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Bubble Nebula and open cluster NGC 7635

Technical card

Resolution: 4884x3352

Dates: Oct. 11, 2015Oct. 16, 2015

Frames:
11x600" ISO800
Hutech IDAS 2" LPS-D1: 10x480" ISO800

Integration: 3.2 hours

Darks: ~20

Flats: ~20

Bias: ~50

Avg. Moon age: 15.23 days

Avg. Moon phase: 5.84%

Astrometry.net job: 1002714

RA center: 350.650 degrees

DEC center: 61.318 degrees

Pixel scale: 0.930 arcsec/pixel

Orientation: -24.039 degrees

Field radius: 0.766 degrees

Locations: NASA Skywatchers dark site, New Kent, VA, United States; Staunton River State Park, Red Oak, VA, United States

Description

Here is my capture of the Bubble Nebula from two nights last October. The first night was from the NASA Skywatchers dark site which was a perfect night as far as sky conditions were concerned, but a total calamity for imaging during the 3+ hours I spent on this target. Right out of the gate, I botched the focus before I realized about an hour later. Recovering from that misstep, I caught some good frames before the meridian flip. At that point I decided to change the power settings on my laptop from the usual astro-imaging setup (turns off monitor after 2 minutes, never sleeps) to the plugged in power settings since I had access to A/C power. What I didn’t realize is this power setup puts my laptop into hybernation mode after 20 minutes. So while I was gabbing away with an astrobuddy for well over an hour, my imaging gear did squat for most of that time. I was lucky to get just 80 minutes of good subs that night. Lessons learned though.

The remainder of subs were acquired during the Friday night of the Fall Staunton River Star Party at Staunton River State Park in Virginia. Skies were supremely clear that night, though seeing was all over the place. It was such a nice treat to have above-average dark skies that I ditched my light pollution filter and exposed for 10 minutes at ISO800, hoping to capture buckets full of H-alpha emission while compromising with star color in the NGC7635 star cluster.

Similar to my last image or two, I used David Aults’ noise reduction technique and Emanuele Todini’s multi-scale method for bringing out the H-alpha nebulosity. For the first time, I applied color saturation while the data was still linear which I think helped maintain color in the nebulosity after stretching.

I really like the Bubble Nebula and open cluster in the same field of view. It is kind of busy and I didn’t bother with star size reduction because I wanted the open cluster to pop out. Still, I think it is easy at first glance for the viewer to identify the primary subject (the Bubble) and secondary eye-candy (the open cluster).

Revision B is just a close-up of the Bubble.

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Processing Workflow (PixInsight)
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1. Initial crop (Dynamic crop) .
2. Neutralize background (BackgroundNeutralization).
3. Set white balance (ColorCalibration).
4. Set luminance coefficients to 0.333333 for RGB channels (RGBWorkingSpace).
5. Non-linear noise reduction using David Aults’ recipe for mask generation and process parameters. I skipped chrominance noise reduction with MMT below.
5a. Reduce background noise (TGVDenoise with inverted luminance mask).
5b. Reduce background luminance noise (MultiscaleMedianTransform to Luminance)
5c. Reduce background luminance noise (MultiscaleMedianTransform to Chrominance)
6. Increase color saturation (CurvesTransformation to Saturation).
7. Non-linear stretch (HistogramTransformation, lower midtones slider aggressively).
8. Lower blackpoint (HistogramTransformation, raise blackpoint slider).
9. Stretch again (HistogramTransformation, lower midtones slider mildly).
10. Lower blackpoint (HistogramTransformation, raise blackpoint slider).
11. Increase brightness, color saturation and contrast using Emanuele Todini’s multiscale processing technique (reference below)
11a. Duplicate image and extract small scale structures (ATrousWaveletTransform, 5 layers, deselect residual)
11b. Create new image of large scale structures by subtracting previous image from original image (PixelMath, deselect rescale).
11c. Stretch image of large scale structures (HistogramTransformation, lower midtone slider with white and black clipped luminance mask from original image)
11d. Increase color saturation of large scale structures (CurvesTransformation to saturation with previous mask).
11e. Merge images of large scale structures, small scale structures and original image together (PixelMath, rescale selected).
11.f. Repeat steps 11a-e except using white and black clipped luminance mask from large scale structure image and additionally adding contrast curve.
11g. Repeat steps 11a-e except using star mask and LocalHistogramEqualization, kernal radius 200.
12. Reduce green cast (SCNR to green, Amount 1.0)
13. Increase local contrast (LocalHistogramEqualization, Kernel Radius 90, Contrast Limit 1.2, Amount 0.60, Circular Kernel deselected; w/mask protecting stars)
14. Increase brightness of H-alpha nebulosity (CurvesTransformation, increase red channel where dominated by nebulosity w/ mask from extracted red channel).
15. Sharpen nebulosity (MultiscaleMedianTransform, 6 layer total; biases of +0.08 and +0.01 on layers 5 and 6, respectively w/ mask from extracted red channel with stars removed).
16. Sharpen nebulosity and stars (UnsharpMask StDev 2.0, Amount 0.4 w/ lightness mask strongly selecting nebulosity and stars).
17. Lower background brightness slightly (CurvesTransformation to RGB/k).
18. Increase color saturation (ColorSaturation increasing red-orange-yellow and blue).
19. Final brightness reduction of background to median value near 0.11 (CurvesTransformation to RGB/k).
20. Final crop (DynamicCrop)
21. Set ICC profile to sRGB for web publishing (ICCProfileTransformation).

References:

“M42 PixInsight Tutorial” by David Ault for linear noise reduction
http://trappedphotons.com/blog/?p=642

“M42 & NGC1975 – Multilayer Processing (First Part)” Tutorial by Emanuele Todini
http://www.stelleelettroniche.it/en/2014/09/astrophoto/m42-ngc1975-multilayer-processing-first-part/
“M42 & NGC1975 – Multilayer Processing (Second Part)” Tutorial by Emanuele Todini
http://www.stelleelettroniche.it/en/2014/09/astrophoto/m42-ngc1975-multilayer-processing-second-part/

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Author

Jason Tackett
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Bubble Nebula and open cluster NGC 7635, Jason Tackett

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