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Crescent Nebula: Bicolor with RGB Stars

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#1 airscottdenning


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Posted 16 September 2020 - 01:10 PM

Crescent Nebula NGC 6888 in Cygnus -> Sept 4 - 13 2020


Borg 125SD f/8 with ASI1600 MM Pro on iOptron CEM40EC guided with Kowa scope and ASI290 mini

FL = 1050 mm; guider FL=100 mm


Shot over about two weeks while dodging fire, smoke, and even snow in between.

Bortle 5/6 on the best nights, but these were NOT the best nights!


Captured in KStars/Ekos on Rasp Pi


After deleting bad frames  using subframe selector, I kept

33x600s Ha; 34x600s O3; 10x60s R; 10x60s G; 10x60s B

total exposure = 700 min ~ 11.7 hours

30 darks; (30 flats, 30 flat-darks for each filter)


Processed in PixInsight:

Calibrated, cosmetic corrected, aligned, subframe selected


Integrated with ESD rejection


Processed B image to reduce "blue bloat"
star mask aggressive MT erosion (6 iterations)

star mask HDR MT


Channel combined RGB to obtain star image
Linear processign including aggressive MLT denoise
Light histogram stretch with some dark clipping for very clean RGB stars


Processed Ha and O3 following C Foster workflow:

Crop, skipped the DBE
MURE Denoise using multiple previews
Deconvolution of each master while still linear

Light histogram stretch of each NB master with ~ 0.1% black clipping


PixelMath combination of NB data following

R = Ha
G = ((Oiii*Ha)^~(Oiii*Ha))*Ha + ~((Oiii*Ha)^~(Oiii*Ha))*Oiii
B = Oiii


Created Synthetic Luminance from Ha + 2*O3

LRGB Combined NB with SynthL

StarNet++ to remove stars from combination


Aggressively stretched  clipped L mask blured with Convolution
Apply to combined NB starless image
Very aggressive TGV and heavy MMT to clean dark regions of artifacts


PixelMath combination of RGB stars with starless NB image:
RGB/K = max(stars, (starless ^ ~starless)


Final Curves tweaks 




This is about 12 hours of exposure over two weeks in September, with horrible nights of choking smoke and even a couple nights of snow in between.


The diaphanous bits are fluorescent space plasma (hydrogen and oxygen -- this is how water is created!). They show up even against the backdrop of the full moon because they're shot through emission-line filters that only pass 3 nm of spectrum.


The stars though -- very different from last year's version. They are shot in "normal" broadband light, so you can see the actual colors of the myriad suns and imagine the solar systems dancing around them. A rainbow of spherical sparkles in and around the water-generating fountain of ionized spectacle!




The Crescent Nebula, an extremely hot and violent star blowing its atmosphere into space in the constellation Cygnus (Swan, Northern Cross).


I prefer the name "Brain Nebula" because duh! The nebula is 5000 light years away, so we see it about the time of the first Pharaohs of ancient Egypt. It's 25 light years across, about 6 times as far as from here to Alpha Centauri!


At the center of the nebula is a very unusual "Wolf-Rayet" star that's hundreds of thousands of times brighter than the Sun! Like all super-massive stars, it lives fast, dies young, and leaves a good-looking corpse. Something like 300,000 years ago, it ran out of hydrogen and began fusing helium in its core, which made it get so hot that it swelled up to a diameter greater than the orbit of Mars and started losing the outer parts of its atmosphere, which were so far from the core that gravity couldn't hold on.


The filamentous blue haze around the nebula is oxygen, lost from that long-ago "red giant" stage. More recently the core collapsed and the star got even hotter. The star is literally blasting its guts out into space, and where the ejected supersonic plasma smacks into the previously shed layers a shock front ionizes the old gas. The red in the image is hydrogen and the white is both hydrogen and oxygen (ie, H2O).

Attached Thumbnails

  • NGC.6888.thumbnail.jpg

Edited by airscottdenning, 16 September 2020 - 01:12 PM.

  • Mert, markdudley, PhotonHunter1 and 3 others like this

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