| This visual back H-Beta line filter threads onto the 2” rear cell of 11” and larger Celestron Schmidt-Cassegrain telescopes and 10” and larger Meade Schmidt-Cassegrains and Ritchey-Chrétiens. It duplicates the 2” rear cell threads of those telescopes, allowing all visual accessories to be attached securely to the filter instead of the scope. It allows eyepieces to be changed quickly in a star diagonal without having to change the light pollution filter from one eyepiece to another. The filter has a very narrow 9 Ångstrom passband centered on the nebula emission line of hydrogen-beta at 4861Å. This very narrow passband completely blocks 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 brightness of H-Alpha emissions) needed to view a 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” t0 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 with individually-measured transmission data. 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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