Combining images from narrowband filters is often done using the
Hubble tri-colour palette, in which SII, Ha, and OIII are assigned to
R, G, and B, respectively. I took this image in 2006 with 20 minutes
exposure per filter from a remote access observatory in New Mexico.

Martin Nicholson, Daventry, England.

My website is at http://www.martin-nicholson.info/1/1a.htm
My informal Astronomical Blog is at http://ukastronomy.livejournal.com/