NGC 7331 and the Deer Lick Group

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September 16, 2023

NGC 7331 is the largest galaxy in this image. NGC 7331 is located in Pegasus, and is about 40 million light years away, and has a diameter of about 120,000 light years. It is the biggest member of the “Deer Lick Group,” several other members of which are strewn around (mostly to the right of) NGC 7331.  These galaxies are about 10 times further away, or 500 million light years (yes, half a billion light years!). See the annotated image which identifies most of the galaxies. I have imaged this galaxy in a wider field of view; you can find it here.


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 13-15, 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.

26x5m Red = 2hr10m
24x5m Green = 2hr00m
25x5m Blue = 2hr05m
60x5m Lum = 5hr00m

Total: 11hr15m

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.

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 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. 

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 image to make pleasing images. Approximate background level after stretch was 0.10 for RGB and Lum.

Nonlinear Processing

Luminance addition: LRGBCombination was applied to replace the lightness of the Colour image with the Luminance master.

Star Removal:  StarXterminator was used to remove the stars from the resulting LRGB image, with Unscreen selected.

Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the 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 galaxy slightly. 

Contrast Enhancement: LocalHistogramEqualization was applied three times.  A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 40, strength 0.25; scale 90, strength 0.25; scale 150, strength 0.18).

Sharpening: A mask was used to select brighter regions of the galaxy  for sharpening with MultiscaleMedianTransform (Layers 1 – 5 with strengths of 0.01, 0.02, 0.04, 0.02, and 0.01, respectively). 

Stars-only steps: MorphologicalTransformation was used in Morphological Selection mode to shrink the stars through a mask made by extracting the Luminance from the stars-only image. CurvesTransformation was used to add saturation 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 created with StarXterminator.

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.