M109, Barred Spiral Galaxy


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March 8, 2015

M109 is a barred spiral galaxy that lies around 85 million light years away (give or take).  It is the largest galaxy in the M109 group of galaxy, several other members of which are also shown in this image (streaks and fuzzy round objects).  This galaxy is quite small for my imaging equipment.  I will come back to it one day.  I can use this image to provide colour information and combine it with more luminance data from a longer focal length scope.  M109 is tucked up quite close to the bright star Phecda, the bottom Big Dipper bowl star that is closest to the handle.  Phecda is so bright that even though it is well out of the field of view, you can still see diffraction spikes from it (diagonal faint bluish spikes in the upper right quarter of the image).

It has been so bad for imaging since mid-December (cold, windy, cloudy and moonlit, all of the above) that I needed 12 nights to collect the 18+ hours of light contained in this image.

SBIG STL-11000M camera, Baader RGB filters, 10″ f/6.8 ASA astrograph, Paramount MX.  Guided with STL-11000’s external guider and 80 mm f/6 Stellar-Vue refractor.  Acquistion, guiding and calibration done using Maxim-DL.  Focusing with FocusMax.  Automation with CCDCommander.  Registration, integration and all processing in PixInsight.  Shot from my SkyShed in Guelph, Ontario.  moderate moonlight for RGB, no moon for L; Full moon for Ha.

18x10m R, 16x10m G, 17x10m B, 40x10m L and 9x20m Ha unbinned frames (total=18hr10m).

Creation and cleanup: Ha, L, R, G and B masters were cropped.  R, G and B were combined to make an RGB image which was processed with DBE and ColourCalibration.  The Ha image was also processed with DBE and the NB-RGB Combine script was applied to blend the Ha into the RGB.

Stretching: HistogramTransformation was applied using autostretch settings from ScreenTransformation tool.

Synthetic Luminance:
Creation and cleanup: The cropped L, 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). DBE was applied to neutralize the background.

Deconvolution:  A star mask was made to use as a local deringing support. A copy of the image was stretched to use as a range mask. Deconvolution was applied (100 iterations, regularized Richardson-Lucy, external PSF made using DynamicPSF tool with about 20 stars).

Stretching: HistogramTransformation was applied using autostretch settings from ScreenTransformation tool.

Combining SynthL with RGB:
The luminance channel of the RGB was extracted, processed and then added back into the RGB image as follows:
1. Extract luminance from the RGB image.
2. Apply LinearFit using the SynthL channel as a 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
Dynamic Range Adjustment and Stretching: HDRMultiscaleTransform was applied at 5 pixel scales, protecting bright stars with a mask. TGVDenoise was applied in RGB/K mode with default settings, followed by HistogramStretch.  A range mask was made that protected stars and background, and LocalHistogramEqualization was applied to the galaxy.

Boosting Ha:  The Ha master was copied and the copy was stretched to make an HaBoost Mask.  PixelMath was used to subtract the Deringing Support mask; this removed the bright stars from the HaBoost Mask.  Colour saturation and red channel were increased in the areas exposed by the mask — primarily the Ha regions in the galaxy.

Final Steps: There was a slight pink halo around bright stars.  To remove them, a Contour Mask was made using the DeRinging Support Mask as a starting point.  StaMask was applied with the Contour checkbox selected.  This made a mask that was black except for a ring around bright stars.  Colour saturation was reduced through this mask,  Background colour saturation was also reduced slightly.   A blurred range mask was made and used to increase colour saturation and contrast in the galaxy slightly.

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

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