Original Post - 10-22-2021:

My first run at Andromeda is a two night shoot.  The second night had the camera slip partway through the night and caused me to crop more agreesively than I had planned.  I take that as an excuse to do another shoot of this fabulous target.  Stars are a bit on the blue side, but I decided I liked it for this version.  I have another process method to try out and hope to post a non-blue stars version soon.

Update - 02-23-2023:

Back to totally revamp and fix the blue stars.  Same data, new experience and software to apply.  I was able to extend the perimeter a bit more with some mask work where the rotated frames created edges.

Target Workflow (L,R,G,B, Ha subs):

Galaxy Process:
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Blink
- Review all subs and remove visually bad frames

WBPP
- Cosmetic Correction Applied
- Pedestal added to avoid clipping in later processes = 800DN
- Register, Local Normalization, Astronomic Solution, Image Integration

Subframe Selector
- Sort by FWHM, PSF Signal Weight, and Star Count individually to weed out subs that stray far below median values

Drizzle Integration
- Use Normalization Files

Dynamic Crop
- Trim rough edges

Linear Fit
- use Statistics to find the lowest channel Median value for reference sub = R

DBE - L, R, G, B, L, and Ha separately
- Apply Subtraction with Tolerance 2.5, Smoothing Factor = 0.65

Blend Ha with R via PixelMath
- Create Ha layer to add to R
- RGB/K = ((Ha*80)-(R*7))/(80-7)
- 7 is filter value in nm
- Save as new_ha
- Add new_ha layer back to R
- $T= (new_ha-Med(new_ha))*5
- Set active window ($T) to be R
- Apply and Save Ha_R

LRGB Combination
- Uncheck L channel and load Ha_R, G, B
- Uncheck Chrominance Noise Reduction
- Save as HaRGB

Image Solver Script
- Search and set target from database = M31

Spectrophotometric Color Calibration
- White Ref = Average Spiral Galaxy
- QE Curve = Ideal QE Curve
- Set ZWO Ha_R, G, B filters
- Make very small background preview
- Set Background Neutralization->Region of Interest->From Preview->Select the background preview

BlurXTerminator - HaRGB and L separately
- Run PSFImage script and get average of the FWHMx and FWHMy values - HaRGB = 3.74, L = 3.99
- Turn off Auto PSF and set to corresponding numbers
- Check "Correct First" for HaRGB image

Bill Blanshan SCNR
- Amount = 0.30

NoiseXTerminator - HaRGB and L
- Turn on 24-bit STF lookup tables
- Apply with Defaults

StarXTerminator
- Remove stars from HaRGB and save as HaRGB_starless
- Remove stars from L image using default settings and save as L_Starless

Generalized Hyperbolic Stretch on HaRGB_Starless and L_Starless
- Multiple iterations to zero in and finalize with Linear at end

Histogram Transformation on L_Starless
- Darken Background without clipping

Curves Transformation
- Apply L_Starless as mask
- Adjust saturation, contrast and colors to taste
- Use Bill Blanshan cyan color mask to enhance galaxy colors
- Remove mask

LRGB Combination
- Apply L_Starless to HaRGB_Starless
- Save as HaLRGB_Starless

Exponential Transformation
- With L_Starless mask applied and background protected
- Apply with 0.5 amount

Curves Transformation
- With L_Starless mask applied and background protected
- Adjust saturation
- Invert mask and adjust background saturation

Range Mask
- Limit to very center core of galaxy
- Apply to HaRGB_Starless with background protected
- Adjust colors (R,G) to bring out glow

Local Histogram Equalization
- Create and apply CIE mask from HaLRGB_Starless
- Large structures - KR = 150, CL = 1.3, Amount = 1.0, 8-bit
- Small structures - KR = 50, CL = 1.3, Amount = 0.25, 8-bit


Starfield Process:
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LRGB Combination
- Reset LRGB Tool
- Uncheck L channel
- Load R, G, B frames from original DBE filter integrations
- Apply Global

Image Solver Script
- Search and set target from database = M31

Spectrophotometric Color Calibration
- White Ref = Average Spiral Galaxy
- QE Curve = Ideal QE Curve
- Set ZWO R, G, B filters
- Make very small background preview
- Set Background Neutralization->Region of Interest->From Preview->Select the background preview

StarXTerminator
- Check 'Generate Star Image' for RGB
- Generate stars from RGB image and save stars

ArcsinhStretch
- Partial stretch (about 1/3 full stretch)

GeneralHyperbolicStretch
- Multiple iterations to zero in and finalize with Linear at end

CurvesTransformation
- Adjust star saturation

PixelMath to reduce galaxy core stars
- Subtract Range mask core of starless L galaxy from stars
- RGB_stars - L_starless_rangemask


Final Image Process:
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Pixel Math to Re-add Stars
- Formula = ~((~HaLRGB_Starless)*(~RGB_Stars))

Dynamic Crop
- Final Crop to preference

CurvesTransformation
- Final color tweaks
- Use range and luminance masks