Stephan’s Quintet

Click image for full size version

September 23, 2023

Stephan’s Quintet is a group of galaxies in Pegasus, not far from NGC 7331 and the Deer Lick Group. Four of the five galaxies in the centre of the image form an association about 290 million light years away, while the 5th, PGC 69270 — the one that looks the bluest in this photo — is much closer, at “only” 40 million light years. A beautiful curved tidal tail hangs off the bottom of NGC 7318.

I’ve seen Stephan’s Quintet several times visually through my 20″ telescope, but this image shows much more detail than I can see visually, and also shows colour. See the annotated image which identifies most of the galaxies. I have imaged these galaxies in a wider field of view; you can find the image here.

Tekkies:

Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Primalucelab low-profile 2″ Essato focuser and ARCO rotator. Equipment control with PrimaLuce Labs Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Data acquired September 15-20, 2023 in a moonless sky.

Celestron 14″ EDGE HD telescope at f/11 (3,912 mm focal length)  and QHY600M camera binned 2×2 with Optolong filters.

10x5m Red = 0hr50m
10x5m Green = 0hr50m
10x5m Blue = 0hr50m
120x5m Lum = 10hr00m

Total: 12hr30m

Image scale 0.4 arcsec per pixel
 
Finder Chart and Annotated Image: 
   
 
 
Data Reduction and Linear Processing

Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization and integration of all frames. DrizzleIntegration was performed on the Lum data (1x, drop shrink=0.9, Gaussian).

Colour master: A colour master was made from the Red, Green and Blue masters using ChannelCombination in RGB mode.

Gradient Removal: DynamicBackgroundExtraction was applied to the RGB and Lum masters.

Colour Calibration:  SpectrophotometricColorCalibration was used to calibrate the RGB master.

Deconvolution: BlurXterminator was used on the Lum and RGB masters with Automatic psf , and star sharpening set to 0.10 with “Correct First” selected. 

Star Removal:  StarXterminator was used to remove the stars from the L and RGB images, with Unscreen selected.

Linear Noise Reduction: NoiseXterminator was applied to each master image with settings Amount=0.9 and Detail=0.25

Stretching:  HistogramTransformation was applied to the RGB  and Lum masters to make pleasing images. Approximate background level after stretch was 0.10 for RGB and Lum. The same was done to the two stars-only images.

Nonlinear Processing

Luminance addition: LRGBCombination was applied to replace the lightness of the Colour image with the Luminance master. The same was done with the stars only images.

Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the LRGB starless image with settings Amount=0.9 and Detail=0.2

Histogram Stretch:  HistogramTransformation was used to reset the black point and increase brightness in the galaxies slightly. 

Contrast Enhancement: LocalHistogramEqualization was applied twice.  A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 40, strength 0.25; scale 130, strength 0.25).

Sharpening: A mask was used to select brighter regions of the galaxy  for sharpening with MultiscaleMedianTransform (Layers 1 – 5 with strengths of 0.01, 0.03, 0.05, 0.03, and 0.01, respectively). 

Stars-only steps: MorphologicalTransformation was used in Morphological Selection mode to shrink the stars through a mask (the Lum stars-only image) CurvesTransformation CIE c* channel was then used to add color to the stars using the same mask.

Star Restoration: PixelMath expression combine(starless, stars, op_screen()) was used to combine the starless and stars-only images.

Final Steps: Background, galaxy and star brightness, contrast and saturation were adjusted in several iterations using CurvesTransformation with masks as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.  The finder chart was made using the FindingChart process.