The Lunt LS35THa 35mm solar refractor puts serious Hydrogen-alpha solar viewing within the reach of virtually every astronomer. You can piggyback this lightweight (3 pound) solar scope on a larger existing telescope or mount it on a photo tripod or altazimuth mount to use as a quick grab and go scope for spur-of-the-moment observing at lunchtime or when on vacation. Because this Lunt scope is small and compact (only 16.35” long and 62mm in diameter at its widest), the LS35THa it is ideally suited to being mounted in pairs side by side for binocular viewing.
The H-alpha filter in the Lunt 35mm solar refractor shows you much more of the living Sun than ordinary glass or Mylar white light solar filters that only show you sunspots. With the economical Lunt LS35THa, you see the violent ever-changing tapestry of multiple prominences leaping off the edges of the solar disk, as well as sunspots and some surface detail.
The Lunt LS35THa is a fully multicoated 400mm focal length achromatic refractor with a 35mm clear aperture and an f/11.4 focal ratio. Its built-in Hydrogen-alpha solar filter is centered on the 6562.8 Ångstrom H-alpha solar emission line. The filter has a narrow <0.75 Ångstrom passband for high contrast. The two-part filter uses a permanently mounted 35mm etalon behind the objective, with a 4mm blocking filter for primarily visual use built into the standard equipment 1.25” star diagonal.
A tuning mechanism built into the system lets you tilt the etalon to center the passband precisely on the H-alpha line for maximum contrast. The tuning mechanism also lets you perform off-band observations of Doppler-shifted disk features to determine whether they are moving towards you or away from you.
The 4mm clear aperture of the H-alpha blocking filter in the star diagonal is matched to the focal length of the telescope and designed primarily for visual observing. It gives you a full disk image of the Sun at prime focus with almost any optional 1.25” eyepiece you might care to use. A typical 10mm Plössl, for example, will give you a magnification of 40x and a 1.25° actual field. This gives you plenty of dark sky background around the solar disk to show off the prominences. A 20mm 1.25” Plössl will give you a magnification of 20x and a little over a 2.5° field of view compared with the 0.5° diameter disk of the Sun, but will brighten the prominences. A 6mm Plössl will give you 67x magnification and about a 0.77° field, letting you get up close to the Sun, although the resulting very narrow 0.52mm exit pupil may start dimming the prominence detail more than you might like to see.
If solar imaging is in your plans, consider the L35TPAK deluxe upgrade. In addition to adding a Sol Searcher finder, Vixen-style dovetail, and Lunt 10mm eyepiece to the basic scope, the L35TPAK upgrade changes the 4mm blocking filter to a 6mm aperture filter that is better suited for solar imaging.
The diagonal’s 1.25” eyepiece holder has a built-in helical focuser that makes focusing easy, even if you are observing in the dead of winter while wearing gloves or mittens. Simply rotate the eyepiece to focus it sharply on the solar image.
The Lunt LS35THa comes with a pair of mounting rings that have collimation screws so you can precisely line up the scope with the optical axis of any larger scope on which it might be piggybacked. The rings are attached to a mounting bar with a 1/4”-20 thread hole for mounting the scope directly on a photo tripod for a quick peek at the Sun. The bar also lets you mount the scope on a dovetail plate and piggyback it on a larger scope or install it on an altazimuth mount with slow motion controls for easier extended observing and tracking of the Sun.
Most solar telescopes exhibit a “sweet spot” in their field of view where subtle prominences pop into clearer view and the larger prominences show more contrast and detail. Experimentally moving the Lunt LS35T/Ha around so that the Sun’s limb and prominences move to different parts of the field will soon reveal where your particular scope’s “sweet spot” is located.