NGC 7538

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November 20, 2023

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 (see the very red natural colour image at left), oxygen and sulphur also contribute, as shown in the narrowband version, which is based upon the Hubble palette.

Acquisition, focusing, and control of Paramount MX mount with N.I.N.A., TheSkyX and PHD2. Primalucelab low=profile 2″ Essato focuser and ARCO rotator. 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. Data acquired under moderate moonlight between October 4 – November 15, 2023.

Celestron 14″ EDGE HD telescope at f/11 (3,912 mm focal length) and QHY600M camera binned 2×2 with Optolong filters.

13x5m Red = 1hr05m
12x5m Green = 1hr00m
7x5m Blue = 0hr35m
39x5m Ha = 3hr15m
38x5m O3 = 3hr10m
38x5m S2 = 3hr10m

Total: 12hr15m

Image scale 0.4 arcsec per pixel

Finder Chart:

Data Reduction and Linear Processing

Preprocessing: The WeightedBatchPreProcessing script was used to perform calibration, cosmetic correction, weighting, registration, local normalization and integration of all frames. DrizzleIntegration was included for the narrowband filters (Gaussian, 1x, drop shrink=0.9).

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 , and star sharpening set to 0.10 with “Correct First” selected.

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.

Linear Noise Reduction: NoiseXterminator was applied to the RGB and narrowband masters with settings Amount=0.9 and Detail=0.35

Stretching: HistogramTransformation was applied to the RGB and narrowband masters to make pleasing images. Approximate background level after stretch was 0.10 for RGB and 0.08 for the narrowband masters.

Nonlinear Processing

HaRGB Image Creation: PixelMath was used to blend Ha into the RGB image using the following expression with parameter values a=1.2 and b=0.06.
Red: max($T[0], a*Ha)
Green: $T[1]
Blue: iif($T[0]<a*Ha, $T[2] + b * Ha, $T[2])

Hubble Palette Image Creation: The S, H and O masters were combined using ChannelCombination in RGB mode.

Nonlinear Noise Reduction: NoiseXterminator was used to reduce noise in the background areas of the HaRGB and SHO images with settings Amount=0.9 and Detail=0.2

Contrast Enhancement: LocalHistogramEqualization was applied three times to the HaRGB and SHO images. A Contrast Limit of 1.5 and 1 iteration was used for each LHE application (scale 20, strength 0.35; scale 90, strength 0.25; scale 150, strength 0.15).

Sharpening: A mask was used to select brighter regions of nebulosity in each image for sharpening with MultiscaleMedianTransform (Layers 1 – 5 with strengths of 0.03, 0.05, 0.08, 0.06, and 0.03, respectively).

Brightness Enhancement: Using a mask to protect the background, the nebula was brightened in each images with ExponentialTransformation and CurvesTransformation.

Stars-only steps: MorphologicalTransformation was used in Morphological Selection mode to shrink the stars through a mask made by extracting the Luminance from the stars-only image. CurvesTransformation was used to add saturation to the stars.

Hubble Palette and Foraxx Colour Adjustments: CurvesTransformation was used with the Hue tool to adjust the colour tones in the two narrowband images.

Star Restoration: PixelMath expression combine(starless, stars, op_screen()) was used to combine the three starless images with the RGB stars-only HaRGB and SHO images created with StarXterminator.

Final Steps: Background, nebula and star brightness, contrast and saturation were adjusted in several iterations in each image using 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.