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    How flat do sextant mirrors need to be?
    From: Bill Morris
    Date: 2009 Jan 26, 22:20 -0800

    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
    Navigation List archive: www.fer3.com/arc
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