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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

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% 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


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.

Processing Workflow (PixInsight)

1. Calibrate light frames using bias, darks, flats.
2. Integration using Windsorized Sigma Clipping rejection

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.



Jason Tackett


Sky plot

Sky plot


Iris Nebula, NGC 7023, Jason Tackett

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