NGC 7331 and Stephan’s Quintet
December 25, 2018
The big galaxy to the lower left of this image is called NGC 7331. It is the biggest member of the “Deer Lick Group”, several other members of which are obvious around (mostly beneath) NGC 7331. NG C7331 is in Pegasus, and is about 50 million light years away. The smaller galaxies around NGC 7331 are around 10 times further away, or 500 million light years (yes, half a billion light years!). There are many other galaxies in this image, some are very small and faint (look for the fuzzy-looking “stars” and the faint “stars” that seem to have a slight fog around them). Also see the annotated image which identifies most of the galaxies.
To the upper right of the image is a group of galaxies called Stephan’s Quintet. Four of the five form an association about 290 million light years away, while the 5th one is much closer, at “only” 40 million light years. The odd one out is the bluest of the grouping. There is a beautiful tidal tail visible that looks like a little curl hanging off the bottom of the quintet. I’ve seen Stephan’s Quintet several times visually through my 20″ telescope, but this image shows much more detail than I can see, and shows colour.
LUMINANCE DATA (Oct 2014): SBIG STL-11000, Baader L filter, 10″ f/6.8 ASA astrograph, Paramount MX. Guided with main camera’s remote guide head with a Stellar-Vue SV80 refractor. Acquistion and guiding with TheSkyX. Shot from my SkyShed in Guelph, Ontario. Excellent transparency, good seeing, no moon. 41x10m L (total 6hr50m).
COLOUR DATA (Oct-Nov. 2011): 6x5m R, 5x5m G and 6x5m B unbinned frames (total=1hr25m). QSI583wsg, Astrodon RGB filters, 10″ f/3.6 ASA astrograph, MI-250 mount. Guided with a SX-Lodestar guide camera using the QSI off-axis port. Acquistion and guiding using Maxim-DL. Shot from my SkyShed in Guelph, Ontario. The RGB was shot with plenty of moonlight, and poor to average transparency.
Total acquisition time 8hr15m. Image scale is 1.1″ per pixel
Data Reduction and Cleanup
The BatchPreprocessing script was used to perform calibration, cosmetic correction and registration of all frames. ImageIntegration was used to make the L, R, G and B masters. DynamicCrop was used to crop all the masters identically.
Creation and cleanup: The R, G and B were combined to make an RGB image. DynamicBackgroundExtraction was applied first with Division, then with Subtraction. The image was colour-balanced with PhotometricColorCalibration.
Linear Noise Reduction: MultiscaleLinearTransform with internal Linear Mask active was used to reduce noise in the RGB image. Layer settings for threshold and strength: Layer 1: 3.0 0.8 Layer 2: 2.5, 0.7 Layer 3: 2., 0.6 Layer 4: 1.0, 0.2.
Stretching: HistogramTransformation was applied to make a pleasing, bright image. Curve was applied to boost contrast and saturation.
Cleanup: DynamicBackgroundExtraction was applied first with Division, then with Subtraction.
Deconvolution: A star mask was made to use as a Local Deringing Support image. A copy of the image was stretched to use as a range mask. Deconvolution was applied (50 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).
Linear Noise Reduction: MultiscaleLinearTransform with an internal Linear Mask active was used to reduce noise in the background areas. Layer settings for threshold and strength: Layer 1: 3.0 0.7 Layer 2: 2.0, 0.6 Layer 3: 1.0, 0.6 Layer 4: 0.5, 0.22.
Stretching: HistogramTransformation was applied to make a pleasing, bright image.
Combining Luminance and Chrominance
LRGBCombination was used to apply the Luminance master to the chrominance image.
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode with a mask to reduce noise in the background areas.
Compression of Galactic Core: Using a mask to protect stars and background, HDRMultiscaleTransform was applied twice (scales of 6 and 4) to a clone of the image. The clone was blended back into the LRGB at 50%.
Sharpening: Using the same mask as above, , MultiscaleLinearTransform was used to sharpen Layers 2 and 3 (strengths of 0.1 and 0.1, respectively).
Contrast Enhancement: Using the same mask as above, LocalHistogramEqualization was applied twice (scale 46, max contrast 1.5, strength 0.15, 1 iteration, followed by scale 150, max contrast 1.5, strength 0.1).
Final Steps: Background, galaxy and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. Medium-sized stars were slightly reduced in size using MorphologicalTransformation’s Morphological Selection tool with a blurred star mask (selection 0.15, strength 0.2, 2 iterations).