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Iris Nebula, NGC 7023

Contains: NGC 7023

Technical card

Resolution: 3774x2696

Dates: Aug. 24, 2014Aug. 25, 2014Aug. 26, 2014Aug. 27, 2014

Frames:
CLS-CCD: 1x445" ISO400
CLS-CCD: 1x688" ISO400
CLS-CCD: 26x780" ISO400
CLS-CCD: 1x228" ISO800
CLS-CCD: 1x291" ISO800
CLS-CCD: 2x360" ISO800
CLS-CCD: 1x381" ISO800
CLS-CCD: 23x390" ISO800

Integration: 8.9 hours

Darks: ~17

Flats: ~30

Flat darks: ~30

Bias: ~50

Avg. Moon age: 14.68 days

Avg. Moon phase: 1.14%

Astrometry.net job: 361054

RA center: 315.365 degrees

DEC center: 68.191 degrees

Orientation: 88.126 degrees

Field radius: 0.749 degrees

Locations: Backyard, Hampton, VA, Hampton, VA, United States

Description

The Iris Nebula is a reflection nebula in the constellation Cepheus surrounded by clouds of light-absorbing dust. I imaged this object over four nights, three of which having excellent transparency, and the fourth was below average due to high-level haze. I chose to image this object because recently I broke my Canon 1000D and decided to replaced it with a Canon 550D. Since I had a week to wait until I could send it to Gary Honis for a Baader modification and because the Iris Nebula does not boast of H-alpha emission, I figured it was a good target.

The Achilles heel of this project was light pollution. To ameliorate its effect, I chose ISO 400 for many frames to leave some dynamic range to spare after background subtraction. Also I used my Astronomik CLS-CCD filter which became the limiting factor because the red channel was not as well exposed as the others, leaving a broad noisy distribution of red pixels that had to be cleaned up in post-processing (see step 19). Even with these efforts and nearly 9 hours of exposure time, the dynamic range lost due to light pollution limited the amount of stretching I could apply before the nebulosity became tattered. Also, several stars were overexposed so I need to go back and image this scene with shorter exposure times, rather than hide them under the rug in processing. Not sure when I will get to that with so few clear nights each month.

Still, the Iris Nebula is a lovely object and I enjoyed the four nights I spent staring at that one star on my monitor, wondering what treasures lie buried at the bottom of the photon well.

Revision B removes the blue/green cast of the original version.

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Processing Workflow (PixInsight)
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Pre-processing
1. Calibrate light frames using bias, darks, flats.
2. Integration using Windsorized Sigma Clipping rejection

Post-processing
1. Crop dithering borders (DynamicCrop)
2. Reduce background gradients (DynamicBackgroundExtraction, subtraction, normalize).
3. Reduce background gradients (DynamicBackgroundExtraction, division, normalize).
4. Neutralize background (BackgroundNeutralization).
5. Reduce image brightness by 10% to alleviate overexposed stars somewhat (PixelMath -> 0.9*$T)
6. White balance (ColorCalibration)
7. Reduce image brightness by 8% since ColorCalibration increased brightness of overexposed stars (PixelMath -> 0.92*$T)
8. Set luminance coefficients to 0.33333 for RGB channels (RGBWorkingSpace).
9. Noise reduction (MultiscaleLinearTransform w/ luminance mask).
10. Non-linear stretch (MaskedStretch; iterations, target median, shadows clipping: 100, 0.1, 0.01).
11. Non-linear stretch (MaskedStretch; iterations, target median, shadows clipping: 200, 0.2, 0.02).
13. Non-linear stretch (MaskedStretch; iterations, target median, shadows clipping: 200, 0.25, 0.025).
14. Non-linear stretch (MaskedStretch; iterations, target median, shadows clipping: 200, 0.3, 0.03).
15. Lower blackpoint (HistogramTransformation).
16. Brighten midtones (HistogramTransformation).
17. Contrast curve (CurvesTransformation w/ star mask).
18. Increase local contrast (LocalHistogramEqualization w/ star mask).
19. Reduce red noise:
19a. SCNR to red, 50% w/ star mask.
19b. Raise shadows of red channel slightly so red histogram has similar width and similar mode as green and blue channels (CurvesTransformation w/ star mask).
20. Noise reduction (ACDNR, luminance and chrominance w/ luminance mask).
21. Increase color saturation by extracting luminance and recombining while increasing saturation (LRGBCombination).
22. Increase star color saturation (CurvesTransformation w/ star mask selecting stars).
23. Reduce green (SCNR w/ star mask).
24. Increase reflection nebulosity, darken dust absorption regions (CurvesTransformation w/ star mask).
25. Attempt to repair blown-out stars which have a saturated red hue (InterChannelCurves, target channel c, reference channel a w/ star mask selecting stars).
26. Add color to star cores:
26a. Clone image, apply Convolution, saturate colors with ColorSaturation.
26b. Combine 50% cloned image and 50% original image through star mask with PixelMath.
27. Reduce star sizes (MorphologicalTransformation w/ star mask selecting stars).
28. Sharpen nebulosity (UnsharpMask w/ mask selecting only nebulosity).
29. SCNR to green.
30. Reduce residual blue cast (Curves, HistogramTransformation).
31. Lower blackpoint to around 0.1 (HistogramTransformation).
32. ICCProfileTransformation to sRGB.

Comments

Author

Jason Tackett
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Iris Nebula, NGC 7023, Jason Tackett

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