NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2012 Apr 1, 16:38 -0700
Ken
you wrote:
>...switching the arm (alidade), or micrometer drum would
completely negate the original test certificate.<
I'm not sure I can agree with this. The accuracy of a micrometer sextant resides mainly in the centring of the rack and the division of the teeth. Dismantling a SNO-T cannot disturb these, as the bearing is machined directly into the frame. Periodic pitch errors in the micrometer worm might lead to minor errors of the order of arc seconds distributed around the micrometer drum and they might vary a little from worm to worm, but not I think by an amount that would be of practical importance.
I cannot see how switching the index arm could affect the instrumental accuracy.
Most other sextants have the bearing attached as a separate piece to the frame and in general I would say that this ought not to be disturbed, even though it is usually a very close fit in the frame. If it is necessary for some reason to do so, it should be replaced in the same position it occupied before. Otherwise a new centring error may be introduced.
Bill Morris
Pukenui
New Zealand
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