"False color" or "pseudocolor" generally refers to a mapping scheme whereby visible wavelengths in one band are mapped to a completely different band, or where invisible wavelengths are mapped into visible bands. "True color" mapping (in reality there is no such thing as true color) refers to mapping where visible wavelengths are mapped to new bands that correspond to similar wavelengths. All multiple channel images are mapped in some way.starsurfer wrote:I used to feel that way about narrowband a long time ago. However, just because an image contains narrowband data doesn't necessarily make it false colour, it depends on the processing.
Pure, bright Ha is perceived by the human eye as a deep, saturated red. Pure, bright [OIII] is perceived by the human eye as a saturated cyan. In broadband color images of most nebulas, we see neither of these in isolation, but rather we see them contaminated by essentially white light (blackbody emissions from stars). The more white, the more unsaturated they appear. And of course, as you note, mixing the wavelengths because of multiple emission bands can create entirely different colors.The colour of OIII is generally blue but can also be green or turquoise. In many nebulae, there are areas where both Ha and OIII overlap and their colours are generally pink or purple.
