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    Re: Wharton & Field's Hydrographical Surveying
    From: Fred Hebard
    Date: 2004 May 9, 23:34 -0400

    On May 9, 2004, at 4:52 PM, Robert Eno wrote:
    
    > Fred Hebard wrote (a quote from Wharton & Field's):
    >
    >> For these observations in an artificial horizon, Wharton recommends an
    >> eypiece shade rather than the swing-out sextant shades.  He also
    >> recommends multiple eyepiece shades of varying optical density.  He
    >> discusses examination of errors caused by swing-out sextant shades.
    >>
    >
    > Robert responds,
    >
    > I don't know if I buy the bit about swing out sextant shades
    > introducing
    > errors. Perhaps this was true back when that book was written, however
    > can
    > the same be true for modern sextant shades?
    
    and Fred:
    
    You bet.
    
    I heard of an example not too long ago of a modern, "quality"
    manufacturer who shipped un-parallel shades.
    
    The eyepiece shade avoids any swinging shade effects, including ones
    that aren't there :)
    
    At appropriate altitudes, swinging shades also will reflect a pretty
    strong image of the sun right into the field of view.
    
    Fred
    
    
    

       
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