Sharpless 2-235 in Auriga

Sh2-235 is an emission region in Auriga. It’s rarely imaged on its own, but there are images with its much larger and fainter neighbors Sh2-231 and -232. Sh2-235 likely is brighter because it's being lit up by the 11th magnitude O9 star BD+35 1201 at its core. Trailing to the left of the main nebula are Sh2-235A, B and C. A was at one time thought to be a planetary nebula, and has the designation PP40. This is a tricolor narrowband image, though the OIII channel was very weak.

Exposure: Total exposure time about 29 hours, 29:26:32 SII:Ha:OIII. All bin 1x1. Captured in December 2018 and January 2019.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of subs about 2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% reduction)

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 5nm Ha/SII, 3nm OIII, 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: PHD2.6.6, 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 1.85

1. Calibration
BatchPreProcessing with flats, darks and bias, using Cosmetic Correction with master dark

2. Stack and Mure Denoise
Blink to preview and reject a few frames
Subframe Selector to confirm selections
Image Integration
Mure Denoise
Dynamic Crop

3. Linear Processing
Dynamic Background Extraction
Deconvolution with Dynamic PSF and star mask (on Ha and SII only)

4. Stretching
Histo Trans
Curves Trans
Ha only: Exponential Trans on background, using a mask
Ha and SII only: Local Histogram Equalization on core
Noise Reduction following David Ault’s Astro Imaging Channel tutorial:
TGV Denoise
Aggressive Multiscale Median Transform to remove lumpiness in background, using an inverted and blurred range mask to protect highlights

5. Color merge
Merged narrowband channels in PixelMath, mapping SII:Ha:OIII to R:G:B
Color shifting toward blue-gold:
Color Mask script (magenta mask to remove magenta halos, then green, cyan and yellow)
Curves to shift colors, using each mask
Mild Local Histogram Equalization in bright areas to enhance contrast

6. Star Reduction
Star removal, following David Ault’s technique
Create version of image with less stretching to emphasize stars
With PixelMath command, add star image to starless nebula to star image

7. Photoshop
Repair remaining slight gradients in three corners:
Create artificial flat by removing stars
Subtract flat from main image, masking into corners with gradient tool
Repair irregular-shaped halo around brightest star with burn and dodge tools

8. Final
Mild Curves Transformation
Slight TGVDenoise to remove remaining noise in bright core
Multiscale Median Transform to remove remaining lumpiness on left side (using mask made in Photoshop)
sRGB
Downsize 50% and save as JPG