M13, The Hercules Cluster

M13Click image for full size version

May 31, 2018

There are about 150 globular clusters orbiting the main part of the Milky Way, and they are common around other galaxies too.  These objects are star clusters of a few hundred thousand to a few million stars and they are very old objects.  The Hercules Cluster, M13, is arguably the finest globular cluster visible from the northern hemisphere but lots of people, including me, have other favourites, like M5 and M22.

Regardless of which is the “best”, they are all beautiful sights in a telescope or binoculars.  M13 is visible to the naked eye in moderately dark skies, obvious in binoculars, pretty in a small scope and stunning in a big scope. It’s one of my favourite objects to look at through the eyepiece.

Sharing the limelight with the likes of M13 must be tough, but there are many faint galaxies in the distant background that are visible in this shot, some of which are highlighted in an annotated image.  M13 lies about 22,000 light years away from us, and is about 2/3 the width of the full Moon.

This is the first light image using a Sky-Watcher Esprit 150 f/7 refractor with a QHY 16200-A camera. These products are on loan from Sky-Watcher USA and QHY CCD, respectively. I am still waiting for the electronic focuser to arrive, so focusing was done manually for this image.


Sky-Watcher Esprit 150 f/7 refractor, QHY 16200-A camera, Optolong L, R, G and B filters, Paramount MX. Acquisition with TheSkyX unguided. Focused manually. Automation with CCDCommander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Nearly full moon, average transparency and fair seeing. Data acquired May 25-29, 2018.

37x3m L. 14x5m R, 15x5m G and 16x5m B, unbinned frames (total=5hr36m). 

Data Reduction and Cleanup
The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. ImageIntegration was used to make the L, R, G and B masters. DynamicCrop was used to crop all the masters identically. DynamicBackgroundExtraction was applied to each master.

Synthetic Luminance
Creation and cleanup of SynthL: The 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).

Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the SynthL image. An internal mask was used, with layer settings for threshold and strength as follows:Layer 1: 3, 0.7   Layer 2: 2, 0.5  Layer 3: 2, 0.45  Layer 4: 1.0, 0.10.

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

RGB Creation
Creation and cleanup: ChannelCombination was used to make color image from the R, G and B masters. The RGB image was processed with PhotometricColorCalibration using a small preview of background sky as the background reference.  

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the RGB image. An internal mask was used, with layer settings for threshold and strength as follows: Layer 1: 3, 0.85   Layer 2: 2, 0.75  Layer 3: 2, 0.55  Layer 4: 1.0, 0.25.

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

Combining SynthL with RGB
The processed SynthL was applied to the RGB image using LRGBCombine.

Additional Processing
Noise Reduction and Re-Stretch: TGVDenoise was applied in L*a*b* mode (1000 iterations with convergence on and set to 0.004) with a range mask used to protect high signal areas. HistogramTransformation was used to raise the black point (but with no clipping).

Final Steps: Background, cluster and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. Slight blotchiness of the background was reduced by increasing the brightness of dark pixels with the PixelMath expression:

                  iif($T<0.05, $T+0.5(median($T)-$T), $T)

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