The Crescent Nebula - NGC 6888

The Crescent Nebula: NGC6888

Object: NGC 6888

Type: Emission nebula
Constellation: Cygnus
Distance: 5000 light years

Date: September 3rd. and October 9th. 2018
Equipment: ATIK 460EX, Vixen 114mm f5.3 ED114 refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 20 x 300s H-alpha, 16 x 450s OIII, 20 flats for each channel, hot pixel removal in Astroart (no darks).

NGC 6888, the “Crescent Nebula”, is a cosmic bubble of rarefied gas about 25 light-years across, formed by stellar winds from its central Wolf-Rayet star (WR 136).  Wolf–Rayet stars are massive stars that have completely lost their outer hydrogen shell and are fusing helium or heavier elements in its core. This star is shedding its outer material in a strong stellar wind, ejecting the equivalent of the Sun's mass every 10,000 years.  The nebula's complex structures are the result of this strong wind interacting with material ejected in an earlier out-gassing phase.  Burning its helium fuel at a prodigious rate and nearing the end of its stellar life, this star will ultimately end in a supernova.

The above image uses narrowband image data obtained by passing the light collected by the telescope through interference filters that isolate light from hydrogen and oxygen atoms in the nebula.  The "forbidden" ionised oxygen atoms produce the blue-green hue enshrouding the detailed folds and filaments of the main nebula.

NGC 6888 is located near Sadr, the central star in the Northern Cross of Cygnus, within the "Summer Triangle" of the bright stars Deneb, Vega and Altair.  The whole area is suffused with wisps of emission nebulae, that show up well in monochrome H-alpha images (below)

Wide field - H-alpha only

A crop of the area shows up the filamentary structure of the nebula...

Crop

These images are the first I have obtained from my new Atik 460EX CCD camera.

Needless to say, any investment in shiny new toys results in conditions that prevent you from playing with them.  Nearly a month elapsed between the collection of the H-alpha data and the OIII frames, due to a combination of moonlight, high haze and horrendous local dew and mist problems.  The night I finally managed to collect the OIII frames was cut short as the humid conditions led to veritable waterfalls of dew running down the outside of the 'scope and the insides of the observatory walls, to the extent that I was worried about shorting out the electrics!

Processing consisted of sigma stacking in AstroArt (which eliminated the aircraft light trails in several of the subs - another challenge associated with imaging from the Medway valley region, which often lies under the Heathrow/Gatwick/London City airport flight paths), with hot pixel removal set at 70%, followed by noise removal and curve adjustment in Paintshop Pro.  The Ha and OIII stacks were taken back into Astroart for registration, and a false green channel then generated in PSP by layering the two narrowband stacks and "multiplying".  The three channels were then RGB combined in Astroart, with a bit of tweaking in PSP to give the final colour image above.

I still feel that the final colour image is a bit flat, and would benefit from some RGB data to give some colour to the field stars.

It's still an improvement upon my first attempt at imaging this nebula, from some 13 years ago...