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The Crescent Nebula NGC 6888

The Crescent Nebula rides on the Swan’s neck in Cygnus in a dense swathe of Milky Way stars - an ideal target for my first bi-colour image with my new Astrodon 3nm Ha and OIII filters and Esprit 150 telescope.

The nebula was discovered by William Herschel on 15th September, 1792 and he described it as ‘A double star of the 8th magnitude with a faint south preceding milky ray joining to it.8’ long by 1.5’ broad’. This double star is not the prominent star with an apparent companion close to the heart of the nebula, rather, it is ADS13515 at the 2 o’clock position on the nebula’s bright periphery. The bright star off-centre of the nebula is particularly significant as this is the star that is powering the emissions from the surrounding gas cloud. This magnitude +7.5 star, HD192163, is of the Wolf-Rayet type and is also designated WR-136.

Charles Wolf and Georges Rayet first described wolf-Rayet stars in 1867 following detection of their broad emission lines. The Wolf-Rayet stage applies to stars with an original mass in excess of 30 times our own Sun’s mass. This stage comes late in the star's evolution when a rapidly expanding shell of hot gas is powered outwards by the stellar wind only to collide with the much slower-moving gas clouds that were ejected thousands of years previously when the star entered its Red Giant phase. These forceful collisions produce a shock wave that generates an enormous amount of energy including wavelengths within the light spectrum, allowing us to observe them. This complex process displays as an arc of bright nebulosity that we identify as the Crescent Nebula. Long exposure images fill in this arc producing a crab-shell shaped nebulous region rich in Hydrogen Alpha and doubly ionised Oxygen emissions. WR-136 is fated to go supernova at some time in the future – watch this space!

Image Stats

Mount: Mesu 200
Telescope: Sky-Watcher Esprit 150 ED Pro
CCD Camera: QSI 683 WSG-8
Sampling: 1.04”/pixel
Guiding: OAG/LodeStar
Filters: Astrodon 3nm Ha and 3nm OIII
Exposure: 30 x 1800 sec Ha, 15 x 1800 sec OIII
Date: 11/06/17 + 19/06/17 – much of which was under Lunar illumination
Calibration: Bias, Darks & Flats

Object Stats

RA: 20° 12’ 04.6”
Dec: 38° 30’ 46.0”
Magnitude: +10.0
Distance: 4700 light years

The Crescent Nebula – NGC 6888


Comparison of Ha and OIII data

We imagers (well me anyway!) tend to think that Ha is the all-powerful emission line in nebulous objects but it is interesting to compare the Ha and OIII data for this structure as there is an enormous amount of OIII emission present in The Crescent Nebula.


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2 hours ago, DaveS said:

I may have a look at this in [NII].

The 3nm Ha filter does block this emission which is why some imagers choose the 5nm version but as I already have a 7nm Ha filter, I can continue to use this when imaging planetary nebulae which often benefit from the NII content.

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This looks great and I love the comparisons as well. I did similar with the Crescent and it was such a surprise as we are normally used to seeing target with next to no OIII! It's a difficult image to balance as well.... nice one Steve :)

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1 hour ago, Tyson M said:

Incredible!   Well presented too. 

Couldn't agree more.  Tights stars and sharp Crescent with its filamentary detail without being forced; well defined OIII envelope. Great :icon_salut:.

Very interesting to see the comparison especially for new imagers to help them gauge data for new targets.

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Super. The Wolf Rayets do often produce strong OIII emission and OIII envelopes are a treat to image since they don't just enhance an image, they change it entirely. There is an under-imaged OIII shell around the Jellyfish, for instance.

Great job here, Steve, with everything in place.


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On 22/06/2017 at 20:46, ollypenrice said:

The Wolf Rayets do often produce strong OIII emission and OIII envelopes are a treat to image since they don't just enhance an image, they change it entirely.

The extreme velocity stellar winds from these W-R stars are amazing, almost hard to imagine the intensity and the effect is shown here in the OIII generation. As for changing the image entirely, I agree and I hope that the two mono images show this well - the Ha version is how I normally visualise this object but the OIII displays a very different beast and a somewhat extended shape.

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