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    Re: Just getting started
    From: Fred Hebard
    Date: 2007 Jan 18, 09:35 -0500

    Gary,
    
    I believe objects for adjusting the index correction have to be
    several miles away before parallax is insignificant.  You could work
    it out as the distance between the index and horizon mirrors being a
    chord on a circle.  The angle subtended by the chord at the center of
    the circle would need to be less than 0.1 arcminutes.  So find the
    radius of the circle that determines this.  This is a fairly simple
    problem in plane geometry; easier than spherical geometry.
    
    The lake may also not be large enough to give a nice horizon (several
    miles).  There is a dip short table in Bowditch to accommodate this
    problem.  Bowditch is available on line, I believe at the U.S. Naval
    Observatory web site.
    
    Fred
    
    
    On Jan 18, 2007, at 9:00 AM, Gary wrote:
    
    >
    > Many Thanks to everyone for the welcome.  I find the challenge of cel.
    > extremely exciting and have been working through sun sight problems in
    > getting started.  I purchased an Astra IIIB and this weekend when I
    > get
    > out to the lake will be taking the first actual sights.  Having just
    > received my sextant on Monday in the mail I have noticed that making
    > the adjustments seems to depend upon the distance of the object.  If I
    > am adjusting basis the back of the couch I look at the rooftop across
    > the street and things are out of adjustment.  Is there a certain
    > distance you should use for making these adjustments?  figured I would
    > just do it at the lake for sure.
    
    
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