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    Re: Simplified Bris Sextant
    From: Greg Rudzinski
    Date: 2012 May 10, 09:32 -0700


    There are no problems with the camera or lens. Lens distortions and geometry are compensated for. The trouble is with the home made Bris Sextant and it's orientation relative to my camera lens. I will attach a graph that I use to get a minutes of arc per pixel multiplier for my 50 mm lens. Unfortunately it is no good for other 50 mm lenses. Another issue is over exposure which I will fix on my next set of observations by using a pair of linear polarizers with the Bris (see picture). I have done another rough calibration using the polarizers and came up with 17* 22.6'.

    Greg Rudzinski

    P.S. The camera and 50 mm lens works very good at determining angles up to 25* and is not in question for these trials. Nor is the 200 mm lens at small angles under 2*. Many hard hours went into this a few years ago.

    [NavList] Re: Simplified Bris Sextant
    From: Alexandre Eremenko
    Date: 10 May 2012 10:27

    > I only compared 6 observations by counting pixels
    > from the center of the Sun down to or up to the horizon.

    Are you saying that cunting pixels from a limb to the horizon
    gives worse results?

    > I was
    > disappointed in the inconsistencies.

    Well the camera must be distorting too.
    And it is not easy to find how much.

    And the glas might be non-flat.
    (The original Bris has very small and relatively fat glass.)

    > Another try will be made with the 50 mm lens which will give a bigger
    > field of view to better determine the reflected image low point
    > relative to the horizon.

    Remember: bigger field of view implies bigger distortion.
    A piece of sphere cannot be mapped on the plane without distortion,
    and bigger the piece bigger the distortion.
    I am afraid that measuring this distortion might be a non-trivial task.


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