What might be the cause of the problem around the bright stars? The image of M45 is 37x10min sub-exposures using Astrodon Ha filter. The moon was still bright last night 11-16-08, but the dome probably shielded from direct moonlight.
Any suggestions?
Dave Bailey

Hi Dave,

You don't say what kind of camera this is. If memory serves, I used to see something quite similar on an ST8. In that case, the pattern was thought to be due to diffraction from the metallization interconnects between the pixels in the sensor itself.

As you know, the stars of M45 are quite bright. I used shorter exposures and a rotation and combine technique to eliminate most of the artifacts. You can see hints of what you are seeing in the link on the unrotated image here: http://www.hiddenloft.com/M/M45rot.htm

It is an old image but I think it shows the effect you are seeing.