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    Re: Sextants with Polarizing filters
    From: Bill B
    Date: 2006 Jan 26, 02:18 -0500

    > Along the way to testing this for myself, one practical problem was
    > immediately apparent when 3 polarizing filters were laid side by side on a
    > lightbox (a sheet of well-lit white paper will do):
    
    A caution if trying this at home. If using a "well-lit" sheet of white paper
    (front instead of back lighted) and laying the filter on the paper, the
    density/color shift of a normal filter will double.  Light has to pass
    through the filter to reach the paper, then the light reaching the paper is
    reflected by the paper--back through the filter--to reach the observer.  How
    might this work with a polarizing filter? I am fearful of hazarding a guess.
    
    > one alone: about 2 f/ stops
    > two aligned for max. transmission: about 3 f/ stops.
    
    Tried your method on a professional light table, thinking the angle of the
    incandescent lighting might have partially polarized the light.  Same
    results. 1.5 and 2.  These filters are perhaps 30 years old, so...
    
    Supposedly, the polarizing filters do not change transmission from tungsten
    to daylight, unlike colored filters or the "proportional" Mired shift.
    
    Bill
    
    PS
    
    While we are on optical, how the heck can folks produce a prime 35mm
    telephoto lens with very little flare, chromatic or spherical aberration,
    little pin-cushion or barrel distortion, plus aperture control and mount for
    a few hundred; yet produce a monocular for a $1400 sextant with the optical
    quality of a chrome hubcap that shifts side error/IE by several tenths of a
    minute if you refocus it?  The sextant can measure 0.1 to 0.2 of a minute of
    and arc--0.00000463 to 0.000000259 of a circle, and the optics can shift the
    image by more than that!
    
    Bill
    
    
    

       
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