The 1.25″ Baader OIII Filter is specially designed for the observing the night sky and offers high contrast views of gas and planetary nebulae that are emitting light from narrow OIII wavelengths. For many objects, the Baader O-III reveals a level of intricate detail that rivals the best deep sky photographs, the Baader O-III perfectly isolates the two doubly ionized oxygen lines (496 and 501nm), through a very sharp and narrow bandpass . Unlike other O-III filters, the Baader O-III completely blocks longer and shorter wavelengths, this is why some O-III filters appear red, instead of the correct 500 nm Teal colour. The result is the highest contrast achievable in an O-III filter. The Baader filters are designed to be stacked.