Hi:
I'd like to submit the attached image of NGC 7635, the Bubble Nebula, to the February contest. This is a reprocessed version of an earlier rendition that I posted in the Gallery in December. The main change was to boost the saturation of the star colours, which were not well handled in the earlier version. PixInsight was used exclusively for the image processing, except for use of CCDInspector to evaluate the subexposures and to monitor overall specs during processing.
Image capture details
Telescope: PlaneWave CDK17, with a focal reducer at f/4.5.
Mount: Paramount ME.
Camera: SBIG STL-4020M.
Exposures: Approximately 12 hours total: about 300 minutes luminance unbinned, and 100 minutes in each of red, green, blue, and Halpha, all with 2x2 binning; 10 minute sub-exposures in all channels.
Flat field correction: using twilight flats.
Image capture: TheSkyX, MaxIm DL, FocusMax.
Image scales: Unbinned pixel size 0.78", frame about 25' on a side.
Imaged on three nights in summer 2011 from rural British Columbia, Canada.
PixInsight processing highlights
LUMINANCE:
Blended Luminance-Halpha (LHa) composite made with Vicent's formula - and tutoring by Harry ; Deconvolution (reduced Aspect ratio and nonzero Rotation to correct for guiding problems on gusty nights); MultiscaleMedianTransform to reduce noise on first two layers; Two applications of HDRMultiscaleTransform, to 6 layers and then 3 layers, with stars masked; MorphologicalTransformations applied separately to small and bright stars (2-way selections to circularize and erode), followed by mild UnsharpMasks; CurvesTransformation to reduce bright star halos; CurvesTransformation to improve contrast on the bubble edge and the most textured part of the nearby nebula, using two "custom" masks; Very mild UnsharpMask to entire image.
CHROMINANCE:
GradientsMergeMosaic used to combine two Halpha sets with different framing (consequence of finding a suitable auto-guide star before/after meridian flip); Blended Red-Halpha (RHa) composite using Vicent's formula; RHaGB ChannelCombination, BackgroundNeutralization, and ColorCalibration; L* channel of RHaGB matched to LHa using LinearFit; LRGBCombine (Lightness=0.5, Saturation=0.25); Histograms and CurvesTransformations for final blackpoint, colour saturation, and contrast.
Best wishes to all!
Howard.
