Re: ATM Alt-alt telescope mount - and image rotation

[MLThiebaux <mlt@ns.sympatico.ca>]

Here's a formula for this rotation rate:

We can think of this system as an equatorial mounting with the polar axis grossly misoriented.  Say the polar axis is pointing at a fixed point T in the sky with declination  t. (If Dobsonian, read azimuth axis instead of polar axis). 

Suppose we are tracking a star with declination  s.  Let  H  be the hour angle of the star minus the fixed hour angle of T.  H is increasing at the constant rate  w = 15 deg/hr.

Let A = cos(s)tan(t)  and   B = sin(s).
Then the apparent rotation rate in the field of view of the telescope is

	w(B-AcosH)/[(sinH)^2 +(A-BcosH)^2)].

Note that the rate is time-varying; a positive rate corresponds to a counter-clockwise rotation in a 2-mirror telescope.

Martial Thiebaux
Rawdon Hills, Nova Scotia