Narrowband imaging uses special filters which transmit a very narrow wavelength of light. Most popular are three filters: Ha, SII, & OIII. These three transmit the most prevalent ionized gases: hydrogen alpha, sodium, and oxygen. These separate images are combined in one of many ways, to equate to red, green, and blue. Two common ways to "map" these colors are called the Hubble Palette and CFHT (Canada France Hawaii Telescope) Palette. So called because of the origin of their use. Hubble images map SII to red, Ha to green, and OIII to blue. Most pleasing to my eye is the CFHT Palette so most of my images are in this format where Ha is red, OIII is green, and SII is blue. My narrowband imaging efforts begin September 2008.