This Lunt Solar Systems 50mm clear aperture H-alpha solar filter provides a highly detailed full disc solar image of flares, prominences, and disc detail with virtually any telescope up to 1800mm in focal length. It is best suited to scopes in the 1200mm to 1800mm range, however, as use with scopes much shorter than 1200mm focal length may result in ghost images at the edges of the field and will provide no visual advantage. 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.
With catadioptric telescopes, the front filter element must be mounted off-axis to avoid being blocked by the telescope’s secondary mirror obstruction. This effectively limits this 50mm filter’s most efficient use to the larger catadioptric scopes, even though Lunt makes adapter plates to fit just catadioptric scopes with apertures as small as 90mm.
For example, a 1250mm focal length 5” Schmidt-Cassegrain has about a 44mm clear expanse between the secondary obstruction and the edge of the corrector plate. This would cause some loss of contrast because of the diffraction caused by the intrusion of the scope’s secondary into the filter’s light path. Even a 1525mm focal length 6” Schmidt-Cassegrain, which has a 47mm clear expanse between its secondary and the edge of its corrector, would suffer a minor loss of contrast.
The filter’s 18mm clear aperture B1800 blocking filter is mounted in your choice of either a 1.25” or 2” right angle star diagonal. Please specify in the comment section of the order form which size diagonal is desired when ordering your filter. Versions of this 50mm H-alpha system with larger and smaller clear aperture blocking filters are also available, depending on the focal length of your telescope. These provide unvignetted full solar disk images with telescopes from 600mm to 3200mm in focal length.
Depending on what telescope(s) you have or may plan on buying, choosing the B3200 blocking filter version for scopes up to 3200mm in focal length 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.
You can also 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 resolution 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 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.