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    Re: sextant precision.
    From: Bill B
    Date: 2005 Jun 20, 16:08 -0500

    > "Does that all make sense?"
    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.
    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.
    This would require adjustment of the index mirror to line things up again.
    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?
    I did try the method of bringing the moon edges into tangency then swinging
    shades in and out with questionable results.  I could not distinguish and
    difference with my Astra.  I found it difficult as overlap was difficult to
    establish when one of the images was much brighter than the other, and
    wonder what affect irradiation will have on the whole process when we move
    from two bright images to one bright and one dim image.

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