In any scope where an axis does not point exactly to the celestial pole, a slow rotation of the field of view occurs. Field rotation is independent of the size of the field of view, for instance, a large scope at high power will experience the same angle of field rotation as a small rich field telescope at low power. Field rotation varies greatly over the sky, non-existent in the east and west, terrible at the zenith, and approximating the sidereal tracking rate when pointed at the meridian. A motor can be added to slowly rotate the focuser to compensate for field rotation.
For visual imaging, a field de-rotator motor is not needed. For CCD imaging, exposing for 5-20 minutes in many sections of the sky with typical chips, a field de-rotator motor is not needed. If you have a CCD chip with very large numbers of pixels, or planning to do prime focus astrophotography, then you will want to add the field de-rotator. The control program shows the field rotation in real time, so that you can plan exposures accordingly. Here's how I judge field rotation: for a CCD chip that is several hundred pixels on a side, the total number of pixels on the perimeter is roughly 1000. That means that we can tolerate a field rotation of 360 degrees/1000 resolution units, or about 1/3 degree. I watch the scope track in realtime, noting the amount of field rotation change over 10 seconds or so. If it looks like I can image for my desired exposure time, then I go ahead. If not, I wait until the object is better positioned in the sky, or adjust my exposure time.
Since the field de-rotator unit need only rotate slowly with finite steps, drive and motor requirements are much more modest than the telescope's main axis drives. A simple single chip driver circuit is all that is needed, with step and direction inputs. To keep vibration minimized, I suggest a field rotation per step size of 1 arcminute. This is also fine enough to prevent field rotation from showing on fine grained 35mm film. For a stepper motor field de-rotator, use the MC3479 chip in place of the discontinued SAA1042 chip.