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    Re: Suitable Sextants - Mirrors
    From: George Huxtable
    Date: 2005 Oct 14, 15:24 +0100

    In a message posted earlier today, I wrote the following, about the sizing
    of sextant index mirrors-
    >the allowance that Frank rightly emphasises, in-
    >>... the index mirror ... it helps a lot if it's  longer (bigger in the
    >>dimension along the index arm) because it will be  foreshortened when the
    >>is set to a large angle.
    >For a sextant which is designed with optimal geometry, the enhancemant in
    >the length of the index mirror should be  (1 / cos 30), or 15%, which
    >allows for that foreshortening over the full range of observed altitudes
    >from 0 to 120 degrees.
    That figure of 15% was based on sloppy thinking on my part, and I would
    like to withdraw it, please. At high altitudes, to keep the apparent
    vertical aperture of the index mirror to be effectively equal to its
    required diameter would call for a much greater enhancement than 15% in its
    lengthwise dimension, and would require a quite-impractical length for that
    mirror. Instead, the sextant user just has to accept that at high
    altitudes, any practical index mirror will result in a field of view shaped
    like the horizontal slot of a letter-box. For that reason, there is indeed
    a case to be made for fitting a long (but not wide) index mirror; longer by
    much more than my 15%.
    Sorry about that.
    I stand by the arguments in the rest of that posting, however.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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