Cocoon Nebula Wide Field
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January 5, 2024
This glowing patch of gas in Cepheus is known as the Cocoon Nebula, a.k.a. Sh2-125. The image shows a complex field full of a combination of emission nebula (red), reflection nebula (blue) and dark dark nebula (brown lanes) and dim, glowing dust. Glowing red, hydrogen is evident all around the image, particularly at lower left and centre right.
The Cocoon Nebula is often misidentified as IC 5146, which is actually the faint star cluster at the heart of the Cocoon. The Index Catalog (IC) a supplement to the New General Catalogue (the NGC) that lists deep sky objects discovered between 1888 and 1907. They tend to be very faint, which is why many of them weren’t discovered earlier. IC 5146 is quite young, with the bright star near its centre being only about 100,000 years old. The cluster and the Cocoon are about 4,000 light years away, relatively close to us in cosmic terms. This field is permeated by faintly glowing gas and dust, and dark nebulae that block the starlight behind them.
The blue reflection nebula to the right of the Cocoon Nebula is vdB 147 (van den Burgh 147). At the same time I was acquiring the image above, I also photographed this object with my C14, which has a 4m focal length and gave a very close-up and detailed view of the Cocoon.
Tekkies:
Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Manual focus. Guiding with PHD2. Equipment control with Primalucelab Eagle 4 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Data acquired under variable moon illumination with average or better transparency and average seeing August 31-September 4, 2024.
Takahashi FS-60CB telescope at f/6.2 (372 mm focal length) and QHY367C Pro camera with Optolong L-Quad Enhance filter.
246 x 5m OSC = 20hr 30m
Total: 20hr 30m
Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization, integration and Drizzle integration of all frames (CFA Drizzle checked, Scale=2x, Drop Shrink=0.9).
Gradient Removal: SpectrophotometricFluxCalibration was applied, followed by MultiscaleGradientCorrection at default settings.
Colour Calibration: SpectrophotometricColorCalibration was used to calibrate the OSC master.
Deconvolution: BlurXterminator was applied using an automatic PSF, star sharpening set to 0.5 and non-stellar sharpening set to 0.9.
Linear Noise Reduction: NoiseXterminator was applied with Amount=0.95 and Detail=0.15
Stretching: HistogramTransformation was applied to make a pleasing yet bright image. Approximate background level after stretch was 0.10
Nonlinear Processing
Nonlinear Noise Reduction: NoiseXterminator was applied with Amount=0.9 and Detail=0.15
Star Removal: StarXterminator was used to remove the stars, with Unscreen selected.
Stretching: HistogramTransformation was applied to adjust the black point and increase brightness and contrast. Approximate background level after stretch was 0.10
Contrast Enhancement: LocalHistogramEqualization was applied twice. A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 40, strength 0.35; scale 150, strength 0.35).
Sharpening: A mask was used to select just the nebula. MultiscaleMedianTransform was applied using Layers 1 – 5 with strengths of 0.01, 0.03, 0.05, 0.03, and 0.02, respectively.
Dark Structure Enhancement: The DarkStructureEnhance script was applied with default settings, except for amount = 0.25.
Stars-only steps: The CurvesTransformation Saturation slider was used to add colour saturation to the stars through a mask made by extracting the Luminance from the stretched stars-only image.
Additional Noise Reduction: The background showed some mottle which was reduced using ACDNR with default settings, except that StdDev was increased to 6 and 8 for the lightness and chrominance channels, respectively. SCNR was applied to the upper left quadrant of the image using a mask to protect the remainder.
Dust Enhancement: Using a mask to protect the nebula, ExponentialTransformation was applied to increase the brightness of the dust (Screen-Mask-Invert with strength 0.2 and highlight protection turned on).
Blue Nebula Enhancement: The Seti-Astro FAME script was used to select the region including and surrounding vdB 147. CurvesTransformation’s RGB and Saturation sliders were used to enhance the colour, brightness and contrast of the object.
Star Restoration: PixelMath expression combine(starless, stars, op_screen()) was used to combine the starless and stars-only images created with StarXterminator.
Final Steps: Background, nebula and star brightness, contrast and saturation were adjusted in several iterations using CurvesTransformation with masks as required. The image was down-sampled by a factor of 2, which restored it to the original resolution (2.9″/pixel). ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg. The finder chart was made using the FindingChart process.
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