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    Polariser (photo) filter as a sextant shade - my experience.
    From: Tony Oz
    Date: 2019 Apr 27, 02:05 -0700

    Hello.

    Being an "armchair navigator" eager to "improve" the result of long history of sextant design I replaced the lightest horison shade with a polariser filter. The idea was to obtain the possibility to enhance the Moon-sky contrast (for lunars) and to more adaptively remove the glare off the seas. The filter used was a professional grade photographic piece by a reputable manufacturer.

    The test result: a complete failure.

    Though the filter surfaces are flat they are NOT parallel! As I was rotating the filter to accomodate to the glare - there was ±3 arcminutes image shift introduced. The filter occured to be a sort of an optical wedge/prism.

    Further I wanted to also use a pair of such (linear-polarising, not circular!) filters as an adjustable shade - now that is out of question!

    Warm regards,
    Tony

    60°N 30°E

       
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