A few general comments on flats.
At first they can be a struggle, but once you figure them out, they are one of the strongest tools in the toolbox. The hole idea about flats involves some very simple algebra. If you shoot a frame of a white surface that is "perfectly" evenly illuminated, in theory it should look like that on the exposure. But it doesn't. First of all there are vignetting (the image is darker towards the edges), and there are small grains of dust in the optical path.
Lets (very simplified) say that you have white in the middle of the exposed flat frame , and 50% grey at the edges. That will be the same on a light frame of the night sky. With a stack of the flats, you can very easily compensate that. You (PixInsight in most cases) simply divide the value of each pixel in the light frame with your flat stack. In the centre of the image you divide by one (white), but at the edges you divide by 0.5 (or 50%). Back in school we learned that dividing with a fraction is the same as multiplying with the inverted fraction, so here dividing by 1/2 is the same as multiplying with 2 (or 2/1). At the edges of the image, where the chip only receives 50% of the light, that signal is now multiplied by two getting the value back to one.
This is all there is to it. If you do things right, you will have a perfectly flat field after using flat calibration. This is very important with astrophotography, as we basically try to image something that is "black", and it makes the task of enhancing the object you're imaging a lot easier if your field is flat. Some people don't use them, but especially with large chips (like DSLR) they are invaluable.
I normally shoot flats using the sky or even a white wall or the ceiling. I cover the front of the telescope with a peace of white cloth, and use a large rubber band to hold the cloth in place and even out wrinkles. It's is very important not to change focus (or anything else in the light path) between the time you shoot lights and flats. If you change focus, the dust bunnies will move, and after flat calibration you will see them as embossed circles. Also focusing the telescope on something lightyears away, will make the wall/sky/ceiling defocused while you shoot the flats and that is good. Look at the histogram while shooting and find an exposure time which creates a spike a little bit to the right of the centre of the histogram. Your flat frames should look grey without stretching them. Not burned out, and not too dark.
Another important thing about flats is that they introduce noise (and we don't like noise!). Typically exposure time of each flat frame is well below one second, and that gives a lot of noise. The are several ways around that. Shoot many flats. I normally shoot 100-200, but recently I've started denoising the flat stack as well, using TGVDenoise with default settings. Especially if you don't use dithering on the mount this has a major positive effect. Even if you use dithering, you'll see an increase in SNR, by denoising the flat stack. You don't have any usable information in the flat stack on pixel level, except for uneven sensitivity of each pixel, and my experience (based on measuring SNR) is that it's better to have a noise free flat stack.
A very easy way to check wether you are doing things right, is to try to calibrate a series of flats using your flat stack. That should give a perfectly flat (grey) frame if you are doing it the proper way.
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