The Dumbbell Nebula is a planetary nebula in the constellation Vulpecula. It was my main imaging effort on Friday night at the East Coast Star Party. The sky transparency wasn’t the best for the first part of the night but improved spectacularly for the last hour before astronomical dawn.

Several processing steps below follow a YouTube M27 processing example (link below) but I modified the steps as needed for my image. The color balancing and image stretching technique I learned from the video worked very well to give a broad histogram mountain with a neutral background in this dense star field. I believe the heritage of this technique is from Rogelio Bernal Andreo. The revised version includes a correction to my calibration routine where I had set the Auto limits on CosmeticCorrection too aggressively, which wiped out some of the red in the nebula and altered the PSF of the bright stars. After fixing this, much more red is visible and the stars are more colorful.

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Processing Workflow (PixInsight)

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Pre-processing

1. Calibrate light frames using darks, bias, flats

2. Integrate using Linear Fit Clipping rejection method (ImageIntegration)

Post-processing

1. Crop image to remove dithering borders (DynamicCrop)

2. Remove gradients (DynamicBackgroundExtraction; subtraction)

3. Set luminance coefficients to 1.0 for RGB channels (RGBWorkingSpace)

4. Balance colors and stretch by setting midtones of R, G and B channels to median linear values of respective channels (Statistics, HistogramTransformation)

5. Stretch by setting midtone of RGB/K channel to 0.85 (HistogramTransformation)

6. Saturate colors and reduce chrominance noise by extracting luminance image and recombining with RGB image while lowering saturation slider and applying noise reduction (LRGBCombination)

7. Remove green color cast (SCNR)

8. Sharpen nebulosity (MultiscaleMedianTransform; target M27 with mask)

9. Increase overall contrast slightly (CurvesTransformation “S” curve)

10. Increase red contrast in nebula by extracting R, G and B channels then applying “S” curve to M27 in R channel then recombining all three channels (ChannelExtraction, CurvesTransformation; target M27 with mask, ChannelCombination)

11. Increase color saturation in M27 (CurvesTransformation; target M27 with mask)

12. Reduce star sizes (MorphologicalTransformation with contour star mask)

13. Sharpen star cores (ATrousWaveletTransform with small-scale star mask)

14. Reduce blue halos due to chromatic abberation (ColorSaturation with contour star mask)

14. Sharpen medium scale nebulosity (MultiscaleMedianTransform; target M27 lightness with mask)

15. Darken background slightly (CurvesTransformation)

16. Reduce chrominance noise in background (InterChannelCurves; target channel CIE c, reference channel CIE L with range mask protecting stars and nebulosity)

References:

“Processing M27 in PixInsight” by garbz01

https://www.youtube.com/watch?v=trR5g3riAk8