M51, The Whirlpool Galaxy

M51, The Whirlpool Galaxy

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April 22, 2016

I have taken images of the Whirlpool Galaxy before, several times in fact, with different telescopes and cameras over the years.  This shot is my best result yet, and contains data acquired during March and April 2016.

M51 is the galaxy in which spiral structure was first seen, by Lord Rosse in 1845 using a 72-inch telescope known as the “Leviathan of Parsonstown.”   The galaxy was discovered in 1773 by Charles Messier.  It’s fairly bright and I’ve seen it in binoculars in a reasonably dark sky.

Left of the Whirlpool Galaxy is NGC5195.  The two are interacting, and you can see that the smaller galaxy has distorted M51’s arms.  M51 has also smeared out its companion, towards upper left and lower right.  The pair lie about 30 million light years away, beneath the end star of the handle of the Big Dipper.  The Whirlpool Galaxy is about 60,000 light years across and has a mass of around 160 billion times that of our Sun.  The pink features in M51 are emission nebulae, similar to the Rosette Nebula in our galaxy.  The knots in the blue arms are star clusters, similar to M35 and NGC2158.

M51 and NGC5195 are the largest members of the M51 Group of galaxies.  You can see several other members throughout this field.   This group and two others (the M101 group and the NGC5866 group) may be part of a single, large, loose group of galaxies, since all lie at similar distances.


SBIG STL-11000M camera, Baader HaLRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with QHY5 and 80 mm f/6 Stellar-Vue refractor.  Focusing with FocusMax.  Acquisition with TheSkyX.  Automation with CCDCommander.  Calibration, registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  Moderate moonlight for RGB, no moon for L; gibbous to full moon for Ha.

22x10m R, 19x10m G, 17x10m B, 21x15m L, 14x5mL and 18x20m Ha unbinned frames (total=22hr).

Creation and cleanup: Ha, R, G, B and both L masters were cropped.  R, G and B were combined to make an RGB image which was processed with DBE and ColourCalibration.  The Ha image was also processed with DBE and the NB-RGB Combine script was applied to blend the Ha and RGB.

Stretching: HistogramTransformation was applied to make a pleasing, bright image.

Cleanup:  DBE was applied to neutralize the background of both the 5m and 15m luminance masters.  HDRCombination was used to combine the masters.

Deconvolution:  A star mask was made to use as a local deringing support. A copy of the image was stretched to use as a range mask. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).

Stretching: HistogramTransformation was applied to make a pleasing, bright image.

Dynamic Range Adjustment and Stretching: HDRMultiscaleTransform was applied at 6 and 4 pixel scales, protecting bright stars with a mask. TGVDenoise was applied in RGB/K mode with default settings, followed by HistogramStretch.  A range mask was made that protected stars and background, and LocalHistogramEqualization was applied to the galaxy at a scale of 50, with a strength of 80%.

[Note – I initially tried using a SynthL for luminance, made by combining all the colour channels, but the results were better using the Luminance alone.  This is likely due to significant moonlight during acquisition of the RGB data.]

Combining Luminance with RGB:
The luminance channel of the RGB image was extracted, processed and then added back into the RGB image as follows:

1. RGBWorkingSpace was applied to the RGB image with all channels equal to 0.33
2. Extract luminance from the RGB image.
3. Apply LinearFit to the extracted luminance using SynthL as the reference.
4. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 3.
5. LRGBCombine was then used to make a SynthLRGB image.

Final Processing:
UnsharpMask was applied to the galaxy using a mask. Contrast, brightness and saturation were adjusted in several iterations with the Curves tool, with separate adjustments were made for background and stars using a mask.

Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.