September 11, 2017
The Coathanger is an asterism — a pattern of stars — that looks unmistakably like its namesake. It is also catalogued as Brocchi’s Cluster and Collinder 399. I’ve looked at this hundreds of times in binoculars; it is a summer favourite. It can even be seen with the naked eye in a dark sky. But this is the first time I have imaged it. The pretty star cluster at right in the image is NGC 6802. Sh2-83 is the small and dim patch of pinkish nebulosity to the lower left of the Coathanger.
The Coathanger was long thought to be a star cluster — a group of gravitationally-bound stars. However, it has recently been concluded that it is a chance alignment of stars. I could not find catalogue numbers for the many tendrils of dark nebulae in this image. They are part of the Milky Way’s dust lane.
Moravian G3-16200 EC camera (on loan from O’Telescope), Optolong R, G and B filters, Takahashi FSQ-106 ED IV at f/3.6, Paramount MX, unguided. Acquisition with the SkyX, focused with FocusMax. Scripted with CCD Commander. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. No moon; average transparency and seeing.
12x3m R, G and B unbinned frames (total=1hr48m).
The BatchPreProcessing script was used to perform calibration, cosmetic correction and registration of all frames. Blink and SubframeSelector were used to identify the best single exposure for each filter. LocalNormalization and ImageIntegration were applied using the best frames as reference frames. DrizzleIntegration was used to make the channel masters. During ImageIntegration and DrizzleIntegration, the local normalization files were used. The high large-scale removal tool was used in ImageIntegration.
Creation and cleanup: The R, G, and Bmasters were cropped. DynamicBackgroundExtraction was applied to each master. R, G and B were combined to make an RGB image which was processed with PhotometricColorCalibration, using a background area as the background reference.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the RGB image. The linear mask feature was used to protect bright stars and brighter areas of the nebulae. Layer settings for threshold and strength: Layer 1: 4.0 0.75 Layer 2: 3.0, 0.65 Layer 3: 2., 0.41 Layer 4: 1.0, 0.25 Layer 5: 0.5, 0.1.
Stretching: HistogramTransformation was applied to the RGB image to make a pleasing, bright image.
Creation and cleanup of SynthL: The linear 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 background areas of the SynthL image. The linear mask feature was used to protect bright stars and brighter areas of the nebulae. Layer settings for threshold and strength: Layer 1: 4.0 0.65 Layer 2: 3.0, 0.5 Layer 3: 2., 0.3 Layer 4: 1.0, 0.1.
Stretching: HistogramTransformation was applied to the SynthL to make a pleasing, bright image.
Combining SynthL with RGB:
The processed SynthL was applied to the RGB image using LRGBCombine.
Noise Reduction and Re-Stretch: TGVDenoise was applied in Lab mode with 300 iterations with a range mask used to protect high signal areas. This was followed by a HistogramTransformation to raise the black point (but with no clipping).
Final Steps: Background and star brightness, contrast and saturation were adjusted in several iterations using Curves with masks as required. The image was downsampled by a factor of 2 to restore the original resolution.
Image scale is about 3.2 arcsec per pixel for this camera/telescope combination.