Monkey Head Nebula

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March 17, 2021

The Monkey Head Nebula (NGC 2174; Sh2-252) surrounds open star cluster NGC 2175 and lies about 6,400-7,200 light years from us. The nebula is a stellar nursery, where new stars are forming. In addition to the reddish emission nebula, it contains bluish reflection components. One is at lower right of the nebula, and another is around the brightest star inside the nebula. This field is located in the northern part of Orion, just below the feet of Gemini. Being set in the winter Milky Way, the image is full of stars that show a range of colours varying from orange-red to blue-white based on their temperature (blue is hottest; red coolest).

I have also separately posted a narrowband image using the Hubble Palette. Although I find the natural look above more aesthetically pleasing, the narrowband photo gives us a look at how the gases vary throughout the object, and give it a more three-dimensional appearance. 

Acquisition, focusing, and control of Paramount MX mount, and guiding with TheSkyX. QHY5III-290 guide camera. 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 transparency and seeing. Data acquired March 2-14, 2021 in a moonless sky.

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

H-alpha 26x10m = 4hr20m
Chrominance 106x5m = 8hr50m
Total: 13hr10m
Image scale 1.15 arcsec per pixel (based on Luminance)

Data Reduction and Initial Processing
Preprocessing: The WeightedBatchPreProcessing script was used to create H-alpha master frames (from the mono camera) and register the files from the one-shot colour camera. Integration followed by DrizzleIntegration with CFA Drizzle and 2x scaling was applied to make the Colour master

Gradient Removal: DBE was applied to H-alpha and RGB masters using Subtraction.

Colour Balancing: PhotometricColorCalibration was applied to the Colour master.

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, strength and iterations were as follows: Layer 1: 5.0 0.85, 1 iteration; Layer 2: 3.5 0.75, 1 iteration, Layer 3: 2.0, 0.5, 1 iteration.

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

Deconvolution:  A star mask was made from the H-alpha master to use as a Local Deringing Support Image. A copy of the image was stretched to use as a range mask. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using the PSFImage script; Global dark deringing = 0.02).

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas of the H-alpha image. Layer settings for threshold, strength and iterations: Layer 1: 3.0 0.85, 1 iteration; Layer 2: 2.0, 0.75, 1 iteration; Layer 3: 1.0, 0.25, 1 iteration.

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

Combining Colour and H-Alpha Images
Alignment: StarAlignment was used register the Colour image to the Ha master.

Make HaLRGB: PixelMath was used to add Ha to the Colour image, using the following expressions for the R, G and B channels:

     R:  max($T[0], 1.1*Ha)
     G:  $T[1]
     B:   iif($T[0]<Ha, $T[2] + 0.08*Ha, $T[2])

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 and nebula (max. 1,000 iterations and convergence selected for both lightness and chrominance). 

Contrast Enhancement: LocalHistogramEqualization was applied with a scale of 50 (max contrast 1.5, strength 0.2, 1 iteration), followed by a scale of 150 (max contrast 1.5, strength 0.25, 1 iteration).

Sharpening: MultiscaleLinearTransform was used to sharpen Layers 2 and 3 with strengths of 0.3 and 0.1, respectively. A mask was used to select only bright nebular features for sharpening.

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 run with 0.2 for the Amount. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving in jpg format.