Link to Detailed Annotated Image

The Leo Trio, or Leo Triplet, is located about 35 million light years away and is part of the Virgo Cluster. The galaxies are interacting, the most noticeable result of which is the 300,000 light year tail of NGC 3628, containing clusters of stars flung out of the galaxy. NGC 3628 has an X-shaped central bulge, possibly also a result of interactions. M 65 is low in gas and dust and has little star formation, except for some relatively new stars in the odd “floating” arms, which may also result from interactions with the other galaxies. M 66 appears to have survived these interactions relatively unscathed.

Messier discovered M 65 and M 66 in 1780. NGC 3628 was discovered by William Herschel in 1784.

Exposure: Total exposure time about 16 hours, 336:46:47:49 x 2 minutes LRGB. All bin 1x1. Data collected in February and March 2023.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border)
Seeing: FWHM of integrated luminance 2.4 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 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)
Although this imaging system is mercifully free of the ring artifacts produced by the moving-mirror SCT in my previous imaging system, I still didn’t have perfect flats, probably because light from the LED panel I use to take flats leaked in the open back of the Newtonian. I’ll block that off in the future, but for this image I removed the light leaks by creating and subtracting an artificial flat, which is simply an image of the light leaks in the image.
a. Create an image of the background by removing stars with StarXterminator. SXT removes most of the background galaxies as well.
b. Clean this image up in Photoshop, removing any leftover galaxies and stars
c. Blur this background image slightly (otherwise in the next step you’ll remove all the noise, creating an unnatural-looking noiseless image) with a 5-pixel dust and scratches filter
d. Subtract the background image from the main image (adding an offset, to avoid having a pure black background), using a mask so that only areas of the image showing the light leaks were subtracted

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