NavList:

Interesting.  I will try it with my Astra IIIB.  I suspect you're right about the micrometer moving slightly in the direction perpendicular to the frame under the various conditions.  For the index arm to move at all, there must be some clearance between it and the arc.

This movement would then be carried into the main angle of the index arm by the tilt of the rack teeth.

It sounds like the takeaway is that it's prudent to do IE estimates with the sextant in a similar attitude as when taking lunars.  I think I normally have the index mirror upward, and I just choose one of the two objects to place in the horizon path to make that happen; it's a little more comfortable that way.  But if the two objects are within 10 or 15 degrees of vertical, I might go the other way.  Perhaps the weight of the index arm is less of a hazard in that case.

Cheers,

Peter

ps: it should be possible to measure the movement of the index arm.  Place the sextant horizontally on a stiff flat surface, then monitor the index arm with a height gauge / dial test indicator.  Gently lift the index arm and turn the micrometer the appropriate way.  Multiply by the "leverage angle" of the rack teeth and perhaps it will be on the same order as the 0.3' you're seeing.