Greg wrote:
Actually, field rotation is a very complicated function of altitude and azimuth. For example, exactly at the zenith, the rotation rate is infinite.
You've discovered "Dobson's Hole".
if your field is circumpolar, the rotation stops and reverses at two points in its 'orbit' around Polaris (its eastern and western most points). The one rpd thing would only work if your field were centered exactly on the north celestial pole (because then the altitude and azimuth would remain constant).
My description was very basic. I'll only disect the difficulties if someone is actually planning to buy one ;)
We derived the whole thing in my astrophysics class at the U (rather nasty derivation, I needed help from the professor to figure it out).
Are there any astronomers on-staff these days? When I went there (over 25 years ago), the place was thick with physicists, but no astronomers. Do they offer an astronomy degree yet? (BYU wouldn't take me...something about my haircut...or lack of one at the time...) Your reminder of the nuances of a field-derotator has convinced me that an equatorial mount or platform is the path I will explore on my next large scope. One less piece of hardware to have to tote-along. I will say that if you use any design by Mel Bartels, you're off to a great start if you do decide to stick with an alt-az mount & derotator. Good luck on your studies & ATMing Greg, and thanks! C. __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/