M17, The Swan Nebula
August 11, 2016
Messier 17 is known as the Swan Nebula, owing to its appearance in the eyepiece. In a moderately sized telescope, only the brightest part of the nebula can be seen easily, and looks distinctly swan-like. Another nickname is the Omega Nebula; personally I see only a very limited resemblance to that Greek letter. The Swan swims in the Sagittarius Milky Way about 5000-6000 light years away. It is about 15 light years a cross and its total mass is about 800 times that of the Sun. From our vantage point, its location in the sky is not far from the Eagle Nebula.
The above image blends the red, green and blue colour channels with data collected through a deep red H-alpha filter. The black and white image at right shows the H-alpha data on its own. A full size H-alpha image is available by clicking on the image.
SBIG STL-11000M camera, Baader Ha, R, G and B filters, 10″ f/6.8 ASA astrograph, Paramount MX. Guided with QHY5 guide camera and 80 mm f/6 Stellar-Vue refractor. Acquisition, guiding and mount control with TheSkyX. Focusing with FocusMax. Automation with CCDCommander. All preprocessing and post-processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. Nearly full Moon for Ha and little moonlight for RGB. Good to excellent transparency and poor to good seeing throughout acquisition.
6x10m R, G and B and 3x20m Ha unbinned frames (total=4 hr).
Creation and cleanup: Ha, R, G and B masters were cropped and processed separately with DBE. R, G and B were combined to make an RGB image which was processed with ColourCalibration.
Linear Noise Reduction: MultiscaleLinearTransform was used to reduce noise in the background areas. Layer settings for threshold and strength: Layer 1: 3.0, 0.5 Layer 2: 2.0, 0.35 Layer 3: 1.0, 0.2 Layer 4: 0.5, 0.1
Stretching: HistogramTransformation was applied to make a pleasing yet bright image.
Synthetic Luminance and H-alpha only:
Creation and cleanup of SynthL: The cleaned up Ha, R,G and B masters were combined using the ImageIntegration tool (average, additive with scaling, noise evaluation, iterative K-sigma / biweight midvariance, no pixel rejection).
Deconvolution: A copy of the image was stretched to use as a deconvolution mask. A star mask was made from unstretched SynthL to use as a local deringing support. Deconvolution was applied (80 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars; local deringing at 70% and global dark deringing at 0.03).
Linear Noise Reduction: MultiscaleLinearTransform was applied to reduce the noise. Layer settings for threshold and strength: Layer 1: 3.0, 0.5 Layer 2: 2.0, 0.35 Layer 3: 1.0, 0.2 Layer 4: 0.5, 0.1
Stretching: HistogramTransformation was applied to make a pleasing yet bright image. TGV Noise was applied and the image was re-stretched to reset the black point.
Combining SynthL with RGB:
The luminance channel of the RGB image was extracted, processed and then added back into the HaRGB image as follows:
1. Extract luminance from the RGB image.
2. Apply LinearFit using SynthL as the reference.
3. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 2.
4. LRGBCombine was then used to make a SynthLRGB image.
Final Processing of SynthLRGB and H-alpha only:
HDRMultiscaleTrasformation was applied at 6 and 4 pixel scales using a mask. LocalHistogramEqualization was then applied at 50 and 150 pixel scales (maximum contrast 1.5 and strength 0.5). The DarkStructureEnhance script was applied with a strength of 0.25. Contrast, brightness and saturation of nebula, background and stars were adjusted using appropriate masks to protect non-targeted areas. A final application of ExponentialTransformation was used to enhance the faint nebulosity in the H-alpha only image.
Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.