UPDATE FROM MABULA
I have had a very rough 2 months unfortunately health wise. I was struck 3x in a row with bacterial infections. The second infection occurred  after a routine hospital checkup and I became very sick. I had to rest and take a lot of antibiotics. Once I was recovering and restarting work 1 month ago, I again became sick. The infection was not yet gone, so even more antibiotics and rest was needed. Needless to say, it took a lot of my energy and I needed a lot of rest.
Finally, the infection is really gone and my energy is coming back step-by-step now and I have started work again. I am terribly sorry to have kept you waiting for support. I will address all outstanding questions and e-mails step-by step and will be on the forum daily from today.
APP 2.0.0-beta47 will come soon as well, a lot of work was already completed before I became very ill, so the release is also nearly ready for you. Beta47 will be much faster actually. Many workflows will be more than 2x faster, mosaics can even be 10x faster than before because registration really received a major boost... all compared to beta46. Â Before I release it, I will make sure that everything is working properly and then I will release it.
The bias pedestal is an ADU (Analog Digital Unit) offset in your data. This pedestal is always there, in your light, flat, dark and bias frames. So the ADU values up until this offset value is always there and it actually is not real data. All ADU values above the bias pedestal are real data, but should be interpreted as the ADU value minus the Bias pedestal.
Let's look at a master bias frame of a zwo optical asi1600mm-cool camera and it's histogram:
 The histogram shows you the value of the bias pedestal. It's almost exactly located at the peak of the histogram. Try to know this value for your camera's sensor with the ISO/gain and /or offset that you are shooting with.
- For DSLR camera's this value normally is the same for all ISO settings.
- For users of astronomical CCD/CMOS camera's, the bias pedestal corresponds to the offset and, usually, can be set manually depending on the camera's manufacturer.
In the FITS header of the master bias, created by APP, you'll find the ADU value of the bias pedestal, in this case we see:
HDU1 - CALFRAME= 'master bias'       / master bias frame                            Â
HDU1 - INSTRUME= 'ASI Camera (1)'    / instrument name                              Â
HDU1 - CFAIMAGE= 'no     '          / Color Filter Array pattern                   Â
HDU1 - MEAN   = ' 1,53E+02'         / mean of channel                              Â
HDU1 - MED    = ' 1,53E+02'         / median of channel                            Â
HDU1 - SIGMA  = ' 3,12E+00'         / standard deviation of channel                Â
HDU1 - NOISE  = ' 2,93E+00'         / MRS gaussian noise estimate of channel   Â
The median/mean values indicate it: 1,53E+02, which is a scientific notation for 153 ADUs.
To come back to:
 "So the ADU values up until this offset value is always there and it actually is not real data."
This is the fundamental reason, why this bias pedestal is so important to understand.
In light frame calibration this bias pedestal must be subtracted by either
- a Master Bias of the same ISO or gain + offset
- A Master Darks of the same ISO or gain + offset.
- Or a Master Dark of MasterBias subtracted darks + a Master Bias. both of the same ISO or gain + offset.
APP, currently, will not allow the use of a MasterBias shot with a different ISO or gain+offset than the ISO or gain+offset of your lights, since this will lead to suboptimal calibration for the read noise patterns of your sensor.
For flat frame calibration the same holds, flats need to be calibrated using:
- a Master Bias of the same ISO or gain + offset
- A Master Dark of the same ISO or gain + offset.
- Or a Master Dark of MasterBias subtracted darks + a Master Bias, both of the same ISO or gain + offset.
