Sunday, November 30, 2014

The Wizard Nebula (NGC 7380)

The Wizard nebula is an emission nebula. It's in a distance of 8,000 light years from earth and lies in the plane of the milky way (hence the myriads of stars). The stars that are formed by the nebula are relatively young (5 million years).

Caroline Herschel discovered the open star cluster in 1787 (her husband William included it in his catalog as H VIII.77). It was many years later that the nebula itself was discovered and finally cataloged as NGC 7380.

(click image to get see a full resolution version)

This image consist of 3h10min Ha, 5h50min OIII and 23h10min SII data. The H694 chip seems to be no too sensitive to SII - with that much exposure time, I expected a much stronger SII signal. I tried boosting it more, but the red quickly overwhelmed the background and stars. This and all the stars made it quite tricky to process this image. After stacking with CCDStack, I did all the processing with Pixinsight:

  1. Cropping all images
  2. DBE for all individual images
  3. TGVDenoise on all individual images
  4. LRGBCombine
  5. Another DBE to remove remaining gradients that weren't visible in the monochrome images
  6. BackgroundNeutralization
  7. ACDNR for further noise reduction
  8. Initial Stretch
    • I also did a stretch with MaskedStretch which resulted in less star bloat. But the image lost a lot of contrast. I transferred the stars from the MaskedStretch to the other image. I learned about this from one of the many great videos on Harry's Astroshed.
  9. Correcting black point
  10. CurvesTransformation to further drop the background
  11. Using the image itself as a mask, I then increased the red and blue signal in the nebula. Using a mask reduced the effect on the background
  12. Inverting the mask and reducing the red and blue in the background
  13. Using a RangeMask+StarMask I used LocalHistogramEqualization
    • First to bring out the fainter, outer areas, and then
    • (after inverting the mask) to increase the inner layer
  14. Using the image itself as a mask, using ATrousWaveletTransform to increase contrast in the nebula
  15. CurvesTransformation to drop the background
  16. ColorSaturation to increase the blue areas
  17. Final black point correction using HistogramTransformation
  18. Removing the magenta from the stars
It took me many, many attempts to come up with this sequence.