My standard workflow was gleaned (by me) from reading 'Inside PixInsight'.
Warren Keller wrote (1st edition, p. 15, 16):
"Just as the light frames should be bias and dark subtracted, so should the flats. Due to their short exposures of a bright light source, flats are relatively noise-free. A master bias can therefore be used as what's called a flat-dark. Some imagers take specific flat-darks that match the exposure time of the flats themselves. As we'll see, PixInsight recommencds a different method than either of these." He then describes the calibration of the flats with the normal MasterDark and MasterBias using dark frame optimization.
So he describes three different methods of calibrating the flat frames:
a) with MasterBias, not using dark frame optimization,
b) with MasterFlat-Dark, not using dark frame optimization (both 'Calibrate' and 'Optimize' unchecked),
c) with MasterBias and normal MasterDark, using dark frame optimization (both 'Calibrate' and 'Optimize' checked).
The MasterDark or the MasterFlat-Dark shall not be pre-calibrated in these examples (i.e. they still contain the the bias offset).
I also just saw this line in the tutorial; 'Don't waste time acquiring dark-flat frames: IC will rescale the master dark to match the dark noise in your flat frames.'
If your camera exhibits strong "amp-glow", it is not a good idea to take approach c) and scale the MasterDark (= use dark frame optimization). In this case it is also not advisable to use dark frame optimization for the calibration of the light frames. When you don't use dark frame optimization at all, you will not need bias frames and MasterBias. So the capturing of dark-flat frames is no wasting of time at all, you capture flat-darks instead of bias frames. Also note that some CMOS cameras give rise to issues with bias frames / MasterBias.
Do you know where would I find info about the dependency issue regarding camera/ and flat frame exposure times?
So what is the optimal approach? It depends, it is reasonable to differentiate 3 cases:
1) If your camera exhibits strong "amp glow" (e.g. some CMOS cameras like ASI183 or ASI294),
I suggest not to use dark frame optimization at all. This means: you will not need a Master Bias. Take flat frames, dark frames and flat-darks. Integrate the dark frames to the MasterDark and the flat-darks to the MasterFlat-Dark. Calibrate the flat frames with the MasterFlat-Dark and integrate the calibrated flat frames to MasterFlat. Calibrate the light frames with the MasterDark (both 'Calibrate' and 'Optimize' not checked) and the MasterFlat ('Calibrate' not checked).
2) If your camera doesn't exhibit "amp glow" and is not cooled (e.g. most modern DSLR cameras),
Take approach a) (calibrate the flats with the MasterBias).
You can try whether the application of dark frame optimization for light frame calibration yields in a better integration result. DSLR cameras that are not cooled may benefit the most with this approach, cooled CCD cameras probably will not.
3) If your camera doesn't exhibit "amp glow" and is cooled (e.g. most CCD cameras),
If you capture LRGB with a monochrome camera and broad band filters or color images with an OSC camera, the flat frames usually are taken with rather short exposure time. In this case the difference between a flat-dark and a bias frame may be negligible. This might be different when you capture narrow band, due to longer exposure time.
As the case may be, I suggest to try either approach a) (calibrate the flats with the MasterBias) or c) (bias frame, dark frames, scaling the MasterDark when calibrating the flat frames).
For the light frame calibration, dark frame optimization is not expected to yield better results.
Bernd
