Hi Siebren,

Guide is indeed wrong here, but not in the manner you're thinking
of. The field rotation rate at the zenith is (for an instant)
_infinite_. This corresponds to the "Dobson's Hole" problem,
where, as you follow an object through the zenith, you have to
rotate the mount 180 degrees.

If, for example, you center a point at a declination equal
to your latitude, set "Zenith Up", and start animating, you'll
see that the view slowly rotates with time. The exception is
when the center passes through the zenith, whereupon it suddenly
rotates 180 degrees.

The only reason Guide tops out at 5400 degrees/minute is that
it computes the field rotation rate as follows:

(1) Find the parallactic angle one second from now.
(2) Find the parallactic angle one second ago.
(3) Take the difference, and multiply by 30 to get the field
rotation rate per minute. (I.e., find the amount of rotation
over a two-second period and expand that to cover a minute.)

In the "zenith" case, the FOV rotated 180 degrees during
those two seconds, so the average rate was 5400 degrees/minute.
The numerical method described above doesn't recognize that there
was an instantaneous rate of (infinity) degrees/minute, with
the field rotation rate being negligible during the remainder
of the two seconds.

-- Bill