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Re: Celestaire Astra IIIB backlash
From: Peter Monta
Date: 2020 Jun 14, 14:23 -0700
From: Peter Monta
Date: 2020 Jun 14, 14:23 -0700
Can you feel the backlash when turning the micrometer? When you reverse direction on the micrometer, is there any difference in feel for the first two arcminutes? One might expect a slightly freer motion, that is, slightly less torque, and then more torque needed as the screw re-engages.
There are two springs at work here: the spring that holds the micrometer assembly against the rack, and the spring that preloads the screw against motion along the arc. It's the latter that plays the most role in backlash---if it's completely loose, there might be an effect like this. Variations in this mechanism are illustrated in Bill Morris's sextant book; they all seem pretty similar, except those few that don't have a spring and rely only on a good fit. (The Astra doesn't fall in this category.)
A snapshot of my Astra is attached. The preload spring is that large chromed strap on top; the short side of the L preloads the screw to the right. Maybe that small screw on top is loose, and the spring is just floating? Can you move that spring at all by gently pushing it left and right? If so, you would want to press the spring to the right, then tighten the screw.
Maybe a light smear of oil (light machine oil or clock oil) on the arc would help (after first brushing to remove any dust). But that might end up just papering over the root cause. Might also want to look at the micrometer screw to see if there's any buildup of old oil.
Cheers,
Peter
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