M101, The Pinwheel Galaxy 

M101Click image for full size version

February 4, 2017

This galaxy is the 101st entry in Messier’s catalogue, M101, but is commonly referred to as the Pinwheel Galaxy. It is close to the handle of the Big Dipper. I have always found this a particularly difficult target to image well. As a result, I spent nearly three months in 2014 capturing more than 41 hours of data for this image! That makes it my deepest exposure ever.

M101 is large — much bigger than our own Milky Way galaxy — and relatively close (22 million light years or so). The asymmetry in M101 is thought to be due to a near-collision with another galaxy a long time ago. This probably triggered extensive star formation, showing up as bright blue clusters within the spiral arms. The pink structures are nebulae, similar to the Orion nebula in our own Milky Way galaxy. Dozens of other galaxies that lie far in the background are spread throughout this image.


SBIG STL-11000M camera, Baader Ha and LRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX. Guided with STL-11000’s remote guide head and 80 mm f/6 Stellar-Vue refractor. Acquistion, guiding, and calibrationand using Maxim-DL. Image registration, integration and processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. No to little moon for luminance shots gibbous to full moon for colour and HA shots. Average transparency and good seeing for luminance; average transparency and poor seeing for colour shots. Acquired over about 15 nights in 2014.

8x15m R, G and B, 116x15m for L, and 19x20m H-alpha  (total = 41hr20m)

NB-RGB Creation
Creation and cleanup:  R, G, B, Ha and L masters were cropped and processed separately with DBE. R, G and B were combined to make an RGB image which was processed with ColorCalibration. The NBRGBCombination script was run with default settings using Ha for red narrowband. A stretched copy of the Ha channel was used as a mask to blend HaRGB into the original RGB only where the nebula were (this helped preserve the hue of the galaxy.)

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas. Layer settings for threshold and strength: Layer 1: 3.0, 0.75   Layer 2: 2.0, 0.65  Layer 3: 1.0, 0.52  Layer 4: 0.5, 0.25

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

Synthetic Luminance:
Creation and cleanup of SynthL: The cleaned up Ha, L, R, G and B masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection).

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 25 stars).

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise. Layer settings for threshold and strength: Layer 1: 3.0, 0.75   Layer 2: 2.0, 0.65  Layer 3: 1.0, 0.52  Layer 4: 0.5, 0.25 and Layer 5: 0.5, 0.13.

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

Noise Reduction and Re-Stretch: TGVDenoise was applied in Lab mode with 300 iterations with a range mask used to protect high signal areas.  This was followed by a HistogramTransformation to raise the black point (but with no clipping). 

Combining SynthL with NB-RGB:
The processed SynthL was applied to the NB-RGB image using ChannelCombination in Lab mode with default settings.

Additional Processing

Contrast Enhancement: HDRMultscaleTransform (scale 6, median) followed by LocalHistogramEqualization scales (scale 50, strength 0.5, max contrast 2; scale 150, strength 0.25, max contrast 1.5) were applied to the bright portions of the galaxy. ExponentialTransformation was used to boost the fainter outer arms of the galaxy.

Sharpening: The bright parts of the galaxy were sharpened using MultiscaleLinearTransform followed by UnsharpMask.

Final Steps: Background, galaxy and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. A few “hot” pixels were removed with the CloneStamp tool.

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

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