NGC6888 – Crescent Nubula (2015/06/25)

(description obtained from internet) The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away. It was discovered by Friedrich Wilhelm Herschel in 1792.

The sole energy source of the Crescent Nebula is the powerful Wolf-Rayet star HD 192163 (WD 136) visible as a blue star in the center of the shell-like nebula in the image. HD 192163 began its stellar life in the Cygnus OB1 stellar association some 4.5 million years ago as an extremely luminous and hot O- type supergiant. A few hundred thousand years ago it left the main sequence as it began to exhaust its dwindling reserve of hydrogen fuel. Swelling to tremendous proportions it became a red giant, releasing the last vestiges of its hydrogen fuel in a fierce stellar wind at speeds up to 20,000 miles per hour. With its exterior stripped and its inner helium layers laid bare the massive star became unstable. The resulting instability led to the Wolf-Rayet phase of prodigious mass loss from the stars surface. The mass loss occurs at a furious pace in the form of a powerful, high energy stellar wind traveling at speeds up to 3 million miles per hour.

The rapidly expanding shell of hot gas blown out by the stellar wind ultimately collides with the slower moving ambient gas cloud, much of which was ejected thousands of years earlier during the stars red giant phase. The force of the collision creates a complex shock front releasing energy in many different wavelengths including the brilliant colors of the visual spectrum. The result of this dynamic process is the shell-shaped glowing gas cloud we know as the crescent nebula.

The physical structure of NGC 6888 is an oblate shaped shell measuring roughly 25 x 16 light years in dimension. The material lost by HD 192163 during its red giant phase was swept up by the subsequent fast wind during the WR phase. Subsequently the shell thinned and fragmented and became ionized by the ultraviolet flux of the central star leaving the nebula as we see it today. Presently the nebula consists of a network of clumps and filaments which shine from the ionizing field of the central star. The shell appears to be extremely leaky to ionizing emission as only 2% of the photons from HD 192163 are processed by the nebula. The escaping photons ionize the thin neutral clouds which surround the nebula.

This is my first complete LRGB and narrowband project. I spent almost 5 nights to acquire images, at 3 different locations which are hundreds of miles away from each other. But my quest for Crescent Nebula started 2 years ago. At Calstar 2013 I was doing my Herschel 400/Hershcel II 400 list and spending 30 mins on searching it under very dark sky with my 10″ DOB, but without any success. One year later, at Henry Coe state park, with the help of narrowband filter, I finally grasped the crescent shaped (more precisely the Euro shaped) nebula.

LRGB

This is LRGB imaged taken at GSSP 2015.

NGC6888_20150718_LRGB

NGC6888 LRGB (2015/07/18)

Narrow Band (Ha/O3/S2)

I spent 3 nights capturing narrowband image. First I captured some Ha/O3/S2 at Henry Coe and BLM Panoche Hill, but then I realized that I need some more O3 subs as it showed some details which were not in Ha subs, esp those halos around the nebula. So I spent another night to capture more O3 and Ha.

I tried a few palette to map narrowband to RGB color space, finally I chose the following one:
R = Ha + S2
G = Ha * 0.075 + O3
B = O3
L = max (Ha, O3). This does not seems a good choice for L because it seems a little bit noisy but it works.

I prefer this mapping for two reasons:
– The star color looks more natural, very close to the RGB star color.
– The color contrast shows details of O3 emission lines.

Overall I am very satisfied with the result. There are something more I will work on in the future:
– Increase Red weight as the overall color tune is too yellow
– Try different mapping
– Integrate RGB star to narrowband image

NGC6888_20150718_NB_RGB

NGC6888 Ha/O3/S2 narrowband RGB (2015/06/25)

NGC6888_20150718_NB_LRGB

NGC6888 Ha/O3/S2 narrowband LRGB (2015/06/25)

The following is the image using HOS palette (Red = Ha, Green = O3, Blue = S2). The drawback of this palette is that there are lots of magenta stars that look wired. I use ColorMask script generating a mask for magenta color, with which the magenta color is removed via CurveTransformation.

NGC6888_NB_LHOS

NGC6888 Ha/O3/S2 narrowband LHOS (2015/06/25)

These are integrated Ha/O3/S2 image.

NGC6888_Ha

NGC6888 Ha

NGC6888_O3

NGC6888 O3

NGC6888_S2

NGC6888 S2

Date: 06/25/2015
Location: Henry Coe State Park, Morgan Hill, CA
Condition: excellent transparency, excellent seeing
Exposure: Ha – 1200sx15, Dark/Bias/Flat calibrated
Date: 07/15/2015, 07/16/2015
Location: GSSP 2015 – Adin, CA
Condition: excellent transparency, excellent seeing
Exposure: L – 900sx8, R – 600sx6, G – 600sx6, B – 600sx6, Dark/Bias/Flat calibrated
Date: 08/17/2015
Location: BLM Panoche Hill, CA
Condition: excellent transparency, excellent seeing
Exposure: O3 – 1200sx13, S2 – 1200sx6, Dark/Bias/Flat calibrated
Date: 08/25/2015
Location: BLM Panoche Hill, CA
Condition: excellent transparency, excellent seeing, breezy before midnight
Exposure: O3 – 1200sx10, Ha – 1200×5, Dark/Bias/Flat calibrated

Total: Ha – 1200sx20, O3 – 1200sx23, S2 – 1200sx6
Equipment: AT111EDT on CGEM, guided by OAG-8300/ST-i. STF8300M+FW8+Baader LRGB
Software: Pixinsight

One thought on “NGC6888 – Crescent Nubula (2015/06/25)

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