June 18, 2018
This globular cluster is M92. It lies in Hercules, not far from M13, which I posted just last week. Late spring is, after all, globular season! M92 is one of my favourite telescope targets, especially in my 20″ reflector.
M92 lies about 26,700 light years away from us, and appears a little less than half the width of the full Moon. Its mass is about 200,000 times that of the Sun, and its age is estimated at 14 billion years.
There are many faint galaxies in the distant background that are visible in this shot, some of which are highlighted in an annotated 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 with FocusMax 4. Automation with CCDCommander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. No Moon, average transparency and fair to average seeing. Data acquired June 13-15, 2018.
20x1m L, 23x5m L, 22x5m R, 21x5m G and 21x5m B, unbinned frames (total=7hr35m).
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 R, G and B masters. Two luminance masters were made from the 1m and 5m subframes. DynamicCrop was used to crop all the masters identically. DynamicBackgroundExtraction was applied to each master.
Combining 1m and 5m Luminance Masters: HDRCombination was used to combine the 1m and 5m L masters. This was necessary to capture both the core and fainter halo in the same image.
Creation and cleanup of SynthL: The HDR-L, R, G and B masters were combined using ImageIntegration (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.
Stretching: MaskedStretch, followed by HistogramTransformation, was applied to the SynthL to make a pleasing, bright image. Use of MaskedStretch prevented core burnout.
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, 0.75 Layer 2: 2, 0.6 Layer 3: 2, 0.5.
Stretching: MaskedStretch, followed by HistogramTransformation, was applied to the RGB image to make a pleasing, bright image. Use of MaskedStretch prevented core burnout.
Combining SynthL with RGB
The processed SynthL was applied to the RGB image using LRGBCombine.
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.
Image scale is about 1.15 arcsec per pixel for this camera/telescope combination.