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I prefer a traditional split mirror. My SNO-T has it.

The disadvantage of semi-reflective ones is that they absorb some light for the horizon,
and seeing the horizon in twilight observations can be a problem. (I tried some of them).

The disadvantage of half mirror is that it is usually silvered on the back
(I have never seen front-silvered horizon mirrors), and the light passing though the glass
may experience some refraction. In the case of full glass this has no effect because the light from
the horizon experiences the same refraction as it passes through the same glass.
(This is a theoretical objection. I don't think I ever tried this kind of sextants).

Given these two objections, it seems that the ideal solution would be a front silvered half mirror.
But I have never seen such arrangement, and I do not know why.

Alex.
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From: NavList@fer3.com [NavList@fer3.com]
Sent: Sunday, April 21, 2024 8:45 AM
To: eremenko@math.purdue.edu
Subject: [NavList] A POLL on horizon mirrors

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A POLL on horizon mirrors
From: Frank Reed
Date: 2024 Apr 21, 05:18 -0700

Which do you like better on a marine sextant, and why, if you have an explanation:

  *   A semi-reflective whole horizon mirror (horizon glass), like on the Davis Mk 25 and others?
  *   A traditional split horizon mirror, fully mirrored on one side, clear 
glass on the other, like on the Davis Mk 15 and nearly all sextants 
manufactured before... (when? 1975?)?
  *   Half-mirror: similar to the split horizon but with no glass at all on 
the 'clear' side, like on the Davis Mk 3 and a few others?

I used to ignore the whole horizon Davis Mk 25 but it now has a major market 
share. Gotta teach what people buy!

Frank Reed




 

   

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