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To eliminate the effect of parallax in determining the index error, take
the sin of one tenth of a minute of arc (or 6 seconds of arc) the
precision to be achieved, which is 0.0000291. Take the reciprocal of
this by dividing 1 by .0000291 and you get 34377.5. Multiply the
distance between the horizon mirror and the index mirror by this number
and you will find the distance that an object must at in order to
provide an accurate index error determination. For example, if the space
is 3 inches ( one fourth of a foot) then .25 times  343775 equal 8594
feet which is 1.6 statute mile or 1.4 nautical mile. If you are only
trying to achieve one minute of arc accuracy you can do the same
computation and come up with 859 feet. And it doesn't have to be a water
horizon any straight line at the right distance will do such as a roof
line or a telephone pole.

Gary LaPook


Fred Hebard wrote:

>
> 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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