Sky-Watcher UHC filter .This narrowband UHC filter, like the O-III filter, isolates the two doubly ionized oxygen lines (496 and 501nm) in addition to the hydrogen-beta line (486nm) emitted by planetary and most emission nebulae. This filter permits superb views of objects like the Orion, Lagoon, Swan and other extended nebulae. It performs well in smaller aperture instruments owing to its greater light transmission than the O-III, yet still suppresses light pollution well. The UHC filter is the best all-around dark-sky nebula filter. Available in two sizes, threaded to fit either standard 1.25” or 2” astronomical eyepieces. Supplied with storage case.