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.
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
be used together.
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
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