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David,

There may be a small refraction difference between what is seen through naked air and what is reflected off the horizon mirror through the glass. Probably just a few tenths of a minute depending on the altitude observed. I wouldn't be too concerned unless you start doing lunar observations using a 7 power scope from a very stable land platform. The Sun image does pass through quite a bit of glass before arriving at the retina. Multiple filters, mirrors, and scope lenses. A properly performed index error check would account for any of the constant optical influences. I would worry more about height of eye, atmospheric refraction, perpendicularity of the sextant, chronometer error, and index error, than the absence of glass in the horizon mirror.

Greg Rudzinski

[NavList] Re: Compact Lightweight Metal Sextants - Freiberger Yacht vs. Astra IIIB
From: David Fish
Date: 10 Aug 2012 05:41
Greg, I may have this wrong but my impression is that on the Frieberger Yacht the physical horizon glass is half the 'normal' width but edge-to-edge silvered -- so in effect only the silvered 'half' of a traditional horizon mirror is physically there and that the clear view of the horizon is direct without intervening clear glass.

So, I was wondering if this would pose any challenges -- certainly to the extent that one gets some reflection on the non-silvered side of a traditional mirror, that would no longer be present (again if I have the Freiberger Yacht arrangement correct). I'm no optics expert, but the clear glass portion of a traditional horizon mirror must have some refractive index and this would no longer be present -- not sure if is this is negligible, a problem or a plus.
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