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    Re: measuring sextant instrument error
    From: R. Winchurch
    Date: 2000 Sep 13, 8:03 AM

    I have a Navy Mk III Mod. 1 and the manuals as well.  If you want I can
    Fax the relevant pages or if I can scan them I can attach to a note to
    this list.
    Paul Hirose wrote:
    > With regard to Bill Murdoch's question on how sextants are checked, I
    > once saw the military spec on the U.S. Navy Mark 3. The details are
    > hazy, but as I recall the accuracy check required two collimators. (A
    > collimator is an optical device which shows an artificial star or test
    > pattern, apparently focused out at infinity, when you look into its
    > objective.)
    > One collimator was stationary and aimed so its rays struck the index
    > mirror of the sextant, which was on its side and attached to a
    > precision rotary angle measuring table (something like a milling
    > machine dividing head). The sextant was positioned on the table so its
    > index mirror was at the center of rotation of the table. In this
    > manner the artificial star remained within view of index mirror as the
    > table rotated. The second collimator was attached to the table and
    > pointed into the horizon glass, so if you looked through the sextant
    > telescope you'd see its image as the "horizon".
    > To begin the test, the sextant was set to zero and the table turned
    > until the two collimator images coincided as seen in the sextant
    > telescope. The table setting was noted. Once you had the zero
    > established, you could set the index arm to say 30 deg and verify a
    > corresponding table rotation would restore coincidence. With the Mark
    > 3, this was done with and without shade glasses, at several angles.
    > Naturally, all this is out of the question for testing a home made
    > sextant (which is what Bill was asking about) unless you have access
    > to an optical lab!

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