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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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