Narrow Band Filters
Hα (H‐alpha) has a wavelength of 656.281 nm and is visible in the red part of the electromagnetic spectrum, it is the easiest way for astronomers to trace the ionised hydrogen content of gas clouds.
Since it takes nearly as much energy to excite the hydrogen atom’s electron from n = 1 to n = 3 as it does to ionise the hydrogen atom, the probability of the electron being excited to n = 3 without being removed from the atom is very small. Instead, after being ionised, the electron and proton recombine to form a new hydrogen atom. In the new atom, the electron may begin in any energy level, and subsequently cascades to the ground state (n = 1), emitting photons with each transition. Approximately half the time, this cascade will include the n = 3 to n = 2 transition and the atom will emit H-alpha light. Therefore, the H-alpha line occurs where hydrogen is being ionised.
Doubly ionised oxygen (O[III]) is the ion O2+. Its emission lines in the visible spectrum, primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm, are known in astronomical spectroscopy as O[III]. Concentrated levels of O[III] are found in diffuse and planetary nebulae. Consequently, narrow bandpass filters that isolate the 501 nm and 496 nm wavelengths of light, that correspond to greenturquoise-cyan spectral colours, are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly light-polluted terrestrial atmosphere) where the frequencies of O[III] are much less pronounced.
Similarly Ionised sulphur (S[II] is ionised sulphur, returning to the de-excited state it emits a photon at 671.6 nm.
The ‘Hubble Palette’
SII (672nm) mapped to Red, Hα (656 nm) mapped to Green, OIII (501nm) mapped to Blue.
Wren Cottage Observatory 