iraf.net

Got a general question / mystery.I have two frames of the same object taken in succession. The stars in frame A are half as bright as those in frame B. Yet frame A has slightly better seeing, as in a lower FWHM for its stellar profiles, so that apparently is not causing the difference. When I do imstat on the two images, I find that frame A, in spite of having the dimmer stars, has somewhat brighter sky (about 720 ADU, compared to 650 for frame B.)So basically, frame A has dimmer stars, brighter sky, and better seeing. Anybody have any idea what's going on here? I had thought that perhaps there was some moon issue going on, but the moon had set sometime in the hour prior to when the first of these frames was taken.Anybody know if there some kind of cloud issue which might explain this?

I swear I remember this being asked before, this isn't a homework problem is it 8-)?In any case, you didn't post enough information about the images to say anything conclusive. The difference can be explained by many things: exposure time, filter, airmass, ccd gain as well as sky conditions like cirrus.