Celestial hemisphere:  Northern  ·  Constellation: Cygnus (Cyg)  ·  Contains:  Crescent nebula  ·  NGC 6888
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Crescent Nebula in H-alpha, Jason Tackett
Crescent Nebula in H-alpha
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Crescent Nebula in H-alpha

Equipment

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Acquisition details

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Description

The Crescent Nebula NGC 6888 is an emission nebula in the constellation Cygnus, formed by stellar winds which excite surrounding hydrogen gas that was expelled from the central star (Wikipedia). This is a 190 minute exposure taken over two nights using a Hydrogen-Alpha filter which isolates light from the excited hydrogen shell. I am particularly excited about this image because I was able to pull out so much faint detail from this dim object in our light-polluted metropolis while a first quarter moon blared in the sky. Additionally, I used the new PixInsight Drizzle Integration tool which helped round the stars out after I extracted the red pixels from the DSLR frame.

For post-processing, I closely followed Alejandro Tombolini’s NGC 1333 tutorial (reference below) and added dark outlier pixel reduction using Manfred Schwarz’s RemoveDarkPixel script in the beginning and contrast enhancement along with star size reduction at the end.

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Processing Workflow (PixInsight)

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Pre-processing

1. Debayer light frames (Debayer, superpixel)

2. Extract red channel (ChannelExtraction)

3. Drizzle integration using Linear Fit Clipping rejection to take into account variability in sky brightness between the two nights. (ImageIntegration, DrizzleIntegration)

Post-processing

1. Reduce dark pixels (2x PixelMath [Schwarz’s RemoveDarkPixel script]; protect stars)

2. Histogram stretch (MaskedStretch)

3. Lower black point (HistogramTransformation)

4. Histogram stretch (MaskedStretch)

5. Lower black point (HistogramTransformation)

6. Histogram stretch (MaskedStretch)

7. Lower black point (HistogramTransformation)

8. Reduce background noise (MultiscaleMedianTransform)

9. Compress dynamic range of nebulosity (2x HDRMultiscaleTransform; protect stars)

10. Background noise reduction (GREYCstoration; protect nebulosity)

11. Brighten nebulosity (ExponentialTransformation; target nebulosity)

12. Compress dynamic range of nebulosity (LocalHistogramEqualization; target bright pixels)

13. Noise reduction (ATrousWaveletTransorm)

14. Contrast enhancement (2x CurvesTransformation)

15. Star size reduction (MorphologicalTransformation; target stars)

References:

Alejandro Tombolini’s NGC 1333 tutorial:

http://www.pixinsight.com.ar/en/info/processing-examples/9/ngc-1333.html

Manfred Schwarz’s galaxy tutorial:

http://www.astrophoto.at/PixInsight/

Wikipedia Crescent Nebula entry:

http://en.wikipedia.org/wiki/Crescent_Nebula

Comments

Sky plot

Sky plot

Histogram

Crescent Nebula in H-alpha, Jason Tackett