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Open Cluster M16 and the Eagle Nebula, a composite image consisting of a luminance image taken in hydrogen alpha light, and a colour image.

Lying some 7,000 light years distant in the constellation Serpens, close to the borders to Scutum and Sagittarius, and in the next inner spiral arm of the Milky Way galaxy from us (the Sagittarius or Sagittarius-Carina Arm) a great cloud of interstellar gas and dust has entered a vivid process of star formation.  Open star cluster M16 has formed from this great gaseous and dusty cloud, the diffuse Eagle Nebula IC 4703, which is now caused to shine by emission light, excited by the high-energy radiation of its massive hot, young stars.  It is actually still in the process of forming new stars, this formation taking place near the dark "elephant trunks".

The stars in the cluster are only about 5.5 million years old with star formation still active in the Eagle Nebula; this results in the presence of very hot young stars of spectral type O6.  The brightest star of M16 is of visual magnitude 8.24.  The nebula extends to a diameter of over 30', corresponding to a linear size of about 70x55 light years.


Image info

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

Focus:  Prime focus

Filter:  Astronomik Hydrogen Alpha 13nm FWHM, 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 L = 42 mins (14 x 180s), and RGB = 50 mins (25 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.


In processing this image I've tried to bring out star colours.  Not easy because not using an IR block filter, light at that end of the spectrum leaks into both the Green and Blue channels, washing out colour differences.  There should still be some differential response, and the ASINH stretch in IRIS serves to emphasise this.


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