The Perseus Galaxy Cluster

The Perseus Cluster is one of the most massive objects in the Universe, and contains thousands of galaxies immersed in an enormous cloud of superheated gas. The cluster is located 250 million light years away, and anchors the north end of the Perseus-Pisces Supercluster, a filament of galaxy clusters covering 40 degrees of the sky that ends in the Pisces Cloud.

At the core of the cluster is NGC 1275, a cD-type galaxy that is also the radio source Perseus A. The blue filaments in the galaxy, which are actually located 200,000 lights years in front of it, are likely the remnants of a companion galaxy that has been torn apart and is falling into NGC 1275. Red filaments stretching away from NGC 1275, one of which is just visible to the left of the galaxy core, are due to rising bubbles of relativistic plasma generated by the central active galactic nucleus, a supermassive black hole of 800 million solar masses. Each filament holds approximately one million solar masses of gas. They are only 200 light-years wide, are often very straight, and extend for up to 20,000 light-years. NGC 1275 may also exhibit the x-ray signature of fluorescent dark matter.

Exposure: Total exposure time about 20 hours, 418:64:65:61 x 2 mins L:R:G:B. All bin 1x1. Lum was captured in November and December 2016; RGB was captured from October 2018 to January 2019.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of best subs about 2 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arsecs/pixel (50% of full scale)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Celestron CPC1100 for luminance; Paramount MX+ for RGB, 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: PHD2.6.3, 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
Blink to preview and reject a few frames
Subframe Selector 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 Dynamic PSF and star mask for deringing support (on lum channel only, and only on cores of a few bright galaxies)

4. Luminance Stretching
Histo Trans
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

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration

6. RGB Stretching
Histo Trans
Boost saturation on stars with Curves Trans
Curves Trans
TGVDenoise
Make a separate version with lower stretching for stars only (“Stars” image)

7. Color Combination
LRGB Combination to create “Galaxies” image

8. Star Removal
Star removal from Galaxies image using star masks, Morphological Transform, and Multiscale Median Transform on the residual layer. See David Ault’s technique.

9. Photoshop: Background Removal
Create an artificial flat by eliminating the remaining small stars with the Dust and Scratches filter
Subtract the artificial flat from the Galaxies image, masking all of the galaxies (PITA!)
Use Radial Blur filter on a color layer to obscure comatic blue halos on bright stars

10. Merge Images
With PixelMath max command, merge Stars and Galaxies images
Final Multiscale Median Transform, using inverted luminance image as mask, to reduce noise and lumpiness in background
Save to Web as JPG