Crescent Nebula NGC 6888 in Cygnus

Sept 4 - 13 2020

Borg 125SD f/8 (FL=1050 mm) with ASI1600 MM Pro on iOptron CEM40EC guided with Kowa scope and ASI290 mini

Shot over many nights with fire, smoke, and even snow in between!

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

Calibrated, cosmetic corrected, aligned, subframe selected

Integrated with ESD rejection

Processed B image to reduce "blue bloat"

Stretch to clean nonlinear & use StarNet++ to generate Star Mask

star mask aggressive MT erosion (6 iterations)

star mask HDR MT

Channel combined RGB to obtain star image

Linear processing 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

https://thecoldestnights.com/2020/06/pixinsight-dynamic-narrowband-combinations-with-pixelmath/

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 blurred 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)

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Writeup for FaceBook:

I already shot this one last fall, but I'm trying out a very different method even if it looks much the same.

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!

WHAT YOU'RE SEEING:

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!

This 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 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 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)