The Needle Galaxy NGC 4565 is an edge-on spiral galaxy in the constellation Coma Berenices that is bright, large and quite easy to find. I managed to image this galaxy for 54 minutes total at the East Coast Star Party 2014 before the clouds rolled in and sent us all to the box wine that night .

During post-processing, the galaxy colors were easy to dig out of the integrated image, but I had to work to bring out the star colors given that my choice of ISO and sub-exposure length had reduced the available dynamic range of the stars strongly, leaving them borderline anemic. In step 8 below I used a technique developed by Vincent Peris of the PixInsight team to compress the dynamic range of the stars which allowed their point spread functions to vary smoothly in a more Gaussian manner which helped retain their color for later enhancement (see Peris reference below). This technique worked pretty well with my shotty data, though it helped highlight some optical anomalies I need to straighten out. If you look carefully at my stars you’ll see that they are reddish on one side and blueish on the other. This suggests that either my telescope is out of collimation (I’ve since re-collimated it) or my DSLR chip sensor is not parallel to the focal plane. The jury is still out on this issue. Fortunately, the asymmetical colors were washed out somewhat when I reduced star sizes with the morphological selection operator in the MorphologicalTransformation process.

All said, though, I am pleased with the final image. It was a lot of fun choosing the final cropping because the stars remind me of a glowing ladder leading to a hovering UFO. It was hard to resist painting laser beams shooting out of this thing!

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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 rejection method (ImageIntegration)

Post-processing

1. Crop image to remove dithering borders (DynamicCrop).

2. Remove gradients (AutomaticBackgroundExtraction; division).

3. Neutralize background (BackgroundNeutralization).

4. White balance (ColorCalibration using galaxy as white reference).

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

6. Stretch by setting midtone of RGB/K channel to 0.75 (HistogramTransformation).

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

8. Restore star colors by compressing dynamic range of stars using method given by Vincent Peris’ “Dynamic Range and Local Contrast” tutorial. Check out Peris reference below for a complete description of his excellent technique. Here is a coarse summary:

8a. Clone linear image at step 5 above and execute MaskedStretch script with target image median equal to that of the image stretched with HistogramTransformation.

8b. Generate mask to select largest differences between the previous two images by taking difference of these stretched images with PixelMath, ATrousWaveletTransform to remove large scale features (wavelet scales > 2), MorphologicalTransformation (dilation) to expand selection.

8c. Clone both stretched images and keep only residual wavelet layers (ATrousWaveletTransform).

8d. Transfer small scale structures to image stretched with HistogramTransformation through mask generated in step 8b (PixelMath; applied 6 times).

9. Compress dynamic range of galaxy to darken dust lane (HDRMultiscaleTransform; mask selecting galaxy).

10. Increase brightness around periphery of galaxy to recover over-darkening by previous step (CurvesTransformation; mask selecting galaxy).

11. Reduce luminance noise (ACDNR to Luminance; luminance mask to protect highlights).

12. Sharpen galaxy (ATrousWaveletTransform to target luminance; mask selecting galaxy)

13. Increase color saturation.

13a. Extract CIE *L channel (ChannelExtraction).

13b. Recombine L image, increasing saturation by paradoxically reducing the saturation setting in ChannelCombination.

14. Reduce chrominance noise in background (ACDNR to Chrominance; mask protecting galaxy)

15. Reduce green (SCNR)

16. Increase color saturation of galaxy (CurvesTransformation saturation channel; mask selecting galaxy).

17. Reduce chrominance noise in galaxy (ACDNR to Chrominance; mask selecting galaxy).

18. Increase image contrast with “S” curve to RGB/K channel (CurvesTransformation).

19. Lower black point a bit (HistogramTransformation).

20. Reduce star sizes (MorphologicalTransformation; morphological selection with contour star mask).

21. Reduce median background channel to around 0.1 (HistogramTransformation; RGB/K channel).

22. Reduce median red background slightly to 0.1 to match the other two channels (HistogramTransformation; R channel).

23. DynamicCrop.

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

References:

Vincent Peris, “Dynamic Range and Local Contrast” tutorial:

http://pixinsight.com/tutorials/NGC7023-HDR/index.html