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    Re: How flat do sextant mirrors need to be?
    From: W F Jones
    Date: 2009 Jan 28, 08:34 -0500

    From a practical perspective, the mirror must be flat enough to permit sharp
    images over the full range of measurements.  A backsight measurement is a
    good test I propose for gaining some sense of how good your mirror is.  A star
    image must not blur.  I had a set of mirrors in the past rejected during a
    sextant re-certification process because the orignal mirror's fasteners were
    too stiff and caused enough surface distortion to prevent collimation at the
    higher angles.  I should not comment further on this matter since my
    knowledge is quite limited in this matter.  Sorry, you don't get much for two
    cents these days.
    Frank J.
    Rochester, NY
    Subject:            [NavList 7172] How flat do sextant mirrors need to be?
    Date sent:          Mon, 26 Jan 2009 22:20:22 -0800
    Priority:           normal
    Send reply to:      NavList@fer3.com
    > 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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