This 90mm aperture H-alpha filter can be used with virtually any telescope having a 90mm or greater aperture – but is designed to work best with telescopes having a focal length of 1000mm to 1500mm, particularly with refractors. The front filter element (containing the ERF and etalon) is threaded to mount directly into the front cell of TeleVue TV-102 and NP-101 refractors, but will require an optional custom mounting plate for installation on other telescopes. With catadioptric telescopes, the front filter element must be mounted off-axis to avoid being totally blocked by the telescope’s secondary obstruction. This would effectively limit this 90mm filter’s use with catadioptric scopes to those with apertures 11” and larger, as even a 10” scope has only an 80mm clear expanse between the secondary obstruction and the edge of the corrector and would block some of the filter’s clear aperture and limit resolution. This filter package is provided with a 15mm clear aperture blocking filter mounted in a 1.25" right angle star diagonal. With such a combination on many catadioptric scopes, you would not be able to achieve a full disc solar image and would be limited to visual and photographic explorations of only portions of the Sun’s surface at a time. The SM90/30 combination, with a 2” straight through 30mm clear aperture blocking filter, would be a better choice with catadioptrics if full disk views are desired. The filter’s 90mm aperture and <0.7A bandwidth provide exceptionally high resolution and contrast and result in unsurpassed views of the finest surface details and prominences when mounted on a good quality telescope.
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. Coronado includes a T-Max mechanical tuning mechanism with the filter to allow the user to de-tune the SolarMax slightly from the H-alpha line to make such Doppler-shifted phenomena more visible. The T-Max fits between the SolarMax and its adapter plate at the front of the scope, but is not shown in the image. A simple rotary wheel movement accomplishes the tilt necessary to move the passband of the filter. Because of the filter's position at the front end of the telescope, this does not compromise the performance of the filter nor limit the H-alpha field of view as is the case with rear mounted filters.
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