This is an emission cum reflection nebula in Sagittarius, although through the hydrogen alpha filter, only the emission (normally red) part is visible.  The blue reflection nebulosity to the north (top) was not visible in the H alpha image, but is here, although it appears green - for some reason!  

The dark dust lanes that lace M20 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions.  The nebula is about 3000 light years away, and about 50 light years across.

Image info

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

Focus:  Prime focus

Filter:  Astronomik Hydrogen Alpha 13nm FWHM, Celestron LPR for the RGB image.

Mount:  Vixen GPD with Skysensor2000

Camera:  Modified Canon EOS 300D, at ISO800

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

Exposure:  Total 80 mins (20 x 240s) Luminance, and 44 mins (22 x 120s) RGB.

Date:  1st May & 1st June 2005

Processing:  Reused the H alpha image from 1st 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.  The RGB was composited with the Luminance image using a technique described by Rob Gendler (Method 2).   Noise reduction in Neat Image. 

Comments

For the H alpha - It was a hot, humid (29 deg C, 90%) and windless night.  Transparency was middling to poor.  The RGB was also taken in conditions of  poor transparency - in fact high cloud terminated the imaging session.