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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.

Dick

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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