Jack2154 i’m trying to get the right exposure for my flats
When you use AutoRun in ASIAIR to capture flats, you can choose any exposure time. In addition, you can change camera gain to get finer grained exposure levels.
In any case, unless you have very severe multiple f-stops worth of vignetting, flats don't require critical exposure. Just make sure that the tails of the histogram are close to the camera noise.
In general, that means placing the peak of the histogram somewhere close to the middle to the histogram. If the histogram curve falls slower on the brighter side of the histogram, use a slightly lower exposure; and vice versa.
If you have really bad vignetting, you really need to fix the root cause, instead of using flat frames as a crutch. Even if you equalize the flatness to get a more even background, aperture vignetting will still cause stars that are vignetted to show diffraction effects -- most easily seen with bright stars that have dark spikes in the Airy disk.
Chen
