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    Perpendicularity and SNO-T
    From: Alexandre Eremenko
    Date: 2004 Oct 13, 01:54 -0500

    Dear Frank,
    (and all who participated in perpendicularity
    I think I found the reason of the disagreement in
    our experiments.
    Apparently it is indeed the property of SNO-T.
    (I have not seen any other sextant in my life, sorry:-)
    The axis of rotation of the index arm of SNO-T does not
    pass through the front (silvered) surface of the index mirror.
    This silvered surface is off the axis by more than 6 mm.
    This should be irrelevant for the sextant performance,
    but this is relevant for the perpendiculatiry test.
    (And this can explain why the visor come with SNO-T as standard
    The point is that the "right edge of the index mirror"
    (where the images of two visors should come together)
    is NOT at equal distance from the visors.
    Apparently, in other sextants, the axis of rotation
    DOES pass through the silvered surface of the index mirror.
    In this case, when the mirror is perfectly perpendicular,
    the test shows this, independently of the eye height
    over the plane of the arc.
    This is apparently in perfect agreement with what the Tamaya
    manual says. Apparently, on Tamaya sextants, the axis
    passes through the rear surface of the index mirror,
    and usually this rear surface is silvered.
    That's why the Tamaya manual says:
    "IF YOU NEED TO REPLACE your index mirror with a mirror
    that is silvered in the front surface,....."
    It remains to ask the other SNO-T owners to
    make the experiment I described to settle the issue

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