Size: 1.25" High Performance O-lll Telescope FilterThis particular filter allows light transmission in the doubly ionized oxygen wavelengths. This is particularly good for viewing double-star systems where a lot of emissions are in the doubly ionized oxygen range. Oxygen III (O-III) are great for viewing diffuse and planetary nebulae. The views of the Veil, Ring, Dumbbell, Crescent and Orion nebulae are dramatically improved.