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    Re: Basics of computing sunrise/sunset
    From: Hein Bodahl
    Date: 2009 Jun 19, 10:47 +0200

    On 18.06.2009 19:49, Brad Morris wrote:
    > Hi Douglass
    >
    > I use a sextant from about 1920.  A wonderful device, identical to the one 
    used by Worsley on his famous boat journey.
    >
    > Should I be worried about the filtering qualities of my shade glasses?
    >
    > Would there be a way to check to see if my shade glasses are up to the mark?
    >
    > Best Regards
    > Brad
    
    If you got a digital camera you could probably use a IR-pass filter to
    take a picture of the sextant filter with a sunlit background. If it
    blocks out IR the sextant filter would turn up dark in the picture on
    long exposures. IR-pass filters go for about than $15 on ebay. I guess
    650 or 720nm is a good choice.
    
    Hein
    
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