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Crescent Nebula in H-alpha

Technical card

Resolution: 3906x2602

Dates: June 6, 2014June 7, 2014

Frames: Baader Planetarium Ha 7nm: 19x600" ISO1600

Integration: 3.2 hours

Darks: ~50

Flats: ~30

Flat darks: ~40

Bias: ~50

Avg. Moon age: 7.97 days

Avg. Moon phase: 56.18%

Astrometry.net job: 296762

RA center: 303.057 degrees

DEC center: 38.358 degrees

Pixel scale: 1.232 arcsec/pixel

Orientation: 83.933 degrees

Field radius: 0.803 degrees

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

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

Author

Jason Tackett
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Crescent Nebula in H-alpha, Jason Tackett

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