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Joel Jacobs wrote:
"Take a proper Star scope  and couple it with a small size horizon mirror,
and note the results at  twilight."

I think this comment gets to the heart of the matter. In older  sextants,
telescopes rarely had large apertures. They went for small aperture  and long
focal length. The horizon mirror only needs to be as big as the  aperture of the
telescope. Navy sextants from the Second World War had small  horizon mirrors
because their telescopes were usually small in aperture.  Naturally there is
no reason to have a horizon mirror bigger than than the  telescope aperture.

And:
"Also take a sextant with small mirrors and  use what ever scope you are most
comforatble with and try some high altitude sun  sights. Note the results."

And that brings us to the index mirror. This  doesn't need to be any wider
than the horizon mirror, but it helps a lot if it's  longer (bigger in the
dimension along the index arm) because it will be  foreshortened when the sextant
is set to a large angle. You can see this by  setting the sextant to 120
degrees and then looking through the horizon glass at  the index mirror.

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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