M51 is a grand design spiral galaxy in Canes Venatici that is interacting with the irregular galaxy NGC 5195. It is rich in detail and is a must-see visual object. This image was acquired over two nights last March-April and I have struggled with the processing ever since. Despite over three hours of integration time (with another 1.5 hours discarded from a third night contaminated by high-level cirrus), the overwhelming light pollution in my city chomped a huge bite out of the dynamic range recorded. As a result, much of the galactic detail I hoped to accentuate exists at the same spatial scale as the noise in the image so I am left with emphasizing large scale structures and color. This is shame because my imaging rig is quite capable of gathering much more detail given the opportunity. The solution is of course darker skies and longer integration times, but M51 season concluded months ago and will not return again until next spring. At that time I plan to revisit this object a darker location with a Barlow lens to set the record straight and give this deep sky gem the distinction it deserves.

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Processing Workflow (PixInsight)

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Pre-processing

1. Calibrate light frames using darks, bias, flats

2. Integrate using sigma clipping rejection.

Post-processing

For each of the four master light frames;

1. Initial crop (Dynamic crop)

2. Reduce background gradient and neutralize background (DynamicBackgroundExtraction, subtract)

3. Neutralize background (BackgroundNeutralization)

4. Set white balance (ColorCalibration; use entire image since it is all stars)

5. Set luminance coefficients to 0.333333 for RGB channels (RGBWorkingSpace).

6. Non-linear stretch (MaskedStretch process; target median background 0.3).

7. Contrast curve to lower background (CurvesTransformation).

8. Reduce luminance and chrominance noise (ACDNR with luminance mask)

9. Increase color saturation of galaxy (CurvesTransformation w/ mask selecting galaxy)

10. Contrast curve to lower background (CurvesTransformation w/ mask protecting galaxy).

11. Increase small scale contrast (LocalHistogramEqualization w/ mask selecting galaxy).

12. Blend star colors into star cores.

12a. Clone image.

12b. Blur the cloned image (Convolution).

12c. Increase saturation of cloned image (ColorSaturation).

12d. Blend 60% blurred cloned image with 40% original image through star mask selecting stars (PixelMath).

13. Reduce green (SCNR)

14. Sharpen galaxy (MultiscaleMedianTransform w/ mask selecting galaxy).

15. Reduce luminance and chrominance noise (ACDNR with luminance mask).

16. Reduce background chrominance noise (ACDNR with mask protecting galaxy and stars).

17. Another contrast curve (CurvesTransformation).

18. Increase color saturation of galaxy (CurvesTransformation w/ mask selecting galaxy).

19. Increase color saturation of stars (CurvesTransformation w/ mask selecting stars).

20. Lower background median brightness to 0.1 (HistogramTransformation).

21. Reduce star sizes (MorphologicalTransformation, Morphological Selection w/ mask selecting stars).

22. Final Crop (DynamicCrop)

23. Set ICC profile to sRGB for web publishing (ICCProfileTransformation).