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In a message posted earlier today, I wrote the following, about the sizing
of sextant index mirrors-

>the allowance that Frank rightly emphasises, in-
>
>>... the index mirror ... 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.
>
>For a sextant which is designed with optimal geometry, the enhancemant in
>the length of the index mirror should be  (1 / cos 30), or 15%, which
>allows for that foreshortening over the full range of observed altitudes
>from 0 to 120 degrees.

=====================

That figure of 15% was based on sloppy thinking on my part, and I would
like to withdraw it, please. At high altitudes, to keep the apparent
vertical aperture of the index mirror to be effectively equal to its
required diameter would call for a much greater enhancement than 15% in its
lengthwise dimension, and would require a quite-impractical length for that
mirror. Instead, the sextant user just has to accept that at high
altitudes, any practical index mirror will result in a field of view shaped
like the horizontal slot of a letter-box. For that reason, there is indeed
a case to be made for fitting a long (but not wide) index mirror; longer by
much more than my 15%.

Sorry about that.

=====================

I stand by the arguments in the rest of that posting, however.

George.
===============================================================
Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
or from within UK 01865 820222.
Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
5HX, UK.

   

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