Arp 273, The Rose Galaxies in Andromeda

Arp 273 is a pair of interacting galaxies located 300 million light years away in southeastern Andromeda, which probably puts it in the Pegasus Supercluster, a small supercluster detached from the much larger Perseus-Pisces Supercluster. It was first described in the Atlas of Peculiar Galaxies compiled by Halton Arp in 1966. UGC 1810, the larger of the galaxies, is about five times more massive than UGC 1813, the smaller galaxy. The smaller galaxy shows distinct signs of active star formation at its nucleus, and may have passed through the larger one in the past.



Exposure: Total exposure time about 18
hours, 441:34:35:37 x 2 minutes LRGB. All bin 1x1. Data collected in November 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: 0.6 arcsecs/pixel (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)