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    Re: "Vernier acuity" of horizon IC tests
    From: Douglas Denny
    Date: 2009 Jul 7, 01:19 -0700

    "Those statistics strongly suggest (though are not yet conclusive) that
    there's a real difference between the index zeros, depending on the
    telescope that's in use. If that's really the case, can anyone suggest a
    possible physical cause? It eludes me."
    -------------
    The answer most likely is:
    
    Slight prism effect through the telescope system if not viewed directly down 
    the parallactic centre axis.  If looking slightly offset from centre-line 
    then prism effect will shift the apparent axis. It does not require much of 
    an offset from the optical centre to give a large effect of prism in powerful 
    optics.
    
    Douglas Denny.
    Chichester. England.
    
    
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