Also known as the Horsehead Nebula filter, the Lumicon Cassegrain Hydrogen-Beta Filter isolates only the hydrogen-beta line of the spectrum (486nm) in a narrow pass-band just 9 nm wide, providing maximum contrast. The Cassegrain filter-cell threads onto your telescope's SCT rear-cell, and presents male SCT threads for accessory attachment.
Lumicon knows that all sky viewing is not done under the best of weather conditions. The Cassegrain filter cell allows you to change eyepieces without threading/unthreading the filter, and is also easier to change-out than the standard 2" filter cell with gloves on, for cold-weather observing.