This 40mm aperture H-alpha filter can be used with virtually any telescope – but is designed to work best with refractor telescopes having a focal length of under 500mm. The front filter element (containing the ERF and etalon) requires an optional custom mounting plate to attach to your specific telescope. The filter is provided with a 5mm clear aperture blocking filter mounted in a 1.25" right angle star diagonal. This combination allows for a full solar disc image at telescope focal lengths up to 500 mm. For longer focal length telescopes, the #SM40/10 uses a 10mm clear aperture blocking filter with a which provides full disc images at focal lengths up to 1000mm, while the #SM40/15 substitutes a 15mm clear aperture blocking filter with a which provides full disc images at focal lengths up to 1500mm.
With catadioptric telescopes, the front filter element must be mounted off-axis to avoid being totally blocked by the telescope’s secondary mirror obstruction. This would effectively limit this 40mm filter’s unobstructed use with catadioptric scopes to those scopes having apertures 5” and larger, as a 5” Schmidt-Cassegrain has about a 44mm clear expanse between the secondary obstruction and the edge of the corrector, the minimum needed for unobstructed viewing.
The SolarMax will work with the 90mm Meade ETX-90 Maksutov, as the adaptor plate holds the filter off-axis. However, the scope has only a 31mm unobstructed expanse between the secondary obstruction and the edge of the corrector, so the secondary mirror blocks a significant segment of the 40mm filter aperture, reducing contrast. In addition, the telescope would then be working at an effective focal ratio of ~f/34, which is quite slow and yields relatively dim images. For the Meade ETX-90 Maksutov and Meade ETX-60 and ETX-70 refractors, which have a built-in star diagonal and do not have provisions for using an external diagonal such as is used with the SolarMax, Coronado has designed a special blocking filter housing to work in conjunction with these telescopes. Specify the Meade ETX-90 or ETX60/70 blocking filter when ordering a SolarMax for these scopes. The filter’s 40mm aperture and <0.8A bandwidth provide surprisingly good resolution and contrast and result in detailed and visually pleasing views of fine solar 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. The SolarMax 40 comes standard with a T-Max mechanical tuning mechanism that allows 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, 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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