Yes I noticed that, and it started here
http://ccdware.infopop.cc/eve/forums...31/m/428101533 very interesting.
Paul. You stack large numbers of exposures exactly the way I do and I find it works pretty well. If your ADUs are too low and dont overcome read noise, then read noise will add up with the number of subs and the noise wont decrease, althought I have lately stacked subs with very low Ha ADUs and noise did go down.
With lots of noisy subs, poisson data reject works far better than sigma. Its more aggressive, and can distort stars, but stars tend to average out OK with large stacks.
Also increase the reject percentage, I usually use 2 but 5 or even 10 can improve things, again, with large stacks to compensate for increased data loss.
If your guiding is very good, eg with internal or OAG, I found bad pixel colums and hot pixels are in the same place on every sub and they arnt removed. Dithering fixed that totally.
Mean combine rejects bad data by definition, the manual says you dont need to do data reject 1st (althogh you can, I do). If you find noise get worse using data reject and mean combine, then I would guess the settings or method of data reject you use is not optimum.