M31 is too large for our field of view and we didn't want to try a mosaic for it as it would probably have needed four panels so we opted to go for half of the galaxy, in this case, the half where satellite galaxy M32 is located.

My goal for this was to get to control the dynamic range so we could show as much detail as we could toward the core while still having the outskirts bright enough to show up. I also wanted to show the gradual change in the color of the galaxy from redder (well, oranger) toward the center and bluer toward the edge. A lot of my time was spent on the H and the blend to into the RGB. I went through a bunch of revision that I was not quite happy with until I got to this one.

L:
dynamic crop
DBE
blurXterminator
GHS (small stretch)
starXterminator (had to do some stretching first so SXT wouldn't take non-stellar data)
GHS
GHS (inverted stretch with Mae) for halo control
noiseXterminator

H:
dynamic crop
blurXterminator
starXterminator
noiseXterminator

RGB:
channel combination
DBE
SPCC
blurXterminator
(extract R as print for later)
GHS (small stretch)
starXterminator
noiseXterminator
GHS (finish the stretch)
convolution (using mask to blur out some halo artifacts)
LRGBCombination (adding in L)
HDRMT (7 layers, median to tame the core)
LHE (two scales, 75 and 30, very mildly applies to enhance dust lane detail)
Curves (contrast)
pixel math in NB (the continuum-subtracted h-alpha) ($T * ~R + R * mtf(~m, (mtf(m, $T) + mtf(m, nb))))
Curves (color - boost color to make it stand out a bit)
MLT (mild sharpening)
pixel math in stars

HRR:
channel combination (h,rprime,rprime)
background neutralization (using preview on background)
color calibration
pixel math ($T[0] - ($T[1] - med( $T[1]))) producing NB

Here's the website with the continuum subtraction method: https://www.nightphotons.com/guides/advanced-narrowband-combination

Overall, with the exception of the halo on the brightest foreground star, I was pretty happy with the outcome. That halo was still in there enough to be seen though now its presence is not so jarring. That's the problem with these stretches that bright up the really faint detail after extracting stars. There is enough very faint left in the image from the star that big blogs of light appear. I'm getting more practice at coping with them but I'd like to be able to avoid the issue. Still, star extraction has come a long way and I wouldn't have even attempted this (or at least not accomplished it half this well) a year ago.