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Alex-
 I would expect that any sextant will have some backlash, unless *every*
tooth in the sextant arm is cut at precisely the correct same angle on both
sides, referenced to the pivot point on the arm, and the helix on the drum
is similarly machined to exactly the same angle (bevel) on both sides, again
consistantly along the entire length.

 If there is any mismatch at any point, the drum would "rock" a bit at that
particular point, and even spring tension would not zero that out, it would
simply make the error more consistant from one time to the next. At least,
that's the way it appears to me. I know "engine turning" was a big step in
the industrial revolution  and in theory all of those angles and faces
could be perfectly uniform given good manufacturing. Still, I suppose each
manufacturer would have different tolerances allowable in this.

 Presumably as a sextant wore a bit with age, it might "polish in" and the
angles become even more uniform. Or less--if the uneven use of different
parts of the arc, wear from salt grit, etc. made a difference. Perhaps that
is a topic you could explore with Plath when you get to them.

 I have dealt with both Plath and Freiberger via email, and found both firms
to be outstanding examples of professionalism. If only every business
treated their customers the same way!

   

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