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    Sextant mirror silvering.
    From: George Huxtable
    Date: 2006 Nov 8, 17:05 -0000

    I just sent a message, as follows-
    Frank wrote in [NavList 1642] Re: Don't do this to a sextant
    | While I'm thinking of it, sextants usually work without serious
    | even with mirrors that have lost 10 or 20 percent of the silvering.
    Since the
    | focus is at infinity, a few blemishes eight inches away don't really
    cause any
    | problems. I have an octant from c.1830 that works fine despite
    missing  about
    | 20 percent of the reflective material on both mirrors.
    I can imagine a potential problem, in that case, when taking altitudes
    of stars.
    There may be an intended star, reflected as normal in the index
    mirror, which may perhaps be confused with a brighter star, rather
    overhead and somewhat behind the observer's head, seen through the
    holes in the silvering. Can that happen, in practice?
    But on second thoughts, (which are usually wiser ones) I now realise
    that it would be easy to make a distinction. Stars seen by a double
    reflection would appear to stay in the same place, as the aim of the
    sextant is altered. Any stars observed by transmission through the
    index glass would have had only one reflection, and would be very
    sensitive to any changes in the aim of the sextant. It should be very
    easy to distinguish between them.
    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.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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