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    Re: How flat do sextant mirrors need to be?
    From: Jean-Philippe Planas
    Date: 2009 Jan 27, 13:46 -0800
    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{at}clear.net.nz <engineer{at}clear.net.nz> wrote:
    From: engineer{at}clear.net.nz <engineer{at}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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