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    Re: How flat do sextant mirrors need to be?
    From: Ken Gebhart
    Date: 2009 Jan 27, 22:10 -0600
    Actually, I cannot be of much help here.  We took on the sextants back in 1986 based on the perceived quality extant in the factory.  At the time they were testing the flatness by observing fresnel patterns on the mirrors, which to my understanding gave them a sort of topographical view of the landscape of the flatness.  They had their own specs on how they should look, and what would be unacceptable.  If unacceptable, they went back to a grinding machine for improvement.

    Ken
    On Jan 27, 2009, at 3:46 PM, jean-philippe planas wrote:

    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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