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NGC6888, the Crescent Nebula, in hydrogen alpha light.

This nebula in Cygnus is the result of the collision of the stellar wind of the 7th magnitude Wolf-Rayet Star star seen at centre, with material shed during its preceding phase as a red supergiant. Wolf-Rayet stars are very hot (25,000-50,000 K), unstable (and may go supernova soon) and expel their outer layers of gas at high velocities.  The resulting nebula is a bubble enveloped in a skin of emission nebula which is shock-heated by the collision.  It measures 26 x 16 light years and is located in the Cygnus Arm of our Galaxy.  Distance is estimated to be 4700 light yrs away.


Image info

Telescope:  Celestron 9.25" Schmidt-Cassegrain at f/6.3

Focus:  Prime focus

Filter:  Astronomik Hydrogen Alpha 13nm FWHM

Mount:  Vixen GPD with Skysensor2000

Camera:  Modified Canon EOS 300D, at ISO800

Guiding:  700mm refractor guidescope, SAC7 camera, autoguided with Guidedog software

Exposure:  Total 72 mins (24 x 180s)

Date:  22nd July 2005

Processing:  RAW file conversion, Offset and Dark subtraction, flat fielding, registering and stacking done in IRIS.  Only the Red channel was used.  IRIS pre-processing almost all automated with AutoIt scripts. Curve stretching in Photoshop CS.  Background gradient removal in  PixInsight.  Noise reduction in Neat Image.


I have added PixInsight to the set of tools I use for image processing.  In particular, DBE (Dynamic Background Extraction) is a powerful means of removing unwanted vignetting and gradients.

Acquisition was managed by GADFly, an AutoIt script I wrote to automatically nudge the position of the guidestar every few exposures.

There was a full moon that night, but good transparency.



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