After some time with deep sky imaging with CMOS cameras I need to say that calibration for this kind of imaging is even more important, than in CCD cameras. The reason for this is amp glow that comes up for long exposure astrophotography. For cooled CCD cameras one can live without bias calibration, also without flats, can remove hot pixels with dithering. But amp glow in CMOS cameras can be removed effectively only with dark calibration.
If you worked before with CCD cameras then as a first iteration you may calibrate images as usual – using bias, dark and flat frames. But the most important information for CMOS calibration is, that you must have calibration frames for each combination of:
- gain and offset settings
- USB traffic settings
So it is good to choose and adjust some optimum settings preset that we will use for some time and we don’t need to create calibration frames too often. Calibrating CMOS images in classic way works just fine if our light frames have corresponding calibration frames. We should not rely on dark scaling for CMOS calibration, amp glow does not scale good. So if we change exposure time or temperature or gain for light frames and there is no corresponding dark frames in calibration library we must make them and use them. That is why it is good idea to have also dark flats, especially if we need to expose flat frames for several seconds (like for narrowbands).
But when we are sure we do not use scaling there is no actual point in having bias frames. And here comes another calibration approach, that do not use bias frames. In this scenario for calibration long exposure images we use:
- bunch of dark frames (they already contain bias)
- some flat frames (to get rid of vignette and dust donuts)
- bunch of dark flat frames with settings the same as for flats
What about the actual numbers? The more the better, but in general we should have no less than 10 dark frames, 20-40 will be good (that is the number I use). For some cases when we have many light frames (like I had a few times almost 100 light frames, and each was 3 minutes) then it may have sense to increase the number of corresponding dark frames. Some guides advise to have as many darks as we have light frames, and that will work of course as well. Making dark frames can be pretty time consuming, and that’s why it is better to find optimum settings and use them as much as possible, so we do not need to create new calibration set once we decide that our settings are wrong and we need to adjust them. For flats 10-20 frames should be enough.
And then if our processing software can automate the process (like MaxIm DL) then all we need to do is to put all frames to the library and let the program do the work. If we need to calibrate manually, then we need to: create master darks from dark frames, calibrate flat frames with corresponding master dark, create master flat from calibrated flats and then calibrate light frames with master dark and master flat.
I used classic way of calibration for over the year and then tried this “biasless” approach and must say it works pretty well. One may ask “why to get rid of biases – they cost almost no time!”. Well, they cost disk space, because we should have many of them. And they also introduce some small amount of noise, very small, but still. But the main advantage I noticed is that I have no more problems with wrong flats that do not remove vignette correctly, especially for narrowband imaging under light polluted sky.
More on this topics in the following entries: