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M17, the Omega (or Swan) Nebula, a composite image consisting of a luminance image taken in hydrogen alpha light, and a colour image.

This is a region of star formation and shines by excited emission, caused by the higher energy radiation of young stars. Unlike in many other emission nebulae, however, these stars are not obvious in optical images, but hidden in the nebula. Star formation is either still active in this nebula, or ceased very recently. A small cluster of about 35 bright but obscured stars seems to be imbedded in the nebulosity.  

This nebula is more massive than the Orion Nebula and is more than 40 light years across. Distance is estimated to be 5000 - 6000 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 117 mins L (39 x 180s), 46 mins RGB (23 x 120s)

Date:  17th May, 4th June 2005

Processing:  Reused the H alpha image from 17th May.  For RGB, RAW file conversion, Offset and Dark subtraction, flat fielding, registering and stacking done in IRIS.  Used the ASINH function to enhance colours.  Curve stretching and a little unsharp masking in Photoshop CS.  Colour saturation enhancement with Luminance Layering, the RGB was composited with the Luminance image using a technique described by Rob Gendler (Method 2).   Noise reduction in Neat Image. 

Star ovality reduced through making a duplicate layer and offsetting that by half a pixel using the 'darken' blending mode in Photoshop.


This was a hard one to get right.  I had good luminosity and RGB data, with nearly 2 hrs exposure on the L, but colour balance was difficult.  Took great care to calibrate the colours the the white balance indicated by my flats (produced using my white LEB lightbox - which gives a very neutral white light) and finally managed to get good representation of colours. 


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