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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
    
    =====================================================
    
    From:               
    To:                 
    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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