The Night Sky Hydrogen-Alpha Filters are long-pass filters with a 50% transition from blocking to passing at about 640nm. At wavelengths longer than 650nm, transmittance is 80% or above. The filter is used as an extreme anti light-pollution filter, and for contrast-enhancement of emission nebulae and other astronomical objects bright in the infrared. The 1.25" Night Sky Hydrogen-Alpha Filter, threads into the nose-piece of industry-standard 1.25" Digital Imagers. The Astromania H-Alpha cut-off filter enhances the contrast between object and sky, allowing you to take good quality images even under urban skies.