Abell 2153/The NGC 7436 Group in Pegasus

Link to annotated image

The NGC 7436 group, also known as WBL 692 or Holmberg 800, is about 340 million light-years away. In theory it’s part of the galaxy cluster Abell 2153 centered just out of this field of view, but that cluster doesn’t appear to have any bright members other than those in this image. While there are many distant background galaxies in the image, redshifts of all of the galaxies named in the annotated image are located at roughly the same distance, so they appear to be part of the cluster. I found no explanation for the seemingly sharp cutoff of the edge of NGC 7435. Presumably it interacted with one of the other galaxies at some point in the past, but it shows no sign of new star formation. NGC 7436 was discovered by William Herschel in December 1784. NGC 7433 and NGC 7435 were discovered by R.J. Mitchell in 1855 using the 72” at Birr Castle.



Exposure: Total exposure time about 20 hours, 479:30:50:40 x 2 minutes LRGB. All bin 1x1. Data collected November 4 to 14, 2020.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: Average FWHM of subs around 2.3 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 3.1.0.457
Guide SW: PHD 2.6.7, 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
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Weighting and registration with WBPP

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
(No deconvolution on this image)

4. Luminance Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Aggressive MMT for noise reduction of larger-scale messiness in the background, using an inverted, stretched and blurred luminance mask

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration, using Average Spiral Galaxy white reference

6. RGB Stretching
Histo Trans
Curves
Boost color saturation with additional Curves

7. Color Combination
LRGB Combination of luminance and RGB images to create a “Galaxy” image

8. Star Reduction
I followed Adam Block’s star reduction technique:
StarNet to create “Starless Image”
Extract two copies of luminance from main image, then apply MLT to one to create a rough star mask
Binarize to select only the stars
MorphTrans to enlarge stars
Convolution to blur star edges
Pixel Math: subtract luminance image from blurred star mask so that cores are excluded from mask, and on ly halos are represented in the mask = “Halo Mask”
Apply Halo Mask to main image, then run PixelMath to use Starless Image where halos otherwise would be
I ran this procedure twice, selecting different sizes of stars using different layers in MLT and different enlargement settings in MorphTrans.

9. Photoshop
Use CloneStamp to remove halos from Starless Image, then subtract it from main image to remove remaining messy clumps and very faint traces of horizontal banding in the background
Save as TIFF and move back into PI

9. Final
Final Histogram Transformation
Slight deconvolution to sharpen stars
ICC Profile Transform to sRGB
Resample to 50% of scale
Save as JPG
ImageSolve
ImageAnnotation (using custom catalogs for galaxy clusters and quasars)