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    Re: Calibrating a sextant scale
    From: Fred Hebard
    Date: 2007 Nov 20, 17:57 -0500

    On Nov 20, 2007, at 12:46 PM, Ken Muldrew wrote:
    > On 20 Nov 2007 at 12:30, Fred Hebard wrote:
    >> Yes, for star-to-star distances, you have to correct for spherical
    >> aberration when clearing the observed distance, just like the
    >> moon.   It's
    >> been tried.  The problem is getting a precise star-to-star
    >> distances.  I
    >> don't believe anybody claims to have solved that  problem.  The only
    >> solution seems to be to take it to somebody who  has a calibrating
    >> machine,
    >> or roll your own if you have access to  tens or hundreds of
    >> thousands of
    >> dollars of optical equipment.
    > A clever method for measuring micrometer eccentricity is described
    > in a
    > message from a couple of years ago here:
    > http://fer3.com/arc/m2.aspx?i=026728&y=200512
    > Ken Muldrew.
    Yes, but that won't help with scale eccentricity or off-centering of
    the pivot.
    You can check to see whether the sextant frame looks to be perfectly
    flat, not that you can do this easily to high precision.  A bent
    frame is a clear indication of a sextant that won't read accurately.
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