M22, Globular Cluster
August 8, 2016
This image of M22 was taken during the week of the Starfest star party in 2016. We had a long string of clear nights and hot, sunny days. Visited my friends each day and came home each night to image from my observatory. One of the objects I imaged is M22, one of my favourite globular clusters, being set in the thick of the Milky Way. It’s among the nearest such objects to us, at around 10,600 light years, and it covers an area about the size of the full Moon. That makes it around 100 light years across. Many globulars are very old; this one’s age is pegged at around 12 billion years.
M22 might be the most famous northern globular cluster if only it rose higher in the sky for people at mid-northern latitudes. It certainly is in the same league as M13 in terms of splashiness, even though it dimmed by being so much lower in my sky. (Being lower in the sky means the light travels through more atmosphere, which dims and degrades the view. The dimming of starlight by the atmosphere is called “atmospheric extinction..”
SBIG STL-11000M camera, Baader R, G and B filters, 10″ f/6.8 ASA astrograph, Paramount MX. Guided with QHY5 guide camera and 80 mm f/6 Stellar-Vue refractor. Acquisition, guiding and mount control with TheSkyX. Focusing with FocusMax. Automation with CCDCommander. All preprocessing and post-processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. No moonlight, good to excellent transparency, and good to very good seeing throughout acquisition.
8x5m R, 10x5mG, 10x5mB unbinned frames (total=2hr20m).
Creation and cleanup: R, G and B masters were cropped and processed separately with DBE. R, G and B were combined to make an RGB image which was processed with ColourCalibration.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas. Layer settings for threshold and strength: Layer 1: 3.0, 0.5 Layer 2: 2.0, 0.35 Layer 3: 1.0, 0.2 Layer 4: 0.5, 0.1
Stretching: HistogramTransformation was applied to make a pleasing yet bright image.
Creation and cleanup of SynthL: The cleaned up 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 copy of the image was stretched to use as a deconvolution mask. A star mask was made from unstretched SynthL to use as a local deringing support. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars; local deringing at 70% and global dark deringing at 0.03).
Linear Noise Reduction: MultiscaleLinearTransform was applied to reduce the noise. Layer settings for threshold and strength: Layer 1: 3.0, 0.5 Layer 2: 2.0, 0.35 Layer 3: 1.0, 0.2 Layer 4: 0.5, 0.1
Stretching: HistogramTransformation was applied to make a pleasing yet bright image. TGVDenoise was applied and the image was re-stretched to reset the black point.
Combining SynthL with RGB:
The luminance channel of the RGB image was extracted, processed and then added back into the RGB image as follows:
1. Extract luminance from the RGB image.
2. Apply LinearFit using SynthL as the 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 of SynthLRGB:
Background, stars and nebula brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required.
Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.