This Lunt Solar Systems 50mm clear aperture H-alpha filter provides a highly detailed full disc Hydrogen-alpha solar image with virtually any refractor up to 1200mm in focal length. The filter’s passband is approximately <0.75 Ångstrom. This is capable of providing high contrast views of both solar prominences and surface detail alike.
The filter consists of two parts: a combined energy rejection filter and etalon that mounts in front of your telescope’s objective lens and a separate blocking filter mounted in a star diagonal the fits into your scope’s focuser. The front filter element (containing the red ERF and the etalon) requires an optional custom mounting plate to attach it to your specific telescope.
The filter’s 12mm clear aperture B1200 blocking filter is mounted in a 1.25” right angle star diagonal.
Using the filter with a telescope whose focal length is much longer than 1200mm will usually vignette the solar disc, cutting off the prominences if you center the scope on the Sun.
Versions of this 50mm filter with larger clear aperture blocking filters are also available, but provide no visible advantage over the B1200 filter on scopes in the 1200mm and under focal length range.
If you plan on using a binoviewer for solar viewing with a 600mm or shorter scope, however, this B1200 version of this filter would be a better choice than the B600 blocking filter version normally recommended for scopes of that focal length. Binoviewer use requires a Barlow or optical corrector lens in the binoviewer’s 1.25” barrel to reach focus. The focal length amplification of the Barlow will very probably vignette the solar image if you use the B600 blocking filter with a 600mm focal length scope. This B1200 blocking filter version would provide unvignetted images with a binoviewer with such a scope.
Choosing a version of the filter with a larger clear aperture blocking filter than the B1200 might make more sense in the long run. You would be able to use the 50mm etalon on a variety of scopes by simply buying a relatively inexpensive second mounting plate, rather than having to buy both a mounting plate and a considerably more expensive larger aperture blocking filter/diagonal to put the H-alpha system on a longer focal length scope. The B1800 would be the largest blocking filter versions recommended, for scopes up to 1800mm in focal length, as the B3200 blocking filter version may result in ghost images at the edges of the field with scopes 1200mm in focal length and under.
You can add an optional second etalon (thereby double stacking the system) to reduce the passband to approximately <0.55 Ångstrom (depending on the telescope). This will give you higher contrast and higher resolution on disk detail during viewing and/or imaging, although with some loss of faint prominence detail.
Some events on the Sun – notably active flares and coronal mass ejections (CMEs) – are extremely fast moving. If such features occur on the disc of the Sun, rather than on the limb, their high velocity towards the observer results in their wavelength being Doppler-shifted to a shorter wavelength than the H-alpha line. If the velocity is high enough, this Doppler-shifted wavelength could be outside the passband of the filter and the feature would not be observable. The Lunt 50mm etalon has a built-in mechanical tuning mechanism that allows you to de-tune the filter slightly from the H-alpha line to make such Doppler-shifted phenomena more visible. A rotary thumbwheel built into the side of the etalon accomplishes the tilt necessary to move the passband of the filter.
The two-part Lunt filter system comes in a protective transport case. A protective metal thread-in dust cover is provided for the etalon. The image above shows the front element housing, with the red ERF concealing the etalon within the housing. The 1.25” star diagonal containing the blocking filter is not shown in this image. The feature image below shows the filter with its thread-in dust cover and the 2” star diagonal containing the blocking filter.