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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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    Re: Horizon Shade Polariser
    From: Bill Morris
    Date: 2016 May 7, 01:58 -0700

    Concern over risks to the eyes from the sun when using a sextant is an old chestnut that has been chewed over several times before on NavList. Reflect that before sun light can reach the eye it has passed twice through the index mirror (unless first surface), twice through the horizon mirror, through various shades and through at least two lenses. Even if we postulate that polarising filters absorb no U/V, which seems to me to be inherently unlikely, that's still about 20 mm of glass. Most of the long-term damage to the eye through U/V exposure is from around 265 to 275 nm and ordinary window soda glass absorbs about 90 percent of the U/V below 300 nm. Of course, if you choose to observe the sun through an un-filtered astronomical telescope or gaze at it for long periods in a cannabis-induced haze, you can expect sun damage, though most of us would flinch and turn away.

    Bill Morris

    Pukenui

    New Zealand (U/V capital of the World)

       
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