Hi Simon, due to the mount method (altitude/azimuth rather than equatorial) the field of view appears to rotate over time. It is possible to use a de-rotator but I believe these are quite expensive. It could end up cheaper to buy a decent equatorial mount.
Unless of course it is a very large Dob (12"+) in which case the de-rotator would make sense.
See Gary Kopff's post (#5) in this thread:
http://www.iceinspace.com.au/forum/s...ad.php?t=73798