Thanks Stephen. I don't actually know why UV filters do that to night images, and I'd be very interested if someone has the answer. I don't have any examples myself because I was warned about it ahead of time by a pro, so I always shoot with bare glass at night. However I did see a photo taken by somebody else, who didn't know about this problem and left his filter on. He was pretty dejected that all his aurora shots for that night were wrecked.