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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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