Mike Patrick:
your mention of "sensor tilt." I was thinking it might be related to the flattener/reducer, but I'm so new to this I wasn't sure
I probably should have said that better...What I meant was that something in the imaging train MAY have your camera sensor slightly out of square with the light path. It could be coming from anywhere. The only way that I know to fix it is with tiny "shims" that can be used to square up the camera. Not a trivial task, but not real complicated either. ASTAP or CCDInspector can look at a single sub and show you if your sensor is out of square. Easy to diagnose, typically very hard to attribute to one thing in the imaging train, but can be solved by shimming (if you're not using a rotator that is).
Also, it may be that it's so slight that it isn't worth messing with. (With my setup it was so small that I said "The heck with it...")
Tim