The Alstar 2" O-III Filter is comparable to the Alstar UHC filter in its function. The difference is that this filter only allows the light of doubly ionized oxygen through. This means that there is an enormous increase in contrast with diffuse, planetary and extremely dim nebulae. This is a special narrowband filter which allows an even smaller range of wavelengths than the Alstar UHC filter. Even so, there are innumerable astronomical objects which can be first observed in detail with this filter or cannot be seen at all without it. A prime example is the Veil nebula in the constellation of Cygnus. It's a great accessory for observations of nebulae due to its capability to suppress all light that lies outside its 4960-5010 angstrom (496-501 nm) passband. This filter can be used for astrophotography as well. The O-III 2" Filter has a standart barrel diameter so you can use it with a telescope of any brand. Note that this filter gives a green tint to observed objects. The O-III provides an even greater contrast boost on some objects than the UltraBlock narrowband filter. And for nebulas, every little bit helps! It substantially improves contrast and outperforms the UHC for most planetary nebulae while on apertures of 10" or more, it outperforms the UHC filter for all types of nebulae. The Alstar 2" O-III Filter isolates just the two doubly ionized oxygen lines (496 and 501nm lines) emitted by diffuse, planetary and extremely faint nebulae. Thus, these faint objects become much more visible against the blackened background of space. The Alstar 2" O-III Filter produces near-photographic views of the Veil, Ring, Dumbbell and Orion nebula, among many other objects. Performs well under both light-polluted and dark skies.