Below is a crop showing the anomaly in a few of the stars. This effect shows up on all the stars in the full image. To me it looks like the stars are not focused, but I have focused the image using two different methods (bahtinov mask and autofocus) so the focus should be very good. It is not a filter issue because the stars look the same with different filters.
Anybody have a guess what is causing this? This only started recently. A few weeks ago I completed a CCD image and the stars were nice and round and didn't show this anomaly. The only thing that changed was the temperature; it's been much colder the last couple of weeks. Is it possible this is pinched optics due to cold? My equipment is in an observatory and nothing was changed between images.