David Nakamoto wrote:

Remember that if hydrogen is the predominant light emitter, and if it is
excited enough, you will get the three visible wavelength color lines being
produced; red, green, and blue.

However, the stronger emission lines which help in making M42 look
bluish-green are probably the [OIII] pair at 4959 and 5007 Angstroms. The one
at 5007 is about 3.4 times as strong as the H-Beta line, not to mention being
closer to the visual peak sensitiviy of the human eye. The H-Beta line
contributes as well, but for the more greenish cast, the Oxygen lines tend to
be quite important. Indeed, I greatly prefer the use of the OIII filter on
M42 over the H-beta filter, as it shows a larger area of nebulosity (I like
the UHC view the best however, as it takes in all three lines). Clear skies
to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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