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    Re: sextant index error measurement
    From: Bill B
    Date: 2006 Nov 03, 19:31 -0500

    Jim wrote
    
    > According to "Nicholls concise guide"vol.1,a close by object
    > or straight line should not be used to determin index error,because it
    > does not allow for sextant parallax and atmospheric refraction.I can
    > give the textbook explanation if it is of interest.
    
    Thank you Jim.
    
    In the methods proposed by Frank, George, and I think Paul we are attempting
    to null out parallax mathematically or empirically. I am still digesting
    Paul's method.
    
    The gist being that with the laser we are using two dots spaced to represent
    the distance between the scope's line of sight and the index mirror line of
    sight, so parallax is not an issue. Similar with George's lower tech method.
    
    With the laser, find the distance between the scope's line of sight and the
    index mirror line of sight (near target).  When the laser dots are the same
    distance apart a goodly distance away, the rays and therefore index and
    horizon mirror are for practical purposes parallel, which when viewing an
    object at close to infinity translates to no parallax error.  So the setting
    on the sextant is the IE.  Then for IC to paraphrase Frank, "When it is on,
    take it off; when it is off put it on."
    
    I would think refraction would be for all practical purposes identical for
    both laser beams or rays from a distant celestial object.
    
    Bill
    
    
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