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    Re: Lunar Distances with Alex's SNO-T
    From: Bill B
    Date: 2006 Dec 06, 03:58 -0500

    In a recent post, I stated:
    "I also produced a card to sit on the mirror side of the horizon glass to
    indicate dead center (vertically in normal upright sextant position).  This
    idea popped up while trying to determine index-mirror line of sight to
    horizon-glass line of sight distance using George's idea.  Even when the
    sextant was tripod mounted, plumb, and zero-mag sight tube was level my
    measurements ranged from 65mm to 67mm depending on the height of the
    sextant--which was based on eyeballing the perceived vertical center of the
    horizon glass in relation to a target line."
    This has proved very interesting. All this time I have been operating under
    the assumption that I was using the vertical (physical) center of the
    horizon glass for observations.  Once my card was in place dividing the
    physical horizon glass equally top and bottom I did a sanity check and moved
    the index mirror out to its limits, noting its location in relation to the
    card's  dividing line as it moved.  To my surprise the optical center of the
    foreshortened index mirror is about 5.5 mm below the physical center. Using
    George's method with the physical center the lines of sight increased from
    66mm to 73mm.  Adjusting the dividing line to the optical center it
    decreased to 65.5 mm.
    A couple of hours of playing produced results, some supported by my earlier
    thoughts, and some observations.
    1. By tracing the laser path when the pointer is aligned with the scope
    barrel, the scope does not look at the physical center of the horizon glass
    mirror, but rather at a spot below the physical center that coincides with
    the point located by viewing the foreshortened surface of the index mirror.
    2.  It does matter whether the laser beam is is centered on the eye-end lens
    and spot on the horizon glass the scope is looking at.  Keeping the entry
    point centered and using either extreme of the horizon glass/mirror juncture
    changed the point separation by >5 mm at 40 ft.
    3.  A zero-mag sight tube can produce sharp spikes if the beam hits the
    mirror/glass juncture dead nuts on. This is useful at shorter distances
    (10-30 ft), but its value diminishes at longer distances as the spike
    enlarges proportional to beam spread spread at larger distances.  Working
    with a focused scope one can keep the spike sharp at greater distances, but
    vertical and horizontal alignment becomes increasingly demanding.
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