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    Plastic sextants and prejudice. was: GPS shortcomings.
    From: George Huxtable
    Date: 2005 Jun 9, 16:30 +0100

    Peter Fogg wrote, about an experience with a plastic sextant-
    
    >When I've had the chance to examine them I've found them disconcerting to
    >use - for example, the index error was different each time I checked on one
    >instrument, before and after taking a sight. It didn't inspire confidance. I
    >know and accept that they have been used to cross oceans with, and possibly
    >they are ideal for the task of teaching students.
    
    The effect Peter describes is most likely to be a loose mounting of one of
    the mirrors, which can happen with any sextant if the adjustment screws
    have been overslackened, perhaps during adjustment by a "sextant-worrier".
    If a corner of a mirror is no longer being firmly grasped between an
    adjusting screw and a backing spring, what he describes is the inevitable
    consequence. It's easy to check for, by pushing, ever-so-gently, against
    the mirror corners. Was that tried?
    
    As the user of a plastic sextant over many years, I always check index
    errors before AND AFTER a measurement (or series), just in case, and don't
    detect a difference.
    
    Nor have I observed any problem, in either of my Ebbco sextants, with the
    warping Peter has referred to, and wonder which is the sextant model in
    which he has observed this fault, and whether in reality it presented any
    problem.
    
    I don't pretend that such sextants are perfect, and we have discussed some
    defects more than once on this list. But Peter Fogg's comments read more as
    a rehearsal of prejudice than a reasoned evaluation.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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