IC417, The Spider Nebula

IC417Click image for full size version

November 27, 2015, published on space.com Dec 10, 2015; lead image in article in Sky & Telescope June 2016

S_and_T teaser Aug 2016IC417IC 417 is an emission nebula in the constellation Auriga, lying about 10,000 light years away.  The glowing centre of the nebula is energized by the hot, massive blue stars embedded within. New stars are being formed in the ovals above centre and the surrounding bluish nebula above-left of centre.  This nebula is informally known as The Spider Nebula.  The “Fly” lies nearby, and I might image it sometime.

Tekkies:
SBIG STL-11000M camera, Baader Ha, R, G and B filters, 10″ ASA astrograph operating at f/6.8, Paramount MX.  Guided with QHY5 guider and 80 mm f/6 Stellar-Vue refractor.  Acquisition and guiding with TheSkyX.  Focusing with FocusMax.  Automation with CCDCommander.  Calibration, cosmetic correction, registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  Nearly full Moon for Ha and no moon for RGB.   Good to very good transparency and average seeing throughout acquisition.

17x15m R, 18x15mG, 17x15mB and 8x30m Ha unbinned frames (total=17hr).

HaRGB
Creation and cleanup:  R, G, B and Ha masters were cropped and processed separately with DBE.  The R, G and B channels were combined and the resulting RGB image was processed with ColorCalibration.   The NB-RGBCombine Script was applied with default settings to produce a linear HaRGB image.  PixelMath was used to boost the blue by adding 7% Ha.

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, 0.39  Layer 3:  1.0, 0.2  Layer 4: 0.5, 0.11

Stretching:  HistogramTransformation was applied using autostretch settings. TGV Denoise was applied and the image was re-stretched to reset the black point.

Synthetic Luminance
Creation and cleanup: 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 (60 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 using autostretch settings.  TGV Denoise was applied and the image was re-stretched to reset the black point.

Combining SynthL with HaRGB:
The luminance channel of the HaRGB image was extracted, processed and then added back into the HaRGB image as follows:
1. Extract luminance from the HaRGB image.
2. Apply LinearFit using SynthL as the reference.
3. Use ChannelCombination in Lab mode to replace the HaRGB’s luminance with the fitted luminance from step 2.
4. LRGBCombine was then used to make a SynthLHaRGB image.

Final Processing
HaRGB:  Star shapes were smoothed and sizes reduced by applying MorphologicalTransformation through a star mask using the Morphological Selection mode set at 0.35.  UnsharpMask was applied through the same mask with a scale of 1.4 px and a strength of 0.5.  Overall contrast, brightness and saturation were adjusted with the Curves tool.  A final boost of colour saturation was applied to the smaller stars only.

Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.