| This Orion 2” H-Beta line filter for 2” eyepieces has a narrow 12 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband centered on the nebula emission line of hydrogen-beta at 4861Å. The filter typically transmits 90% of the blue-green H-Beta emissions, while completely blocking the rest of the visual spectrum, including light pollution, natural airglow in Earth’s skies, etc. The result is an extreme contrast between the black background of space and the delicately attenuated H-Beta light (about one-third the strength of H-alpha emissions) needed to view a select few extremely faint objects such as the Horsehead, Cocoon, and California Nebulas. In many cases, an H-Beta filter is the only way to view those objects. The filter needs a steady, transparent sky; generally a 10-12” or larger aperture telescope; and a long focal length eyepiece to get you as close as possible to a maximum-efficiency 7mm exit pupil for the best results in viewing these difficult “observing trophy” objects. It is not designed for astrophotography or for observing reflection nebulas. The filter comes in a protective hard plastic case. An individually-measured light transmission curve is supplied with each filter. Enter your telescope focal ratio in the formula box to the right to see what focal length eyepieces will work best with this filter. | 
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