M51, The Whirlpool Galaxy

M51Click image for full size version

June 21, 2021

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.

Right 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.

Tekkies:
Acquisition, focusing, and control of Paramount MX mount (unguided) with N.I.N.A. and TheSkyX. Focus with Optec DirectSync motor and controller. Equipment control with PrimaLuce Labs Eagle 3 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Data acquired May 8-13, 2021 in a moonless sky.

Luminance: Sky-Watcher Esprit 150 f/7 refractor and QHY600M camera with Optolong UV/IR filter
Chrominance: Takahashi FSQ-106 ED IV @ f/5 and QHY367C Pro one-shot colour camera with Optolong UV/IR filter

Luminance 85x5m = 7hr05m
Luminance 41x10m = 5hr50m
H-alpha 17x10m = 2hr50m
Chrominance 135x5m = 11hr15m
Total: 27hr00m
 
Image scale 0.72 arcsec per pixel (based on Luminance)
 

Data Reduction and Initial Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create Luminance and H-alpha master frames (from the mono camera) and a RGB master frame (from the one-shot colour camera). DrizzleIntegration (1x) was applied to the registered RGB images. The RGB master was aligned to the Luminance master using StarAlignment

Gradient Removal: DBE was applied to Luminance and RGB masters using Subtraction.

Colour
Colour Balancing:  Colour of the RGB master was balanced with ColorCalibration.

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold, strength and iterations were as follows: Layer 1: 5.0 0.95, 1 iteration; Layer 2: 4, 0.8, 1 iteration, Layer 3: 3.0, 0.65, 1 iteration.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Luminance
Deconvolution:  A star mask was made from the Luminance master to use as a Local Deringing Support Image. A copy of the image was stretched to use as a range mask. Deconvolution was applied (100 iterations, regularized Richardson-Lucy, external PSF made using the PSFImage script; Global dark deringing = 0.03).

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas of the Luminance-filtered image, using an internal mask to protect bright stars. Layer settings for threshold, strength and iterations: Layer 1: 3.0 0.85, 2 iteration; Layer 2: 2.0, 0.75, 1 iteration; Layer 3: 1.0, 0.25, 1 iteration.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

H-alpha
Deconvolution:  A star mask was made from the H-alpha master 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 the DynamicPSF with about 20 stars; Global dark deringing = 0.02; Global light deringing 0.003).

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas of the H-alpha image. Layer settings for threshold, strength and iterations: Layer 1: 4.0 0.85, 1 iteration; Layer 2: 3.0, 0.75, 1 iteration; Layer 3: 2.0, 0.25, 1 iteration.

Stretching:  HistogramTransformation was applied to the Ha to make a pleasing, bright image, with background set to an intensity of approximately 0.10. 

Combining Luminance, Colour and H-Alpha Images
Creation of LRGB: The Luminance was applied to the RGB image using LRGBCombination with default settings.

Creation of HaLRGB: PixelMath was used to add Ha to the LRGB image, using the following expressions for the R, G and B channels:
     R:  max($T[0], 1.2*Ha)
     G:  $T[1]      B:   iif($T[0]<Ha, $T[2] + 0.05*Ha, $T[2])

Additional Processing
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target the background areas and protect the stars and nebula (max. 1,000 iterations and convergence selected for both lightness and chrominance). 

Contrast Enhancement: HDRMultiscaleTransformation was applied to a clone of the image with scales of 6 and 4. The result was blended back 1:1 with the original using a mask to limit the blend to the core of the galaxy. LocalHistogramEqualization was applied twice using an inverted star mask to protect the stars, first with a scale of 40 (max contrast 1.5, strength 0.25, 1 iteration), followed by a scale of 130 (max contrast 1.5, strength 0.35, 1 iteration).

Sharpening: MultiscaleLinearTransform was used to sharpen Layers 2 and 3 with strengths of 0.1 and 0.08, respectively. A mask was used to protect background and stars and limit sharpening to the brighter galaxy features.

Final Steps: Background, galaxy and star brightness, contrast, and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. MorphologicalTransformation was used in Morphological Selection mode (Strength 0.8; Selection 0.13) through a star mask to shrink the brightest stars. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.