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    Re: Polariser (photo) filter as a sextant shade - my experience.
    From: Tony Oz
    Date: 2019 Apr 27, 13:42 -0700

    Dear Ed,

    It was exactly because I liked the view of the horison through a polarised filter that I decided such an upgrade. My mistake was to (blindly) trust the adverticement. A photographic filter must simply be flat, if it is a wedge still - no problem, no photographer will ever notice that.

    C&P must have spent extra efforts guarantiing their filter's parallelism.

    This exercise proved me that I must check the IE of my sextant for EVERY combination of shades used. It very probably could be that an IE value calculated by observation of Sun's width is not the same when no shades inolved.

    By the way, I found a very sound formula for IE calculation:

    360° - (onscale_value + offscale_value)/2

    where the "onscale values" (to the left of the zero limb mark) are represented as 360°xx', and the "offscale values" are represented as 359°yy', and the xx and yy are read from the drum directly. The resulting value is to be added the the HS with its' sign (algebraically).


    60°N 30°E

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