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May 8, 2016

This is NGC2903, a pretty barred spiral galaxy in western Leo.  It lies about 30 million light years away.  New stars are being formed rapidly in the central region.  Pink emission nebula and blue star clusters are visible in the tightly wound arms near the centre.  The galaxy is a little smaller than our Milky Way, at “only” 80,000 light years across.  Several other galaxies are also visible in this field.

SBIG STL-11000M camera, Baader LRGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with QHY5 on an 80 mm f/6 refractor.  FocusMax for focusing.  CCD Commander to automate the sessions.  Acquistion and guiding with TheSkyX.  All pre-processing and processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario. RGB under moderate moonlight and luminance with no moon.  Average to very good transparency and good seeing.

20x15m R, 18x15m G and B, and 33x15m L unbinned frames (total=22hr30m).

RGB and Luminance
Creation and cleanup:   R, G, B and L masters  were prepared with ImageIntegration.  The R, G and B masters were combined with ChannelCombination, and the resulting RGB image was cropped and processed with DBE, followed by BackgroundNeutralization and ColorCalibration.    The L master was registered to the RGB master, and DBE was applied.

Linear Noise Reduction:  MultiscaleLinearTransform was used to reduce noise in the background areas of the RGB image. Layer settings for threshold and strength:   Layer 1: 3.0, 0.5   Layer 2: 2.0, 0.39 Layer 3:  1.0, 0.25  Layer 4: 0.5, 0.1.  A mask was used to protect high signal areas.

Stretching:  HistogramTransformation was applied to make a pleasing yet bright RGB image. 

Deconvolution:  A copy of L was stretched to use as a deconvolution mask.  A star mask was made from unstretched L to use as a local deringing support. Deconvolution was applied (100 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 25 stars; local deringing at 70% and global dark deringing at 0.02).

Linear Noise Reduction:  MultiscaleLinearTransform was applied to reduce the noise.  Layer settings for threshold and strength:   Layer 1: 3.0, 0.6   Layer 2: 2.0, 0.5 Layer 3:  1.0, 0.4  Layer 4: 0.5, 0.15

Stretching: HistogramTransformation was applied to make an image with similar brightness to the RGB image.  HDRMultiscaleTransformation was applied to the core using a mask to protect the rest of the image.  TGVDenoise was applied using an inverted copy of L as a mask, 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 RGB image as follows:

1. RGBWorkingSpace was applied to the RGB image with all channels equal to 0.33
2. Extract luminance from the RGB image.
3. Apply LinearFit to the extracted luminance using SynthL as the reference.
4. Use ChannelCombination in Lab mode to replace the RGB’s luminance with the fitted luminance from step 3.
5. LRGBCombine was then used to make a SynthLRGB image.

Final Processing
UnsharpMask was applied to the galaxy and mid-sized stars using a mask.  Contrast, brightness and saturation were adjusted in several iterations with the Curves tool, with separate adjustments were made for background and stars using a mask.  

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