From the Thousand Oaks website....
For 30 years we've manufactured the most popular white light filters in the world. With our white light filter, excellent views of sunspots and granulation are easily seen. In H-Alpha light, spectacular prominence activity can be seen at the limb. Flares are also visible on the surface along with the same white light details visible with our standard filters. The two part system is easy to use and can be setup in less than a minute.

Each system includes an energy rejection filter (ERF) and H-Alpha filter unit (HAU) with a tilting (fine-tuning) mechanism. The HAU is a compact 4 3/4" long x 2" diameter and weighs approximately 7oz. It attaches between telescope back and star diagonal or between star diagonal and eyepiece and accepts standard 1 1/4" accessories. The ERF fits over the front of the telescope tube. The HAU is the same for all telescopes, however, the ERF is designed for use with a specific instrument. Telescopes with a secondary mirror obstruction require an off-axis ERF. Refractors use a center axis as pictured below . ERF's can be purchased separately for additional telescopes. The purpose of the ERF is to increase the focal length and reduce the energy entering the telescope while allowing a narrow band of "red light" to enter. The HAU completes the blocking of all wavelengths except 656.28. The HAU and ERF  MUST  be used together.

The system works best at f/15 and above and is compatible with most refractors and Schmidt Cassegrains. The effective focal length of the telescope is increased with the ERF. Example: A four inch aperture f/10 refractor with the included 2" center aperture ERF will have a working focal length of f/20. An eight inch aperture f/10 Schmidt Cassegrain with the included 2.5" off-axis aperture ERF will have a working focal length of approximately f/30. Astro-photography can be done using a standard T-Ring & prime focus\eyepiece projection adapter.

The system does not work well, if at all with Newtonian and reflectors since the focus travel may be too limited (the focus is extended out 3 1/8") or the minimum of f/15 cannot be achieved.