M53, Globular Cluster

M53Click image for full size version

May 28, 2015

M53 is another globular cluster, lying less than a degree from NGC5053 in Coma Berenices, and at a similar distance of about 60,000 light years.  But M53 contains hundreds of thousands of stars, so looks much richer than its dim neighbour.   In fact, its mass is more than 800,000 times the mass of the Sun.  Even so, because it is relatively far from us it still looks considerably less rich than the brightest northern globulars.  There are dozens of tiny galaxies throughout the field, too.

SBIG STL-11000M camera, Baader RGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with STL-11000’s external guider and 80 mm f/6 Stellar-Vue refractor.  Acquisition and guiding with Maxim-DL.  Focusing with FocusMax.  Automation with CCDCommander.  Calibration, cosmetic correction, registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  No moon, good to very good transparency and fair to average seeing throughout acquisition.

8x10m R, 12x10m G and 11x10m B unbinned frames (total=5hr10m).

Creation and cleanup: R, G and B masters were cropped and DBE was applied to each before combining to make an RGB image.  ColourCalibration was applied.

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

Synthetic Luminance:
Creation and cleanup: The cropped 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 copy of the image was stretched to use as a deconvolution mask.  A star mask was made from unstretched L to use as a local deringing support. Deconvolution was applied (50 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars; local deringing at 70% and no global deringing).  The deconvolve image was blended 50/50 with the non-deconvolve image.

Stretching: HistogramTransformation was applied to make a pleasing yet bright image.  TGV Noise was applied and the image was re-stretched to reset the black point.

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

Final Processing
A star core mask was made by applying morphological transformation and convolution to a copy of SynthL. Colour saturation in star cores was increased using this mask.  Colour saturation of the background was decreased using the same mask, inverted.  A mild unsharp mask was applied to the core of the globular cluster.  The Masked Stretch script was used to darken the background.  Contrast was increased slightly with the Curves tool.

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

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