all o.k. no problem! Thanks for your comment! I'm thankful for your tips.
I worked on the two nights separately, first the 2 hours without a filter, then the 2.5 hours with the l-pro. Then add the two results with fitswork. The individual images were processed as follows: first slightly stretched with fitswork until something could be seen, then gradXterminator, then further stretched again with fitswork until the galaxy was clearly visible, then I went to PS and increased the colors, further very slightly stretched, later some parts of the image are slightly softened with Gaussian blur or sharpened with PS. Also tried to emphasize the dust areas of the galaxy. For the core: a bit light and shadow in PS. Then gradXterminator again. I used now starnet v2, and a starmask (with subtract in PS) too.
What strikes me again and again: there are hardly any yellow, orange and above all blue in the galaxy. That's why the galaxy keeps coming out in these weird colors. But the colors are there in the stars. I can't see that I've lost details, o.k. al little bit because of the blurring. I fear that I'm not able to correct the colors.
I tried to do a color calibration: with siril: get a histogram with blue left and right flanks, but the galaxy is even more red, the background is a bit bluer, but not the edge of the galaxy; I also tried REGIM, but again only a redder galaxy. But I have to admit, I didn't do the color calibration in the linear state like it's supposed to be done.
Maybe it's the heavy light pollution (Bortle 7-8) at my location that prevents blue in particular from coming out so weak in my gray sky. Then it surprises me, however, that the colleague in the neighboring sub-forum experienced imaging can work out the colors so well with Bortle 8.
About the reducer: despite guiding, the stars are mostly shifted by one pixel. I checked the adjustment before, with f/4 , no corrections necessary. Therefore I have to correct with the reducer everything by one pixel in both directions, not without the reducer. It is important how exactly, centrically, the reducer is fastened in the focuser with the 2 screws.
I think, in reality there is not so much blue at the edges of m31 as shown in most of the images in this forum, see this example, especially at the south edge of M31, taken in Austria under bortle 3-4 sky:
This is the first 2h file, RAW-SumStack with lights and flats, 38x3,1 min, without any filter: if you are interested...
and this is the stack of 38x4 min with the l-pro filter:
Edited by hobbyknipser, 26 September 2022 - 09:10 AM.