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    Re: sextant precision.
    From: George Huxtable
    Date: 2005 Jun 21, 10:15 +0100

    Bill wrote-
    >Just a basic question about what we are really trying to measure.
    >First, if my basic understanding of refraction is not amiss, if the glass is
    >not perpendicular to the light path (even if it optically perfect with
    >parallel surfaces), light will not follow a straight line through the glass.
    >It will enter, be bent, and upon exit be bent to its original angle but
    >offset.  The amount of offset will be determined by the thickness of the
    >glass, wavelength(s) of the light, and color of the glass.
    Yes, all correct so far.
    >Assuming the above to be true, if we set the sextant so the index mirror is
    >parallel to the horizon glass/mirror with no shades in place, then introduce
    >a shade between the source and index mirror, the light coming out of the
    >shade will strike the index mirror parallel to but offset from the original
    >path if it the plane of the glass is not perpendicular to the light path.
    Well, yes, but the next bit is quite wrong-
    >This would require adjustment of the index mirror to line things up again.
    Think about it. Parallel light from the Sun (or any other astronomical
    object you may choose) bathes the surface of the sextant, the observer, and
    indeed half the surface of the World. The index mirror intercepts a tiny
    sample of that light and sends it down to focus at the observer's eye via
    the telescope. If you put an angled slab of parallel glass, thick as you
    like, into the incoming path of light before it strikes the index mirror,
    the only difference it will make is that the bit of incoming sunlight
    sampled by the index mirror is displaced by a few millimetres. But, as it's
    everywhere parallel, that sample of sunlight is just the same, travelling
    in exactly the same direction, as before. The sextant won't notice any
    difference whatsoever.
    If the sextant were being used to measure the angle to a very near object,
    it would be another matter, as the incoming light would no longer be
    parallel, but slightly divergent.
    If the piece of glass intercepting the light was not strictly
    plane-parallel, but was even-slightly prismatic in shape, then that would
    change the DIRECTION of light passing through it. Then that light would no
    longer be parallel to the incoming rays, and the sextant reading would be
    altered by the amount of the bending. This is the effect we are considering
    when discussing the prismatic errors of shades.
    >If I understand, what we are trying to measure is a)is the glass optically
    >"correct" and/or b) is it exactly parallel to the light path. Yes?
    The important question is a). As for b), presumably Bill really intended to
    say "is it exactly PERPENDICULAR to the light path", and the answer is No:
    that hardly matters at all.
    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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