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    Re: Calibrating a sextant scale
    From: Alexandre Eremenko
    Date: 2007 Nov 22, 10:46 -0500

    Dear Bill,
    1. I am very interested in the results of your testing,
    especially for SNO-T. Do you have a factory certificate
    of this sextant to compare?
    2. If I understand your first procedure correctly,
    a collimator creates a beam of parallel light rays.
    (As you say, it is "focused at infinity").
    You measure the angle between these two beams with
    a theodolite, and then with a sextant.
    Now, how wide is this beam for a typical collimator?
    I mean, if you place your collimators only few meters away
    from the theodolite, how large is the space from which
    both these beams from the collimators are visible?
    If I understand correctly you then replace the theodolite
    by a sextant; the sextant has to be roughly on the
    same place where the theodolite was.
    But the typical distance between the two mirrors of the
    sextant is few centimeters. Will you be able to catch
    the beams from both collimators into the sextant mirrors?
    In other words, how wide are these beams?
    Another problem I see with this setting is that even
    the smallest LED light is not a point source at the
    distance of few meters.
    As I understand from the journal
    description of the arrangement in the Kew observatory,
    the collimators there are of
    very large diameter, and the distance
    to them is not few meters but something like 15 meters.
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