H-alpha filters are essential for imaging nebulae and other objects which are rich in ionized hydrogen. At 656.3nm, a narrow-band emission filter is required to separate H-Alpha from the SII doublet (671.6nm and 673.1nm).
OIII 8nm Narrow Band filters isolate the doublet emission of doubly ionized Oxygen. After ionized hydrogen, this is the next most common emission when imaging nebulae. Doublet emission occurs at 495.9 and 500.7 nm.