Lunt LS50THa solar telescope

di Raffaello Braga

The Lunt LS50THa is the heir to the smaller 35 mm, a very well made solar scope unfortunately no longer available. The LS50 has a 50 mm f/7 cemented doublet lens and an internal etalon – i.e. not in frontal position – with pressure tuning and a declared bandwidth lower than 0.75 Å. The tube, 37 cm long and 2 kg heavy, can be directly connected to a photographic stand, a dovetail for standard astronomical mounts and to a solar finder. The latter, in my opinion, is not necessary, as for pointing to the Sun it is sufficient to look at the shape of the tube shadow on the ground or a wall by moving it until it assumes the minimum size, or to look directly into the diagonal BF from a certain distance until one sees the solar disk at the inside the opening.

The Lunt 35, which was a very nice instrument, however, had a weak point: the eyepiece holder consisting of a rotating helical focuser with a very limited range of focus, which made it suitable for visual observations but which in photography was almost unusable.

In Lunt LS50 the manufacturer succeded in making matters worse by inserting a non-rotating focuser between the tube and the diagonal. This would have been fine if the focuser was of adequate workmanship, but unfortunately it is an economic model of Chinese manufacture which – in the specimen examined but also in others I have been able to use – has both a transversal play that tilts the diagonal and also a play in the sense of the rotation. For visual observation the inconvenience, although annoying, does not affect the use of the telescope, but imaging becomes impossible.

To improve things I locked the focuser in the fully retracted position and replaced the original eyepiece holder on the diagonal with a focusing one, which however must be of a low profile type otherwise there would be no enough backfocus for imaging.

The tuning of the etalon takes place by rotating a 9 cm long knob that contains a piston that compresses the air in the etalon cavity: by compressing (or expanding) the air its refractive index changes and it is possible to precisely set the CWL of the filter. I found the tuning simple and immediate and easily adaptable to changes in atmospheric pressure. According to the manual, the tuning system allows to use the instrument from -150 to +3000 m s.l.m.

I really liked the telescope, at least in terms of image quality. The perfect tuning and good optics lead to a highly contrasted solar disc with uniform detail over the whole field, an element not to be overlooked when observing the Sun without tracking as with a manual altazimuth mount.

By reducing the aperture of my 90 mm refractor to 50 mm, I could compare, at the same magnification, the images provided by a couple of Daystar Quark Chromosphere and Combo to those given by the Lunt 50, finding the latter very similar to the Combo but with less diffused light. But in the Lunt, as I said, the image quality remained more uniform and the vision of the solar disk in its entirety was decidedly more satisfying. Of course with only 50 mm of aperture making high resolution imaging is out of question but if you don’t have high expectations in this respect the small Lunt offers a very good visual and imaging experience.

UPGRADES

I strongly recommend to replace the original focuser with a better one that can be found under several brands as Starlight Instruments and Moonlight. For imaging I also recommend to replace the B400/600 diagonal with a straight-through B1200 which offers a wider and more uniform field.

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