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The Crescent Nebula, NGC 6888 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:  Skywatcher 8" f/5 Newtonian

Focus:  Prime focus

Camera:  SAC7B, uncooled

Exposure:  Mosaic of 2 images 64 and 117 x 25s respectively

Date:  21st July 2003

Processing:  The usual YRGB processing, except that the subtraction of a 'banding flat' (see the Advanced Processing page) was done to remove fine banding noise in both the images in the mosaic.  The left component of the mosaic developed a broad banding pattern which I was unable to remove completely.  The residual can be seen in the final image.


This is probably the limit of my camera in terms of recording faint nebulae.  The right image had a large number of frames and still only recorded the brighter parts.  The left image acquisition was cut short by cloud (hence the incomplete mosaic).  As a result there were not sufficient frames to subdue the banding.  This affected the technique I use to subtract a synthetic sky background and resulted in a blue and green colour cast in part of the left image.  Overall though, an impressive number of stars were recorded!


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