This large emission nebula in Sagittarius has an apparent extension of 90x40 minutes of arc, which is 3 x 1 1/3 the apparent diameter of the full moon, and corresponds to about 140x60 light yrs at its distance of about 5,000 light yrs.

Note the presence of Bok Globules, dark nebulae scattered in the nebula [see detail], which are collapsing proto-stellar clouds with diameters of about 10,000 AU (Astronomical Units).  Some of the more conspicuous globules have been catalogued in Barnard's catalogue of dark nebulae: e.g. long, narrow black B 296 at the south edge of the nebula (lower edge of the detail above).

Image info

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

Focus:  Prime focus

Filter:  Astronomik Hydrogen Alpha 13nm FWHM (Luminance), LPR Filter (RGB)

Mount:  Vixen GPD with Skysensor2000

Camera:  Modified Canon EOS 300D, at ISO800

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

Exposure:  Total 63 mins (Luminance) and 62 mins (RGB)

Date:  20th April & 28th May 2005

Processing:  Reused the H alpha image from 20th April.  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.  Composited with the Luminance image using a technique described by Rob Gendler (Method 2) then colour saturation enhancement with Luminance Layering.  Noise reduction in Neat Image.

Comments

First composite using H alpha luminance information.  The result is quite dramatic with the Ha nebula contrast retained while colour saturation is satisfactory.  Initially I got green halos around bright stars as a consequence of the compositing technique, but this was fixed using the final order of processing described above - ie compositing first before luminance layering.