CXB (formerly Astrodon) 3nm Narrowband NII Filter (Nitrogen II)3nm NII Filter, center wavelength of 658. 4 nm What about NII and H a? This is a bit complicated. It is not well known that most H a filters pass both H a and NII. H a emits at 656. 3 nm and NII emits most strongly at 658. 4 nm (and weakly at 653. 8 nm). These are very close together spectrally. Thus, most H a filters are wide enough (e. g. 4. 5 nm bandwidth and wider) to pass both emission lines as shown for the older CXB (formerly Astrodon) 6 nm