M100, Spiral Galaxy

M100

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April 18, 2015

M100 is a spiral galaxy in Virgo, and is a member of the Virgo cluster of galaxies.  It is one of the largest members of this galaxy cluster, and has a diameter of 107,000 light years.  It is 55 million light years away.  Note the pink emission nebulae and blue star clusters in the arms.  I counted at least 17 other galaxies in this image – they look like smudges or streaks, as opposed to the sharp, round stars.

Tekkies:
SBIG STL-11000M camera, Baader Ha and LRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with STL-11000’s external guider and 80 mm f/6 Stellar-Vue refractor.  Acquistion, guiding and calibration done using Maxim-DL.  Focusing with FocusMax.  Automation with CCDCommander.  Registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  moderate moonlight for RGB, no moon for L; moderate moon for Ha.

11x10m R, 9x10m G, 9x10m B, 30x10m L and 16x20m Ha unbinned frames (total=15hr10m).

RGB:
Creation and cleanup: Ha, L, R, G and B 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 into the RGB.

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

Synthetic Luminance:
Creation and cleanup: The cropped L, Ha, 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). DBE was applied to neutralize the background.

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 (100 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).

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

Combining SynthL with RGB:
The luminance channel of the RGB was extracted, processed and then added back into the RGB image as follows:
1. Extract luminance from the RGB image.
2. Apply LinearFit using the SynthL channel as a reference.
3. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 2.
4. LRGBCombine was then used to make a SynthLRGB image.

Final Processing
Dynamic Range Adjustment and Stretching: HDRMultiscaleTransform was applied at 6 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.  SCNR was applied to the image to reduce a green cast.

Final Steps: Background colour saturation was reduced slightly.   A blurred range mask was made and used to increase colour saturation and contrast in the galaxy slightly.  An UnsharpMask was applied to the galaxy followed by a light noise reduction with ACDNR on the entire image.  A copy of the SynthL was processed with MultiscaleLinearTransformation with 4 layers and residual unchecked.  Convolution (2 px) was applied to the result and this was used as a mask to boost colour saturation in the centre of the medium and small stars.

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

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