Sky Mentor 12" Dobsonian Reflector

Geoff Gaherty
Toronto Centre RASC

For many years the Dobsonian reflector was the ugly duckling of the telescope world. Much admired by serious deep sky observers, it was generally shunned by the average amateur astronomer because of its homemade appearance: cardboard tubes, focusers made from plumbing parts, and homely wooden mounts. The Dob also suffered from a reputation as a "light bucket," implying, often correctly, that its mirrors weren't of good enough quality for high power observation of the Moon and planets. In the last few years two manufacturers in the Far East have given the lowly Dob a makeover, with nicely enameled metal tubes, stylish mounts, high quality components, and, most important of all, vastly improved optics. At first their efforts were concentrated on the smaller 6" and 8" sizes. More recently both have introduced highly successful 10" models. Now one of the manufacturers has upped the ante by introducing a 12" f/5 model, available in Canada from Khan Scope Centre under the Sky Mentor brand. This size begins to enter the realm of the custom Dob makers, but at a remarkably low price. Rarely has a scope been so eagerly anticipated by amateurs on the internet, so I was anxious to get one out for a test drive.

The most obvious thing about this scope is that it is big! The tube, finished in silver gray enamel, is 144 cm (56.6") long, 36 cm (14") in diameter, and weighs 23 kg (50 lb). The base is similarly large: 64 cm (25") in diameter, 65 cm (25.5") tall and weighs 15 kg (35 lb). Pointing at the zenith the scope is 158 cm (62") tall, and the eyepiece is 145 cm (57") above the ground; the assembled scope weighs 38 kg (85 lb). This immediately made me wonder if I could transport the scope, given my bad back. However, the height of the base was a perfect fit for the cargo deck of my Passat wagon, so I could place the mount just behind the car and slide the tube out into its bearings without any serious lifting. Once unloaded, I used a two-wheel hand cart to move it around. To fit the tube in my wagon I did have to fold down one of the rear seats, though.

      

Assembly was straightforward, although the one page instruction sheet was rudimentary. The mount is a larger version of other Sky Mentor mounts with one important difference: instead of the three standard Teflon azimuth bearings there is a plastic plate with 24 small metal roller bearings. This is sandwiched between the base and the bottom of the rocker box. The mount includes an eyepiece rack which holds one 2" and three 1.25" eyepieces, and a sturdy carrying handle. The tube attaches to the mount with two tension springs. In practice I found these unnecessary except when using a heavy accessory like a binoviewer.

The primary mirror is exactly 12" in diameter, not the 12.5" more commonly encountered. It appears to be optical glass rather than the greenish plate glass found in some Dobs. The rear of the mirror blank is ground flat, something not found in some other Dobs. The mirror is supported by a 9-point flotation cell. This cell differs from earlier Sky Mentor cells in having recessed collimation screws, so that the scope no longer rests on the collimation screws when stood on end, something that always worried me. The odd metal plate found in smaller Sky Mentor cells is gone, so that the back of the mirror is wide open for cooling. Finally, the optical centre of the mirror is now marked with a small white ring, somewhat smaller than a standard loose-leaf reinforcer. This makes collimation much easier and saves the buyer the nuisance of disassembling the mirror cell to mark the mirror themself.

    

The secondary mirror is supported by a proper mirror cell, rather than being glued to a flat plate. This is supported by a spider with four thin but rigid vanes. The focuser is a standard 2" rack and pinion style with tension screw. The 2" to 1.25" adapter has a compression ring rather than a simple set screw, a nice touch. The 8x50 finder scope is mounted on a sturdy cast metal dovetail plate. The scope comes equipped with a very nice 2" 32 mm Plössl with a 63° field, producing a 1.3° field at 48x, a 1.25" 9 mm Plössl, producing 139x, and a Moon filter.

Much to my surprise, the skies were clear on the night I received the scope, so I put it to use, simply following the same observing program I would normally have followed with my 11" Starmaster. Seeing was poor, so the views of the Moon and Mars were unsatisfactory. I attempted a star test, but again poor seeing got in the way, though I could see enough to tell me that the optics were pretty good. Most importantly, I could see no sign of the astigmatism that has plagued some of the smaller Sky Mentors, caused by an overly tight secondary mirror cell. The primary mirror required a tiny tweak to fix its collimation, but the secondary mirror was right on. Most of my evening was spent observing variable stars. I was able to see comparison stars down to 12.1 magnitude, quite good considering the light pollution and bright moonlight in my back yard. Along the way I had some looks at some favourite double stars, including Polaris, Epsilon Lyrae, Epsilon Bootes, Albireo, and Gamma Delphini, all of which were cleanly split. Despite the bright moonlight, I had nice views of the Dumbbell Nebula (M27) and the globular cluster M13 in Hercules, which was nicely resolved at 170x.

The only problem I encountered came when I switched to high magnification to view the Moon and Mars. The azimuth motion of the scope seemed very erratic and jumpy, making it very hard to centre objects in the field of view. I soon realized that this was due to the roller bearing plate. It had been shipped sandwiched between the ground board and the bottom of the rocker box and, along the way, the roller bearings had made deep impressions in the material covering the mount parts. There were a series of deep grooves in both bearing surfaces, and the rollers caught in these as the scope was moved in azimuth. The next day I replaced the roller bearing plate with several Teflon Super Slidex pads, and from then on the mount performed very well.

    

A few nights later I put the Sky Mentor to the acid test: a side-by-side comparison with my Starmaster 11", with optics by the legendary Carl Zambuto. Once again the seeing was less than perfect, but the star tests in the two telescopes were remarkably similar. Epsilon Lyrae was split cleanly by both scopes, but the star images seemed cleaner and tighter in the 11". On the Moon, the 12" showed the craterlets in the floor of the crater Plato more clearly than the 11". On Mars, the two were close, but the Zambuto optics produced an image with slightly greater contrast and colour saturation; the 12" looked slightly washed out in comparison. But, considering the Starmaster cost three times as much as the Sky Mentor, the latter put in a very good showing.

In summary, the Sky Mentor 12" is a worthy "big brother" to its justifiably popular smaller siblings. Despite its large physical size it handles very easily, both in transport and under the stars. I would only urge the manufacturer to reconsider the roller bearing plate, or to provide it with harder surfaces to move against. With the simple fix of replacing it with Teflon bearings, this scope is an excellent performer and puts quality large aperture within the reach of many.

2003-09-28, rev. 2003-11-26