Sh2-97 Emission Nebula
July 8, 2016
Sh2-97 is a small, extremely faint emission nebula in Cygnus the Swan, and is also designated LBN 151. It is even fainter than Sh2-65, which I acquired during the same time period. From our distance of about 12,700 light years away it appears about 1/3 the width of the full Moon. Although it is in the same region of sky as many other nebulae (part of a huge molecular cloud in Cygnus), Sh2-97 is more than twice as far away. It lies in a rich star field near the edge of the dark rift in the Milky Way in Cygnus. The purplish patch near the bottom of the nebula is blue reflection nebula and red emission nebula together. The star colours give an indication of their temperature: blue and white stars are the hottest; reddish stars are the coolest.
SBIG STL-11000M camera, Baader Ha, R, G and B filters, 10″ f/6.8 ASA astrograph, 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. All preprocessing and processing in PixInsight. Shot from my SkyShed in Guelph, Ontario. Half Moon for Ha and no moon for RGB. Good to excellent transparency and average to very good seeing throughout acquisition.
12x20m Ha, 6x15m R, 6x15mG and 6x15mB unbinned frames (total=8hr30m).
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. The NBRGBCombine Script was applied with default settings to produce a linear HaRGB image. MultiscaleLinearTransform was used to reduce noise in the background areas. HistogramTransformation was applied to make a pleasing yet bright image.
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). PixelMath was used to add the result to 2x the Ha image with no rescaling.
Deconvolution: A copy of the image was stretched to use as a deconvolution mask. A star mask was made from the linear SynthL image to use as a local deringing support image. Deconvolution was applied (90 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.
Stretching: HistogramTransformation was applied to make a pleasing yet bright image. TGVDenoise was applied and the image was stretched again with HistogramTransformation 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. Equalize RGB channels with RGBWorkingSpace and extract luminance from the HaRGB image.
2. Apply LinearFit to extracted luminance 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.
TGVDenoise was applied to the luminance in the nebula, with stars protected. LocalHistogramEqualization was applied twice to boost the nebula using a mask to protect stars and background (scales of 50 and 150, strength of 0.52 and maximum contrast 1.5 for both applications). Contrast, brightness and saturation were adjusted with the Curves tool, with separate passes and appropriate masks for adjusting background, nebula and stars. The DarkStructureEnhance script was applied to increase contrast in the dark portions of the nebula.
Image scale is about 1.1 arcsec per pixel for this camera / telescope combination.