M51, The Whirlpool Galaxy

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June 7, 2026

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.

To the right of the Whirlpool Galaxy in this image 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. 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 NGC 2158.

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.

The image above shows comparable detail to a previous version made with a 14″ SCT, which is impressive considering the current image was made with a 120 mm refractor! I have prepared an annotated image identifying the main galaxies in this field of view.

Tekkies:
Acquisition, focusing, and control of Sky-Watcher Wave 150i mount and other equipment with N.I.N.A. and Green Swamp Server. Primalucelab 3″ ESATTO focuser, ARCO rotator, GIOTTO flat panel and ALTO cover motor. Player One Xena-M guide camera, OAG and Filter Drawer MAX. Equipment control with Primalucelab EAGLE 6 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Acquired under above-average transparency and seeing and moderate to strong moonlight from May 27 – June 2, 2026.

Sky-Watcher Esprit 120, Player One Poseidon-C Pro camera, Optolong L-Quad Enhance filter and L-eXtreme filter.

LQUAD       238 x 5m = 19hr 50m
L-eXtreme    92 x 5m =   7hr 40m

Total = 27hr 30m

Image scale 0.45 arcsec per pixel (x2 Drizzled)
 
 
Finder Chart and Annotated Image:
 
 
 
Data Reduction and Linear Processing

Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, integration and drizzle integration (2X).

Master images: A master Ha image was made by extracting the red channel from the L-eXtreme master. The green and blue channels were discarded. The LQUAD master was renamed as RGB

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

Colour Calibration: BlurXterminator was applied to the RGB master with Correct Only selected, followed by ColorCalibration.

Deconvolution: BlurXterminator was applied to the RGB and Ha masters with Automatic psf , star sharpening set to 0.5, and non-stellar set to 0.9. 

Ha Continuum Subtraction: The Seti Astro Continuum Subtraction Utility was used to remove continuum emissions from the Ha master using the red channel of the RGB master as the continuum reference image. Starless images were used for this process.

Stretching:  MultiscaleAdaptiveStretch was applied to make pleasing images from the RGB and Ha masters. Approximate background level after stretch was 0.1 for the RGB masterand 0.05 for the Ha master.

Nonlinear Processing

Noise Reduction: NoiseXterminator was applied to the RGB and Ha masters with settings Amount=0.9 and Interations=4.

Star Removal and processing:  StarXterminator was used to remove the stars from the SynthLRGB master with Unscreen checked. Colour was increased in the stars-only image by increasing saturation using CurvesTransformation through a star mask.

Supplemental Stretch: HistogramTransformation was used to darken the black point and brighten the galaxy.

Initial Saturation Boost: CurvesTransformation’s saturation tool was used to boost colour in the galaxy in the starless RGB image. A range mask made with RangeSelection was used to protect the background.

H-alpha Blending: The contiuum-subtracted Ha was added to the RGB image using Jurgen Terpe’s CombineHaWithRGB script, using a mask to protect the background.

Nonlinear Noise Reduction: NoiseXterminator was applied to the HaRGB image with settings Amount=0.9 and Iterations=5. 

Contrast Enhancement: LocalHistogramEqualization was applied twice. A Contrast Limit of 1.5 and 1 iteration was used for each application (scale 150, strength 0.25, and scale 40, strength 0.35). 

Sharpening: BlurXterminator was applied (no star reduction, no halo reduction, manual psf=3, amount = 0.1).

Star Restoration: Stars were added back into the image using the PixelMath expression combine(starless, stars_only, op_screen())

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. The annotated image was made with the AnnotateImage script.