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    Re: Compact Lightweight Metal Sextants - Freiberger Yacht vs. Astra IIIB
    From: Bill Morris
    Date: 2012 Aug 10, 13:39 -0700

    Greg and David

    Provided that the sides of the glass are parallel, the entering and leaving rays are parallel, that is to say, light comes straight through without angular deviation, but it is displaced laterally with a displacement d = ti(n-1)/n, where t is the glass thickness, i is the angle of incidence and n is the refractive index. This lateral displacement does not matter a bit, except to designers of optical micrometers.

    The angle of incidence on the horizon mirror is about 18 degrees in a typical sextant, so about 8 or 9 percent of the light gets reflected from the unsilvered glass surfaces (it depends a little on the refractive index and more on the angle of incidence).

    Yesterday, when the sun made a brief appearance I compared the image overlap using Galilean telescopes (as in the Yacht and most other modern sextants). With the "full glass" horizon mirror of the Trommelsextant there is an overlap of reflected and direct images of about 2 1/2 sun diameters, compared to about 1 sun diameter with the half glass of the Yacht sextant. The position is different with Keplerian telescopes, i.e inverting or prismatic telescopes, but that's another story.

    Freiberger Prazisionsmechanic began to make survey equipment in 1791 and was a notable maker of mining survey equipment, as suggested by their trade mark, which shows a tunnel portal, mountains and two stars. As far as I can discover, they made no sextants until after WWII.

    Bill Morris
    New Zealand
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