The renowned, original Lumicon OIII narrow bandpass filter (11nm) for the Cassegrain Rear Cell isolates just the two doubly-ionized oxygen lines (496 and 501nm lines) emitted by planetary and emission nebulae while blocking the rest of the overall spectrum of light. The result is extreme contrast between the black background of space and the delicate photons of OIII light needed for near-photographic views of the Veil, Ring, Dumbbell and Orion nebulae, among other objects.
