Sharpless 2-157, the Lobster Claw, on the Cepheus-Cassiopeia Border

Sh 2-157 is an emission nebula located about 11,000 (some sources say 8000) light years away. The nebula is illuminated up by the Wolf Rayet star WR 157, a member of the small cluster Basel 3, located just below the bright blue arc of the upper “pincer.” Other stars also light this area, including stars in the bright region at the left of the end of the upper “pincer” of the claw. This region is very bright and is often blown out in images of this object, but I kept it toned down to bring out the detail in that area. The small cluster at lower right is NGC 7510, located about 9900 light years away. At lower left, the oval-shaped object with a dark streak through it is cataloged as We 1-12, a planetary nebula, but it only shows in the Ha and SII images, not in OIII, so I doubt it’s actually a planetary. It's listed in the HASH PN database without any commentary. It's not listed at all in SIMBAD or NED.

Exposure: Total exposure time about 27.3 hours, 24:23:35 x 20 mins SII:Ha:OIII, all bin 1x1. Data collected from July to October 2023.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of each integrated channel was about 2.25 arcsecs
Image scale at capture: 0.65 arcsecs/pixel
Scale of presentation: 0.9 arcsecs/pixel (72% of full scale)

Equipment:
Scope: 12” f/4 Newtonian with 3” Riccardi-Wynne coma corrector
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.2 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: ASI6200 MM, connected via NINA’s built-in driver
Filter wheel: ZWO 7x2” EFW
Filters: Chroma 50mm unmounted LRGB
Focuser/Rotator: Moonlite Nitecrawler WR35
OAG: ZWO OAG-L
Guide camera: ASI174MM, 4 second exposures
Automation SW: NINA 2, TheSkyX
Guide SW: PHD 2.6.11
ASCOM: ASCOM 6.6 SP1
Platesolving: ASTAP, failover to Astrometry.net
Processing Software: Pixinisight, Photoshop CS2

Processing Workflow by Workspace in PixInsight 1.8.9:

1. Processing
Calibration, weighting, registration and integration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Dynamic Background Extraction on each channel
Determine PSF using the PSFImage script, and enter this into BlurXTerminator
BlurXterminator using Correct First and manual PSF on luminance and RGB
StarXterminator to remove stars from each image. Star images were set aside for use later.
NoiseXterminator on each channel

2. Stretching
Histo Trans x 3
Curves Trans x 2

3. Background Subtraction
My flats didn’t work perfectly, and the OIII in particular had dark corners with a bright edge. I used a combination of dodging and burning in Photoshop to tone these to match the surrounding areas.

4. Color Blending
Channel combination
Mild Local Histogram Equalization to enhance detail
Color Mask script to create masks for green and cyan. Using these masks, I shifted green to gold and cyan to blue using Curves Transformations
Final Curves Transformation
I made separate stretches of the some of the bright areas to keep them from being blown out, and layered them into the final image with masks in Photoshop

5. Stars
Channel combination of narrowband stars images
Plate solve (this took ages – 67,000 stars were identified!)
SpectroPhotometric Color Calibration in narrowband filters mode
Histo Trans x 2
ColorSaturation boost
Histo Trans
Curves Trans
SCNR to neutralize green, on both max and average
PixelMath to add stars to nebula image, using ((1-$T)*(Stars))+$T
Final Histo Trans to darken background

5. Final
ICC Profile Transform to sRGB
Rescale to 72%
Save final image and annotated image as JPGs