Usually one can get away with several minutes of exposure with a ST6 because field rotation does not exceed a pixel dimension at the corners of the chip over this length of time. Close to the zenith times are shorter.
Check out http://www.efn.org/~mbartels/altaz.html for a couple of images taken unguided with a ST6 + 20" f/4.8 computerized altaz.
Mel Bartels