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.
One reason to take N frames of a subject, rather than one long exposure is that during stacking it is easier to get rid of anomalous things like airplane trails.
With N frames, simple averaging would drop the intensity of an anomaly by 1/N. That is nice, but outlier detection in the stacking algorithm - like kappa-sigma or others can improve on that.
But it obviously takes some number of frames for this to be effective. In the extreme case of N=2, the outlier detection can't really work very well. Even if it did pick the right frame to throw away, you would be throwing away half of your exposure.
My question is what is the minimum value of N where the stacking N frames with outlier detection in APP do a good job? Â
My guess is that it is on the order of N=10, but I am not sure.
The cost of N becoming large is read noise, but many modern CMOS cameras have very low read noise - so low that sky background dominates it in many cases.  Â
About 25-30 frames, give or take, for the best rejection. Less will still work, more works better, but about that amount is the minimum for an optimal correction.
ok, thanks!
