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Re: Sextant arc length
From: Peter Monta
Date: 2015 Sep 18, 11:24 -0700
From: Peter Monta
Date: 2015 Sep 18, 11:24 -0700
I tried some very long Lunars (up to 137d) with my SNO-T and backsights with a pocket sextant (up to 140d). This is very tricky.
I remember playing around with large angles on my Astra IIIB (inspired by the story about Cook's astronomer taking a large lunar distance) and one thing I noticed is that the index mirror gets vignetted down to a tiny sliver---that is, when looking at the index mirror from the point of view of the horizon mirror, it is foreshortened because of the glancing angle. Might be better to go with an auxiliary pentaprism when taking sights larger than 110 degrees or so. That's more hassle, though.
The glancing angle has a couple of consequences: small mirror aberrations get translated into increasingly bad astigmatism, and resolving power is hurt in the direction of the short side of the mirror, that is, in the angular direction that's of primary interest.
Cheers,
Peter
