The NGC 5846 galaxy group is located about 85 million light years away near the eastern border of Virgo. (At this range, this FOV covers 2.5 x 1.7 mly.) It’s one of the Virgo III groups, a chain of galaxy groups extending 40 mly east from the main body of the Virgo cluster. All of the bright ellipticals in this image, and a few outside this FOV, are part of the NGC 5846 group. Like many large elliptical galaxies, NGC 5846 has a large number – one estimate reports 1200 – of globular clusters.

The group also hosts an “ultra diffuse galaxy,” a faint blob to the upper right of NGC 5846 marked on the annotation as UDG1. UDGs have extremely low luminosities due to the absence of star-forming gas, and so are made up almost entirely of very old stellar populations. The first example was discovered in the Virgo cluster in 1984. UDG1, according to one study, is almost entirely composed of stars that formed in globular clusters. MATLAS, a project with the Canada-France-Hawaii telescope to survey the outskirts of nearby bright galaxies, has also discovered hundreds of low surface brightness galaxies in this area, although it’s not clear how many of them are physically associated with NGC 5846. Many of the small diffuse blobs in this image are LSBs listed in that catalog.

The barred spiral galaxy NGC 5850 has a claim to fame in that it was the archetype SBb galaxy used by Hubble in his 1926 galaxy classification system. One paper asserts that NGC 5850 had a high-speed interaction with NGC 5846 in the last 200 million years and shows some distortion as a result. However, it’s not clear that it’s located anywhere near the larger galaxy. Redshift measurements locate it 40 mly behind NGC 5846, and Tully Fisher estimates place it 25 mly in front of NGC 5846. If either of those is accurate, I’m not sure how it could have interacted with the larger galaxy in that time period.

Exposure: Total exposure time about 22.5 hours, 501 x 2 mins Lum, 22:17:29 x 5 mins RGB. All bin 1x1. Data collected from April to June 2023.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated luminance 2.6 arcsecs
Image scale at capture: 0.65 arcsecs/pixel
Scale of presentation: 1.3 arcsecs/pixel (50% 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
RGB Combination for RGB frames
Calibration, weighting, registration and integration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Dynamic Background Extraction on luminance and RGB images
ImageSolve RGB, then run Spectrophotometric Color Calibration, using Average Spiral Galaxy white reference
Determine PSF using the PSFImage script, and enter this into BlurXTerminator
BlurXterminator using Correct First and manual PSF on luminance and RGB
NoiseXterminator on luminance and RGB

2. Luminance/Narrowband Stretching
Histo Trans x 3
Curves Trans
No denoise necessary

3. RGB Stretching
Create a saturation mask: apply ScreenTransferFunction to the stretched luminance, and then to Histo Trans. Clip the mask with Histo Trans and blur slightly with Convolution.
Histo Trans x 2
Curves Trans to boost saturation, using the saturation mask to prevent spurious background colors from being boosted
Histo Trans
Curves Trans to brighten

4. Color Blending
LRGB Combine

5. Background Subtraction (Artificial Flat)
To remove background lumpiness caused by heavy stretching due to my light polluted skies, I create and subtract an artificial flat, which is simply an image of the messy background, with all stars and imaging targets removed.
a. Create an image of the background by removing stars with StarXterminator
b. Clean this image up in Photoshop, removing the leftover galaxies and any leftover stars
c. Blur this background image slightly (otherwise in the next step you’ll remove all the noise, creating an unnatural-looking noiseless image)
d. Back in PI use PixelMath to subtract the background image from the main image (adding a pedestal, to avoid having a pure black background).

6. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Rescale to 50%
ImageSolve
Create annotated images with the AnnotateImage script, using some custom databases to extract quasar redshifts and galaxy clusters
Save final image and annotated image as JPGs