Thanks, William!

I'll fiddle for the next week; I'm desperately trying to finish M100 before it slips below the horizon for a year, so I don't want to lose imaging time (and this annoying problem doesn't seem to have any effect on the quality of images). If my fiddling doesn't solve it, I'll do your method when the moon's too full to image. And, if all else fails, I suppose I can do another Tpoint run/Super Model.

Thanks again.