Thanks for taking the time to upload the data Andy - it's very useful!
Below is what I came up with, using fairly simple workflow - tweak to taste obviously.
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- m31stack.jpg (377.69 KiB) Viewed 10866 times
The good news is that I was able to recover mostly useful colouring - see workflow below.
--- Auto Develop
To see what we got. We can see a blue bias (typical). Oversampling (not unusual with DSLR data), some field flattening issues (elongation towards the corners). Stacking artifacts are also readily visible.
--- Bin
Firstly I bin to improve noise and make use of the oversampling.
Parameter [Scale] set to [(scale/noise reduction 50.00%)/(400.00%)/(+2.00 bits)]
--- Lens
In your case, the field isn't entirely flat. The Lens module can help in some instances and does a pretty decent job here too (one caveat being that diffraction spikes are no longer straight but slightly curvbed). A side effect of using the Lens module is that it (necessarily) cuts of some of the borders to make the data fill the whole image again ('Auto crop'). This has gotten rid of most of the stacking artifacts for us (bonus!), though a little bit remains.
Parameter [Center Y] set to [73 pixels]
Parameter [Curvature Linked] set to [125.42 %]
--- Crop
Getting of remaining artifacts and framing M31 to have its core in the center of the image.
Parameter [X1] set to [16 pixels]
Parameter [Y1] set to [9 pixels]
Parameter [X2] set to [1613 pixels (-50)]
Parameter [Y2] set to [1062 pixels (-52)]
--- Wipe
I roughly mask out M31, so that Wipe won't 'wipe' away any parts of M31. This is especially important at higher Aggressiveness settings.
A 'default' wipe yielded some issues in the corners. I don't know what the cause of this is - are your flats ok? That's why I used the Vignetting preset.
Parameter [Dark Anomaly Filter] set to [4 pixels]
Parameter [Corner Aggressiveness] set to [99 %]
Parameter [Aggressiveness] set to [75 %]
--- Auto Develop
For final stretch. I used an ROI over part of M31
Parameter [Ignore Fine Detail <] set to [2.4 pixels]
Parameter [Outside ROI Influence] set to [8 %]
--- Contrast
Default values, except Parameter [Dark Anomaly Filter] set to [3 pixels]
--- Deconvolution
Automatically generated mask.
Parameter [Radius] set to [1.8 pixels]
--- Wavelet Sharpen
Using same mask that Decon created.
Parameter [Amount] set to [242 %]
Parameter [Small Detail Bias] set to [93 %]
--- Color
Here's where it gets interesting...
I always apply the Color module towards the end (so it can recover the colour hue and saturation that was otherwise mangled by the luminance stretching).
As a matter of fact, it comes up with a pretty good color balance all by itself.
For M31, things to look out for are a yellowish core (present!), bluer outer rim (present!), some purple (or pinkish) HII areas dotted around the disk (also present!), and brown dust lanes (present!). This is not bad at all for LP filtered data. The only issue that betrays the LP filter is the star colors which are either blue or orange with few other temperatures in between. It's not a massive issue though. (as a side note, there are ways to get 'correct' colors and use an LP filter - do let me know if you're interested)
The only thing I had to really do is pull back on the green a little; looking at the image in the MaxRGB mode, we can see that the default values that ST came up with leave some areas that green-dominant. This is almost always a sign that the image is too green, as it is very rare to have such areas in outer space.
I setteled on Parameter [Green Bias Reduce] set to [1.49]
I upped the [Dark Saturation] parameter to about [6.80]; this introduces a bit more color saturation into the darker areas of the image.
Finally, I set Parameter [Cap Green] to [To Yellow], to remove any last bit of spurious green.
--- Wavelet De-Noise
To taste.
Again, this is a simple workflow and you should obviously tweak anything to taste. The big takeway is (hopefully) the usage of the Color module and what to look out for.
Hope this helps and happy holidays!