Medusa Nebula in Three Palettes
RGB with Ha and O3 added
Foraxx palette with RGB stars
Hubble palette with RGB stars
Click an image for full size version
April 7, 2025
The Medusa Nebula lies in Gemini and is well placed for imaging in late winter from my locale. It’s also catalogued as Sh2-274, PK 253+14.1 and Abell 21. The nebula glows mainly due to excited atoms of hydrogen, oxygen and sulfur. Ha and O3 account for the mainly red and teal hues of the nebula in the natural-colour RGB-with Ha-and-O3 image. The other two versions are presented in tone-mapped (i.e. false) colours made using sulfur, hydrogen and oxygen filters. The Hubble palette maps these elements’ emissions to red, green and blue, respectively. This palette shows more detail than the natural-colour version, and reveals some of the chemistry of this dynamic region of space. Each hue represents a different mix of S, H, and O. The Foraxx palette uses the same narrowband data, but gives a completely different result that, in some images, reveals even more than the more traditional Hubble palette.
The Medusa Nebula is about 1,500 light years away and about 8 light years across.
Tekkies:
Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX. Guiding with PHD2. Primalucelab low-profile 2″ Essato focuser, ARCO rotator and Giotto flat panel. 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 under variable moonlight from March 7 – 27, 2025.
Celestron 14″ EDGE HD telescope at f/11 (3,931 mm focal length) and QHY600M-SBFL camera binned 2×2 with Optolong filters.
28x1m Red = 0hr28m
26x1m Green = 0hr26m
22x1m Blue = 0hr22m
1m and 5m S2 totalling 3hr54m
1m and 5m Ha totalling 4hr33m
1m and 5m O3 totalling 4hr00m
Total: 13hr43m
Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization, integration, and auto-cropping.
Colour master: A colour master was made from the Red, Green and Blue masters using ChannelCombination in RGB mode.
Gradient Removal: DynamicBackgroundExtraction was applied to the RGB and narrowband masters.
Colour Calibration: ColorCalibration was used to calibrate the RGB master.
Deconvolution: BlurXterminator was used on the RGB and narrowband masters with Automatic psf at default settings.
Linear Noise Reduction: NoiseXterminator was applied to the RGB and narrowband masters with settings Amount=0.9 and Iterations=3
Star Removal: StarXterminator was used to remove the stars from each of the RGB and narrowband masters, with default settings. Only the RGB stars-only image was preserved.
Hubble Palette Image Creation: The H and O masters were balanced to S using LinearFit and combined using ChannelCombination in RGB mode.
Stretching: HistogramTransformation was applied to the RGB, SHO and narrowband masters to make pleasing images. Approximate background level after the stretches was 0.10 for RGB and SHO and 0.09 for the individual narrowband masters.
Nonlinear Processing
HORGB Image Creation: The stretched H and O masters were combined with the RGB image using PixelMath.
Foraxx Palette Image Creation: The stretched S, H and O masters were combined with Paul Hancock’s Foraxx Palette Utility script.
Background Cleanup: Artifacts that were visible in the three master images were removed using the CloneStamp tool.
Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the RGB, Hubble and Forrax palette masters with settings Amount=0.9 and Iterations = 3
Dynamic Range Compression: The CreateHDRImage script was used to compress the core of the nebula using a mask made with the RangeSelection tool. This was applied to all three images.
Contrast Enhancement: LocalHistogramEqualization was applied twice to each image. A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 150, strength 0.35; scale 50, strength 0.22).
Brightness Enhancement: Using a mask to protect the background and bright parts of the nebula, the faint parts of the nebula were brightened in each master image with ExponentialTransformation and CurvesTransformation.
Sharpening: A mask was used to select brighter regions of nebulosity in each image for sharpening with MultiscaleMedianTransform (Layers 2 – 43with strengths of 0.07, 0.05 , 0.03 and 0.03, respectively).
Hubble Palette and Foraxx Colour Adjustments: CurvesTransformation was used with the Hue tool to adjust the colour tones in the two narrowband images.
Star Processing and Restoration: HistogramTransformation was used to stretch the stars-only RGB image, followed by CurvesTransformation through a star mask to boost saturation using the Saturation slider. The stars were added back into the three master images using the PixelMath expression combine(starless, stars, op_screen()).
Final Steps: Background, nebula and star brightness, contrast and saturation were adjusted using several iterations of CurvesTransformation, with masks, as required. ICCProfileTransformation (sRGB IEC61966-2.1; Relative Colorimetric with black point compensation) was applied prior to saving as a jpg.
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