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    Re: sextant precision.
    From: Bill B
    Date: 2005 Jun 21, 16:16 -0500

    > 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.
    I have thought it (and sketched it) and am not clear I am wrong.  Let's use
    a star, and assume its rays to be parallel for all practical purposes.  We
    make our horizon and index mirrors parallel.  This sets equal angles between
    the scope line of sight/horizon mirror and horizon mirror/a point on the
    index mirror (hopefully at the axis).  The reflected and glass images
    coincide. If we were to offset the parallel ray striking the index mirror
    (or change the geometry of the sextant by raising the index-mirror axis
    vertically) the images would no longer coincide.
    My contention is that a optically perfect shade with faces parallel will
    still offset the ray if not perfectly perpendicular to the ray path.  You
    are probably correct that the offset is negligible in practice, but
    nonetheless does exist.
    > 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.
    Agreed regarding faulty glass and direction, but maintain refraction is also
    a factor--even it is so small it can be ignored for practical purposes.
    Which prompts the question, is refraction by a non-perpendicular shade (if I
    am correct) insignificant?

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