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    Re: Error of Perpendicularity
    From: George Huxtable
    Date: 2006 Apr 18, 16:33 +0100

     Alex wrote (about aligning the index mirror of a sextant for perpendicularity) -
    | That's why visual adjustment (without dominoes) is OK too.
    | Two years ago there was a large discussion on the list about
    | how exactly to do this adjustment, and the general conclusion
    | was that dominoes (or visors) are only recommended for sextants
    | with front-silvered index mirros.
    | For back silvered mirrors, the usual eye method is OK.
    I wonder if Alex misremembers slightly, perhaps, about the "usual eye method", 
    which involves squinting at the aec, and its
    reflection in the index mirror, to check that they align.
    Just to be a pedant, as I recall, the difficulty is this- it only works if the 
    axis of rotation of the index arm passes along the
    reflecting plane of the index mirror. Most sextants are arranged so that it 
    does, but a sextant will work just as well if it
    doesn't. In that case, all that happens is that a slightly different patch of 
    the index mirror is used as the altitude changes,
    moving toward one edge or the other.
    It's when you have a sextant, originally designed for a back-silvered mirror, 
    in which the mirror plane aligns with the axis, has
    its mirror replaced with a front silvered type, that the problem occurs. If 
    the mounting hasn't been redesigned to keep the
    reflecting surface in the same plane, everything will work just as well, 
    except that the reflection of the arc in the index mirror
    will no longer appear as a continuation of the true arc. That problem often 
    arises when a rear-silvered index mirror is replaced by
    a front-silvered one, in manufacture or by the user. But it isn't inherent in 
    the use of a front-silvered mirror, and it may arise
    also with a rear-silvered mirror, if the usual practice for positioning its plane has not been followed.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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