M42 in SII, Hα, and OIII; 15th February 2008

Previously I have imaged M42 by combining pictures taken with Hα and OIII filters.  I have now added an SII filter to my collection.  It is an Astronomik filter with a band width of 13 nm centred on 672 nm.  This is slightly further into the red than Hα (656 nm).  Although these wavelengths are very close, we are seeing light emitted by different atoms and so there is no reason to expect the nebula to look the same through the two filters.
When it comes to combining these pictures, it is a matter of choice how one assigns colours to the three monochrome images.  The natural order would be to make the SII image red, the Hα image green, and the OIII image blue, but this produces a very green image because Hydrogen is more abundant that Sulphur and contributes most to the resulting image.  To me this looked unnatural, so I have assigned red to the Hα image, green to the SII image and blue to the OIII image.  However one does it, the colours are not natural.

M42 in light from Hydrogen (red), Sulphur (green) and Oxygen (blue).  At each wavelength, four pictures were taken using exposures of 1, 3, 7, and 15 seconds which were combined as monochrome images and then the three monochrome images were combined to produce the coloured version.  All processing was, as near as I could make it, the same for each set of pictures.  Finally the coloured version was imported into RegiStax and a slight wavelet enhancement applied followed by a stretch of the histogram.

Date and Time	15th February 2008 20:39 to 21:37 UT
Camera	Atik ATK-1 HS raw modified
Telescope	Skywatcher ST80 refractor with Astronomik H-α, OIII, and SII filters
Capture	K3CCDTools. 1, 3, 7, 15 sec exposures, 85% gain, high gamma, ~30 frames each
Processing	K3CCDTools. Each avi stacked and saved in png16 format. Photoshop. 3 images combined to produce a coloured version. PhotoImpact. Saturation increased to 100%. Registax. wavelet 2 = 5, histogram 10-155. NeatImage. Auto-reduction of noise.

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