Galaxy Cluster Abell 76 in Pisces

Link to annotated image

Abell 76 is a fairly dense grouping of mostly elliptical and lenticular galaxies in southern Pisces. Redshift measurements indicate that it’s located about 540 million light years away and is about 40 million light years across. IC 1565 is the brightest member, at mag 14.4, while the remainder are no brighter than magnitude 15.5. Although this image captures the brightest members, the cluster extends well beyond this frame and includes Hickson Compact Group 5, a bit north of this image. I came across this target in Alvin Huey’s Abell Cluster guide.

There may be more than one galaxy cluster here. The official center of Abell 76 is in the clump of small galaxies just left of IC 1565. But I found no redshifts for any of these galaxies, so perhaps they constitute an independent and much more distant galaxy cluster in the background. There is another galaxy cluster cataloged at about the same location – NSC J003954+064638 from the Northern Sky Optical Catalog, published in 2009 – but the NASA Extragalactic Database for that target reports a redshift that would place it only 340 million light years away, so that seems incorrect. NED also gives redshift information for several star-like specks – which it labels as galaxies – in the same area, none of which appear to be brighter than about magnitude 21. These objects were part of the Sloan Digital Sky Survey Data Release 13 in December 2017, and I assume they were surveyed because Abell 76 is here, but the reported redshifts place them at about 5 billion light years. I’m not sure how to put all this seemingly contradictory information together.



Exposure: Total exposure time 18.4 hours, 409:43:47:53x 2 minutes LRGB. All bin 1x1. Data collected in December 2020 and January 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance around 2.8 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
Deconvolution:
Dynamic PSF to create PSF image
Deconvolution, using a mask created in Photoshop so that only a few detailed spirals were sharpened

4. Luminance Stretching
Histo Trans x 5
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform, using an inverted luminance mask, to remove lumpiness in background

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

6. RGB Stretching
Histo Trans
Boost color saturation with Curves
Curves Trans
TGV Denoise
Aggressive Multiscale Median Transform, using an inverted luminance mask, to remove lumpiness in background

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

8. Background Subtraction
StarNet++ to create starless image
In Photoshop, use the Healing Brush and CloneStamp tools to remove halos from the starless image, leaving only the background
Subtract the starless image from the Galaxy image to remove remaining messy clumps in the background
Save as TIFF and move “Galaxy Image” back into PI

9. Star Reduction
I followed Adam Block’s star reduction technique:
StarNet to create a new “Starless Image”
Extract two copies of luminance from the Galaxy 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 Galaxy image, then run PixelMath to use Starless Image where halos otherwise would be

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