextant precision
    From: Alexandre Eremenko
    Date: 2004 Oct 3, 11:00 -0500

    Jared Sherman wrote on Thu Sep 30 2004 - 20:30:06 EDT:
    >only in one direction, >
    >Alexandre, this is probably a writer's error in the manual.
    This is not the error the the manual, but my incomplete
    or non-precise citation of the manual.
    My own understanding of this recommendation agrees with
    your explanation: I have to rotate in one chosen direction
    immediately before reading:
    > It doesn't matter which way you go
    > --only that it be
    > consistent. And of course, that should be the same
    > direction that you go
    > in when zeroing for index error as well.
    In Russian manuals, the error component explained in your
    message has a special name, literally in Russian "dead movement"
    of the worm. In the web translation of the SNO-T manual it
    is translated as "backlash between sextant worm and frame".
    They always care to include this "dead movement" of each
    particular sextant into its certificate. They guarantee that it
    is to be most 6" (some sextant manuals say 12" depending
    of the year of production)
    but measure it and include to the
    certificate for each sextant.
    For example, in my sextant certificate it is 4".
    They also have a special check for this quantity that they recommend
    to do periodically together with the other standard checks.
    (The procedure is evident: measure the index error twice,
    one time for each direction of rotation,
    and then take the difference).

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