Flocculent Spiral Galaxy in Leo

NGC 3521 is a flocculent spiral galaxy located 26 million light-years away in a relatively barren stretch of southernmost Leo. It has a morphological classification of SAB(rs)bc, which indicates that it is a spiral galaxy with a trace of a bar structure (SAB), a weak inner ring (rs), and moderate to loosely wound arm structure (bc). The annotated image reveals numerous mag 18-19 background galaxies with ranges from 150 million light years to more than 2 billion (the mag 18.7 galaxy, which is PGC 3286109, just below the main galaxy) and mag 20-22 quasars, some at over ten billion light years.

Exposure: Total exposure time about 26 hours, 557:70:80:77 x 2 minutes LRGB. All bin 1x1. Data collected November 2019 to February 2020.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: Average FWHM of subs around 2.4 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% of full scale)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon Type IIe LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.0.0.8
Guide SW: PHD 3.1.0, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2

Processing Workflow by Workspace in PixInsight 1.8.8:

1. Calibration
BatchPreProcessing with flats and bias, using Cosmetic Correction with master dark
Blink to preview and reject a few frames
Subframe Selector for luminance to confirm selections and weight by FWHM and SNR
StarAlign to register frames

2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Crop

3. Luminance Linear Processing
Dynamic Background Extraction
Deconvolution with PSF on luminance frame, using a mask created manually in Photoshop so that only the regions I wanted were sharpened.

4. Luminance Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred luminance frame as a mask to protect highlights
(I made a second, less stretched version to use as a luminance star image)
Slight Local Histogram Equalization to bring out contrast

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration

6. RGB Stretching
Histo Trans
Masked Stretch
Curves
Desaturate background with Curves, using an inverted Range Mask
(I made a second version using masked stretch only, to use for color in the luminance star image)

7. Color Combination
LRGB Combination of luminance and RGB images
Slightly boost blue in outer spiral arms with Curves and a mask created in Photoshop

8. Star Reduction
StarNet to remove stars. I modified the StarNet image in Photoshop so that I removed only the medium-sized stars:
a. Layer StarNet image over LRGB image
b. Select>Color Range to select only the medium-sized stars in the LRGB image
c. Adjust the selection size with Select>Expand/Contract
d. Create a mask from the selection, and slightly blur it with Gaussian Blur
e. Use the mask to mask in the StarNet image, and thus erase the medium-sized stars.

For the bright blue star, I created an 8 bit copy of the image, selected the star with the selection tool, then used Filter>Distort>Pinch by 50%, and copied and pasted the star back into the 16 bit image

An artificial flat, created in Photoshop, was used to flatten the background.

Repair comatic halos: On medium-sized stars, Filter>Radial Blur.

Then return to PI, use MorphologicalTransform to shrink stars and sharpen slightly with Deconvolution, then return to PS and mask in the now-reduced stars.

Save as TIFF and move back into PI

9. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 50% of scale
Save as JPG
ImageSolve
ImageAnnotation (using a custom catalog for quasars)