[SJAAboard] FW: First Impressions: Scope 9 C-11 f/10 Compustar

[Jim Van Nuland]

   There is another consideration with that scope's electronics.

   The first two digits of the date are fixed at 19, so you can't
directly enter (say) 2008.  The trick is to subtract 4 from the 
real year until the result is less than 2000.  For the present 
year, enter 1996.  The resulting error is about 7 seconds of RA 
error per 4 years.  So in 2008, the error is 5.5 arc-minutes of 
pointing error, or 1/5 of the moon's diameter, in a consistent 
direction.

   The error is due to the calculation of local sidereal time 
starting from the date and time.  When the year is off by 1, the 
error is about 3 minutes of time, or 0.75 degrees.  But after 4 
years, the "leap" nearly cancels the accumulated error.  That's 
why the adjustment must be done in 4-year steps.

   One could enter a slightly incorrect time to eliminate the 
error.  As to the amount and sign of the adjustment, I leave that 
as an exercise for the reader, or as an experiment for the observer.

   Paul Barton pointed out restriction to me about 10 years ago, 
and we experimented by setting the year 4 less than real.  It 
worked fine for finding objects.  By now a small guiding 
adjustment would be needed.

Clear Skies!
-- 
Jim Van Nuland, San Jose (California) Astronomical Association
<a href="http://www.svpal.org/~jvn/ ">JVN's web site</a>