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    Re: Calibrating a sextant scale
    From: Paul Hirose
    Date: 2007 Nov 22, 12:12 -0800

    Alexandre E Eremenko wrote:
    > 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?
    That's a good point. The test procedure for the Navy Mark 3 sextant used
    two collimators and a precision rotary table. The sextant was attached
    to the table with its index mirror at the table's rotation axis. One
    collimator was stationary and aimed at the index mirror. The other
    collimator was on the indexing table and aimed into the horizon glass.
    So in any position of the table you got a full beam of light from both
    I think a high power scope was used instead of the regular sextant
    telescope, but I can't remember for sure. It's been years since I saw
    the description of this procedure. (It was in the military specification
    for the sextant.)
    > 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.
    But a point source isn't necessary for the collimator illumination. The
    only collimator I ever used projected a resolution test pattern (sets of
    lines with different spacings). The device was basically a telescope in
    reverse. The test pattern reticle was at the focal plane of the
    objective lens, and illuminated by a light bulb from behind (where the
    eyepiece would be  in a telescope). The distance between the reticle and
    objective lens was carefully adjusted to focus the output at infinity.
    That is, the diverging rays from any point on the reticle would emerge
    from the objective as parallel rays.
    A reflecting gunsight in a fighter plane is a type of collimator. I've
    seen the instructions (ca. 1952) for testing one of these sights for
    correct focus. You moved your head left and right in order to observe
    the reticle across the entire field of view, while watching for any
    movement of the reticle with respect to a fixed object far away. If
    there was movement, the sight was not emitting parallel light and needed
    a focus adjustment.
    > Thank you. It is encouraging to know that someone reads
    > my postings. The "ultimate accuracy" of sextant observations
    > seems
    > to be too exotic subject, even on this list:-)
    Just because people don't reply, it doesn't mean they're ignoring your
    postings. I always read your messages with interest.
    I block messages that contain attachments or HTML.
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