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    Re: Simplified Bris Sextant
    From: Greg Rudzinski
    Date: 2012 May 10, 12:25 -0700

    Alex,

    If you looked at the graph that I attached on a previous post you will see two formula. Each as different as they are produces the same minutes of arc per pixel for my 50 mm lens for any pixel value through the lens field. For technical math details I will have to refer you to Marcel Tschudin who helped me with this project.

    The math stuff actually came later. The first accurate functioning graph was done (without formula) by taking Sun/Horizon images every 2 minutes of GMT time for the full field of view (the camera records gmt to the second for each image automatically) then determining what the sextant altitude should be for each image to get a zero intercept for the location and height of eye of the photographer . Each reduced zero intercept altitude is divided by the measured pixels from Sun to horizon to get minutes of arc per pixel. The minutes of arc per pixel values are graphed vs. the measured pixels. Once a graph is completed then any measured pixel value can be used to get a minutes of arc per pixel value which is to be then multiplied by the measured pixels to get an Hs altitude in minutes of arc. Sorry but that's the best I can explain it.

    Greg Rudzinski

    P.S. A cheat sheet can be made from the finished graph for field use. See attached image of cheat sheet.

    [NavList] Re: Simplified Bris Sextant
    From: Alexandre Eremenko
    Date: 10 May 2012 13:57
    Greg,

    > Lens distortions and geometry are compensated for.

    I don't understand how.
    You cannot have a faithful picture of a piece of the sphere
    on the plane. This is a mathematical theorem.
    There is absolutely no way to measure the angles on celestial sphere
    by measuring the distances on a flat photograph.
    (I mean to have the same scale in all places of the picture).

    Alex.

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