Celestial hemisphere:  Northern  ·  Constellation: Cepheus (Cep)  ·  Contains:  NGC 7023
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Iris Nebula, NGC 7023, Jason Tackett
Iris Nebula, NGC 7023
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Iris Nebula, NGC 7023

Equipment

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Acquisition details

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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

Revisions

  • Iris Nebula, NGC 7023, Jason Tackett
    Original
  • Final
    Iris Nebula, NGC 7023, Jason Tackett
    B

Sky plot

Sky plot

Histogram

Iris Nebula, NGC 7023, Jason Tackett