The Lumicon Cassegrain Comet Filter, a narrow band-pass filter (25nm), isolates the 501nm OIII line and both C2 lines at 511nm and 514nm. 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.

The 1.25" filters employ industry-standard 1.125" x 42 tpi threads (except for Questar/Brandon, which use a thread pattern unique to Questar). The 48 mm filters employ industry-standard 48 x 0.75 mm threading, which thread into 2" eyepieces. All larger filters use the given size with 0.75 mm pitch (e.g. a 72 mm Minus Violet filter has 72 x 0.75 mm threads).