The Pinwheel Galaxy is a spiral galaxy in Ursa Major and is a fantastic visual and photographic object. I imaged this galaxy over two nights. The first night was the night of the Camelopardalids meteor shower, so I set my scope up imaging and relaxed with some of my NASA Sky Watcher astro-buddies awaiting the meteor storm...which never happened. At least I got these images out of that night.
Processing of this image has been a struggle since last May because I had used limits that were way too aggressive for the automatic cold pixel detection in the CosmeticCorrection calibration process which zapped some critical color information from the image and left the galaxy core violet! I spent the last two months suspecting chromatic abberations in my telescope and trying to process my way out of the mistake by selecting the galaxy core and transposing the hue with CurvesTransformation. Needless to say, it never worked well and I was never satisfied. Once I discovered where I messed up, I re-tuned the hot and cold pixel detection thresholds which gave me an integrated image with a nice yellow galaxy core. Back in business!
The processing steps below follow in a broad sense Harry Page’s RGB workflow, but there is a hodge-podge of techniques I have learned from a variety of sources. The final revision has milder MMT noise reduction globally, particularly in the galaxy and a darker background (step 29).
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Processing Workflow (PixInsight)-------------------------------------------------------------------------------------------
Pre-processing
1. Calibrate light frames using darks, bias, flats
2. Integrate using Windsorized Sigma rejection method (ImageIntegration)
Post-processing
1. Crop image to remove dithering borders (DynamicCrop)
2. Remove gradients (AutomaticBackgroundExtraction; division)
3. Remove gradients (DynamicBackgroundExtraction; division)
4. Neutralize background (BackgroundNeutralization)
5. White balance (ColorCalibration using M101 as white reference)
6. Set luminance coefficients to 1.0 for RGB channels (RGBWorkingSpace)
7. Balance colors and stretch by setting midtones of R, G and B channels to median linear values of respective channels (Statistics, HistogramTransformation)
8. Stretch by setting midtone of RGB/K channel to 0.75 (HistogramTransformation)
9. Reduce background brightness (CurvesTransformation)
10. Compress dynamic range to increase brightness of M101 (HDRMultiscaleTransform; protect stars)
11. Reduce luminance noise (ACDNR)
12. Reduce chrominance noise (ACDNR)
13. Increase contrast (CurvesTransformation “S” curve)
14. Reduce star sizes (MorphologicalTransformation with contour star mask)
15. Extract luminance image
16. Increase local contrast of galaxy in luminance image (LocalHistogramEqualization; mask selecting galaxy only)
17. Sharpen galaxy details in luminance image (ATrousWaveletTransform, increasing bias and applying k-sigma noise thresholding with an amount of 0.2 to counteract small scale noise increase)
18. Combine luminance image to RGB image, increasing color saturation (LRGBCombination)
19. Remove green color cast (SCNR)
20. Increase color saturation (ColorSaturation; mask selecting galaxy only)
21. Re-neutralize background (BackgroundNeutralization; target background 0.2)
22. Brighten galaxy and darken background (CurvesTransformation)
23. Increase color saturation of red to bring out HII star forming regions (ColorSaturation; mask selecting galaxy only)
24. Final contrast increase (CurvesTransformation)
25. Large scale chrominance noise reduction (ATrousWaveletTransform; target background)
26. Luminance noise reduction (MultiscaleMedianTransform -> Lightness; target background)
27. Clean up dark pixels (2x Pixel Math [Shwartz script])
28. Clean up hot pixels (Pixel Math [Shwartz script])
29. Reduce green tint (SCNR)
30. Lower background brightness (Curves Transformation)
References:
Manfred Schwarz’ galaxy tutorial:
http://www.astrophoto.at/PixInsight/
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