Cocoon Nebula and IC 5146

Cocoon NebulaClick image for full size version

August 23, 2020

IC 5146 in Cepheus is a small and faint star cluster listed in the Index Catalogue (IC). The IC is 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. This star cluster is embedded in a glowing patch of gas known as the Cocoon Nebula, a.k.a. Sh2-125. You may also notice there are relatively few stars around the nebula and to its upper left. This is a part of dark nebula B168, which is made up of dust that is blocking out the star light behind it. The star cluster within the Cocoon Nebula is quite young, with the bright star near its centre being only about 100,000 years old. The cluster and nebula are about 4,000 light years away, relatively close to us in cosmic terms.

There are some bonus objects in this image.  Above the Cocoon Nebula is a lovely blue reflection nebula, vdB 147 (van den Burgh 147).  There are also a pair of very faint galaxies in the lower-right centre of the image.  PGC167593 (mag. 18.23) is just to the right of the bright white star beneath the nebula.  To its right and slightly lower down is PGC167590 (mag. 18.50). Both are just smudges, but are clearly seen in the full resolution image (click on the pic above).

Acquisition, focusing, guiding and control of Paramount MX mount with TheSkyX. Focus with Optec DirectSync motor and controller. Automation with CCDCommander. Equipment control with PrimaLuce Labs Eagle 3 Pro computer. All pre-processing and processing in PixInsight. Acquired from my SkyShed in Guelph. Average or better transparency and seeing. Data acquired July 30-August 13, 2020.

Luminance: Sky-Watcher Esprit 150 f/7 refractor and QHY 16200-A camera with Optolong UV/IR filter
Chrominance: Takahashi FSQ-106 ED IV @ f/5 and QHY367C Pro one-shot colour camera with Optolong UV/IR filter

Luminance:  69x10m = 11hr30m
Chrominance: 234x5m = 19hr30m
Total: 31hr00m

Image scale 1.15 arcsec per pixel (based on Luminance)

Data Reduction and Initial Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create a Luminance master frame (from the mono camera) and registed RGB frames (from the one-shot colour camera) that were combined withDrizzleIntegration using CFA Drizzle option. Damaged cores of a few bright stars in the Drizzled colour frame were replaced with corresponding pixels from a scaled-up non-drizzled master using PixelMath.

Gradient Removal: DBE was applied to L and RGB masters using Subtraction.

Colour Balancing:  Colour was balanced with PhotometricColorCalibration.

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: 5.0 0.85, 2 iterations; Layer 2: 3.5, 0.75, 2 iterations.

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas, using an internal mask to protect bright stars. Layer settings for threshold and strength: Layer 1: 3.0 0.85, 1 iterations; Layer 2: 2.0, 0.75, 2 iterations.

Deconvolution:  A star mask was made to use as a Local Deringing Support image. A copy of the  luminance image was stretched to use as a range mask. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using PSFImage script with about 30 stars).

Stretching:  HistogramTransformation was applied to make a pleasing, bright image, with background set to an intensity of approximately 0.10.

Combining Lightness and Colour Images

LRGB Combination: The lightness image was applied to the RGB image using LRGBCombination with default settings.

Additional Processing
Nonlinear Noise Reduction: TGVDenoise was used in L*a*b* mode to reduce noise with a mask used to target the background areas and protect the stars (max. 1,000 iterations and convergence selected for both lightness and chrominance). 

Stretching:  Another pass of HistogramTransformation was applied to increase contrast slightly by raising the black point (without clipping) and lowering the grey point to achieve a background intensity of approximately 0.10.

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 30 (max contrast 1.5, strength 0.52, 1 iteration), followed by a scale of 150 (max contrast 1.5, strength 0.22, 1 iteration).

Sharpening: MultiscaleLinearTransform was used to sharpen Layers 2 and 3 with strengths of 0.05 for both. A mask made using the GAME script limited sharpening to high-signal areas in the nebula.

Final Steps: Background, Nebula and star brightness, contrast, and colour saturation were adjusted in several iterations using CurvesTransformation with masks as required. The DarkStructureEnhance script was applied to a clone of the image using default settings. The clone was used to replace pixels in the original using PixelMath and a mask to limit the substitution to the centre of the nebula.  ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.