ABSTRACT
Following along the same lines as the previous chapter on CFA imaging, a whole new world opens up with the introduction of processing images taken through narrowband filters. These filters select a precise emission wavelength and almost completely reject light pollution (and moonlight) with the potential for lowering sky noise and hence deliver a better signal to noise ratio. This permits many astrophotographers to successfully image from light polluted urban areas. Images taken with nar-rowband filters are quite distinct; their raison d’être are gloriously colored nebulous clouds, punctuated by small richly colored stars. These particular goals require a unique approach and flexibility to image acquisition and processing. For starters, the exposures required to collect sufficient pixels are much longer than RGB imaging and will likely demand an entire night’s imaging to each filter. Even so, the relative signal strengths for the common emission wavelengths are quite different and are dominated by the deep red of hydrogen alpha (Hα). Another anomaly is the commonly imaged wavelengths do not correspond to red, green and blue and encourage individual interpretation. Image “color” is whatever you choose it to be. Typically exposures are made with two or more filters and the image files are assigned and / or combined to the individual channels of an RGB file.
