NGC 7538
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December 9, 2024
NGC 7538 is an emission nebula that lies not far from the Bubble Nebula in Cassiopeia, about 9,100 light years away. It has an apparent size of about 8′ x 7′ (the full moon is about 30′ across). NGC 7538 contains the largest known protostar, estimated to be 300 times the size of our solar system. A protostar is a young star that is still growing by gobbling up matter from the molecular cloud that it was born from. Although hydrogen dominates the visible emissions, oxygen and sulphur also contribute, as shown in the narrowband version above, which is based upon the Hubble palette. I also made a natural-looking colour image from the RGB stars combined with the H-alpha and O3 datashown at right.
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. Above average transparency and average or better seeing. SHO data acquired during and after full moon, and RGB data acquired in a moonless sky, between October 4 – November 15, 2023 and October 16-21, 2024.
Celestron 14″ EDGE HD telescope at f/11 (3,931 mm focal length) and QHY600M-SBFL camera binned 2×2 with Optolong filters.
8x5m Red = 0hr40m
7x5m Green = 0hr35m
7x5m Blue = 0hr35m
68x5m S2 = 5hr40m
74x5m Ha = 6hr10m
72x5m O3 = 6hr00m
Total: 19hr40m
Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization, integration, drizzle integration for S, H and O filters (1x, 0.9 drop shrink, Gaussian kernel) 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 Detail=0.35
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.
HOO Image Creation: The H and O masters were combined using ChannelCombination in RGB mode, with H assigned to red and O assigned to both green and blue.
Stretching: HistogramTransformation was applied to the SHO and HOO masters to make pleasing images. Approximate background level after the stretches was 0.10 for both images.
Nonlinear Processing
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 SHO and HOO masters with settings Amount=0.9 and Detail=0.15.
Dynamic Range Compression: The CreateHDRImage script was used to compress the core of the nebula in both the SHO and HOO masters using a mask made with the RangeSelection tool.
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 0.21; scale 50, strength 0.18).
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 – 4 with strengths of 0.07, 0.07, and 0.03, respectively).
Colour Adjustments: CurvesTransformation was used with Curves to adjust the colour tones in the two narrowband master images.
Dark Structure Enhancement: The DarkStructureEnhance script was applied to the HOO image with default settings, except strength= 0.25.
Star Processing and Restoration: HistogramTransformation was used to stretch the RGB stars-only image, followed by CurvesTransformation through a star mask to boost saturation using the Saturation slider. The stars were added back into the SHO and HOO master images using the Screen blending mode in PixelMath with the expression combine(starless, RGB_stars, op_screen()).
Final Steps: Background, nebula and star brightness, contrast, hue 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. The finder chart was made using the FindingChart process.
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