Oh and.....
.... I'll often set my interval rate at the bottom of the drift explorer to between 1000 and 2500ms depending on how bad the seeing is. Sometimes the longer refresh interval helps in straightening out the seeing.
Using the technique Al has described I have successfully imaged for 1800 sec with guiding only in RA, without noticeable trailing after a 45 min drift training session. Normally 15 - 20 min is enough once the technique is perfected. While doing a pointing model on my AN with Gary Kopff one night at Lostock (the year I taugh my young padawan, Grasshopper Al), we used the AN to check my polar alignment. The AN's accuracy measured to 2.5 arcsec if I remember correctly. Elevation was out by 40 arcsec and AN couldn't measure the Az error as it was too small to be measured.