Ken Gebhart from Celestaire could have the answer as he is likely to have specified the ASTRA IIIB sextant index mirror. JPP
--- On Tue, 1/27/09, engineer@clear.net.nz <engineer@clear.net.nz> wrote:
From: engineer@clear.net.nz <engineer@clear.net.nz> Subject: [NavList 7172] How flat do sextant mirrors need to be? To: NavList@fer3.com Date: Tuesday, January 27, 2009, 12:20 AM
I recently checked some old sextant mirrors against a tenth wave optical flat prior to resilvering them and found that the greatest deviation was a half wavelength. 3 mm modern mirror float glass compared favourably, suggesting a cheap solution to replacing deteriorated mirrors in old
instruments.
I have posted some images in my blog on my website www.sextantbooks.com
Can one of our erudite members suggest how flat the mirrors need to be on theoretical grounds? I have vague memories that people aimed for a quarter wave when making the flats of amateur Newtonian telescopes, but this may well be irrelevant.
Bill Morris
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