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) is not visible at all.  

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

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)

Date:  1st May 2005

Processing:  RAW file conversion, Offset and Dark subtraction, flat fielding, registering and stacking done in IRIS.  Only the Red channel was used.  Curve stretching and a little unsharp masking in Photoshop CS.  Star ovality reduced through making a duplicate layer and offsetting that by two pixels using the 'darken' blending mode in Photoshop.  Noise reduction in Neat Image.

Comments

It was a hot, humid (29 deg C, 90%) and windless night.  Transparency was middling to poor.  Stayed up till 5 am for this!