Sh2-132 in Hubble Palette
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January 2, 2022
Sh2-132 is one of the 312 bright emission nebulae catalogued by Stewart Sharpless. This large area of emission nebula in the constellation Cepheus glows from the characteristic red light of ionized hydrogen. Sh2-132 is about 1,200 light years away. Dark patches within it are due to soot blocking out the light. There are also some prominent dark nebulae blocking out the light of the stars behind them outside the main area of Sh2-132. I’ve imaged this a few times before, but not in narrowband.
Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Focus with Optec DirectSync motors and controller. Equipment control with PrimaLuce Labs Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average transparency and seeing. Acquired November 23 – December 19, 2021, mostly under a moderately moonlit sky.
Sky-Watcher Esprit 150 f/7 refractor and QHY600M camera with Optolong H-alpha, O(III) and S(II) 3nm filters
O3: 22 x 20m = 7hr20m
S2: 22 x 20m = 7hr20m
Total: 22hr for narrowband image; 37hr45m for broadband/narrowband blend
Data Reduction and Cleanup
The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration and integration of all frames.
Creation and cleanup: The S2, Ha and O3 were combined to make an RGB image. DynamicBackgroundExtraction and BackgroundNeutralization were applied to the image.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the RGB image. Layer settings for threshold and strength: Layer 1: 5.0 0.85 Layer 2: 4, 0.75 Layer 3: 3, 0.64 Layer 4: 2.0, 0.2 Layer 5: 1, 0.1.
Stretching: HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10
Creation of Synthetic Luminance: ImageIntegration was used to make a noise-weighted average of the three narrowband masters. This was used as synthetic luminance.
Deconvolution: A star mask was made to use as a Local Deringing Support image. A copy of the image was stretched to use as a deconvolution mask. Deconvolution was applied (45 iterations, regularized Richardson-Lucy, Parametric PSF mode).
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright structures. Layer settings for threshold and strength: Layer 1: 4.0 0.9 Layer 2: 3.0, 0.75 Layer 3: 2.0, 0.6 Layer 4: 0.5, 0.2.
Stretching: HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.
Combining Luminance and Chrominance
LRGB Combination: The synthetic luminance was applied to the chrominance image using LRGBCombination with default settings.
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to mainly target the background areas and protect the stars (250 iterations for L and 350 iterations for chrominance).
Contrast Enhancement: LocalHistogramEqualization was applied twice using a mask to select the bright parts of the nebula and protect stars and background. The first pass was with a scale of 50 (max contrast 1.5, strength 0.25, 1 iteration), followed by a scale of 150 (max contrast 1.5, strength 0.27, 1 iteration).
Sharpening: MultiscaleMedianTransform was used to sharpen Layers 2, 3 and 4 with strengths of 0.35 and 0.17 and 0.07, respectively. A mask was used to limit sharpening to high-signal areas and to protect the stars and background regions.
Hue Adjustment: The green tones were transformed towards blue, and yellows towards red, using multiple applications of the CurvesTransformation Hue control. SCNR was applied with a strength of 0.20 through a Green mask generated with the ColorMask script.
Final Steps: MorphologicalTransformation was applied (morphological selection=0.11; strength =0.5; 3×3 kernel) through a star mask to slightly reduce the stars. Background, nebula and star brightness, contrast, saturation and hue were adjusted in several iterations using CurvesTransformation RGB/K control with masks as required. The DarkStructureEnhance script was applied with a strength of 0.2. Excessive saturation around some stars was reduced with specific masks and the CurvesTransformation saturation tool. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.