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    Re: Calibrating a sextant scale
    From: Paul Hirose
    Date: 2007 Nov 20, 12:03 -0800

    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.
    The subject of star separation angles has been discussed on the list, so
    I'm sure several programs exist to compute accurate values. Here's a
    quick and dirty one that I wrote:
    The whole package is free, but it's no good unless you have a Windows
    machine and a C or C++ compiler. And there's no user interface to speak
    of. To change input values you have to edit the source code and
    recompile. However, the program is pretty thorough. It corrects for
    effects such as proper motion, aberration (not spherical aberration --
    that's something else), and refraction. In the example, refraction
    decreases the Deneb - Vega separation angle by 30 seconds of arc.
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