TEC's 5 1/2-inch Apochromat: there's no denying that high-end apochromatic refractors are currently enjoying an unprecedented wave of popularity. Here's one of the reasons.
AT FIRST BLUSH IT WOULD SEEM that pets and telescopes don't have much in common, but in my case I'm not so sure. The odd connection came to mind as I was completing tests of Telescope Engineering Company's TEC 140 APO refractor and preparing for a family vacation. On our vacation checklist were issues concerning our pets, which currently include ones that swim, meow, bark, and gallop (and there's talk of adding a few that fly now that the little gnawing critters have gone on to that big exercise wheel in the sky). While there are dog people and there are cat people, our family loves all animals, which is exactly how I feel about telescopes. Reflectors, refractors, Schmidt-Cassegrains, Maksutovs: I like them all equally. Nevertheless, I know that there are amateurs who have an almost irrational exuberance for certain types of telescopes, particularly refractors. After testing the TEC 140 APO, a 140-millimeter (5.5-inch) f/7 apochromat, I have a better appreciation for their enthusiasm.
Although readers may not be as familiar with Telescope Engineering Company as they are with other scope manufacturers, the Golden, Colorado, firm was established in 1994. TEC president Yuri Petrunin and optician Eduard Trigubov wrote the article about legendary Russian optical designer Dmitri Maksutov in our December 2001 issue (page 52). Rounding out the TEC staff, all of whom are engineers from the former Soviet Union, are Alex Rychenko, Serg Kapinus, and Dmitri Gershengorin. Initially the company offered a line of Maksutov-Cassegrain telescopes along with custom optical-design and fabrication services. Recently it added a line of apochromatic refractors, with the TEC 140 APO reviewed here being the first to enter full-scale production. Petrunin says a 160-mm model (see page 58) will follow in 2004, and larger scopes are planned as well. "At present," notes Petrunin, "our biggest demand is for the apos."
The TEC 140 APO's base price buys you just the optical tube assembly and a pair of dust caps for the objective and focuser. Everything else is optional. TEC loaned us a scope for testing, along with a 7 x 50 finder and bracket ($200), tube rings ($200), and a custom-fitted ScopeGuard case ($379). I did most of my observing with a Tele Vue 2-inch Everbrite mirror diagonal and a collection of eyepieces and Barlow lenses that range from today's premium wide-field designs, including Tele Vue Radians and Naglers, to orthoscopics dating back to the 1960s. All of them worked well with the apo, especially the modern ones, which are designed for telescopes having low-f/ratio, flat, well-corrected fields.
As typically configured for observing, with tube rings, finder, diagonal, and eyepiece, the TEC 140 APO weighs a bit more than 20 pounds (9 kilograms) and thus requires a solid mounting. Most of my testing was done with the scope attached to a Vixen GP-DX German equatorial mount, which is rated for loads up to 22 pounds. I'd consider it the bare minimum for visual work with this scope. A better choice, though still near the minimum end of the scale, is the Losmandy GM-8 or Astro-Physics 400GTO. Better still for visual work, and pretty much a minimum for imaging and astrophotography applications, is the Losmandy G-11 or the Astro-Physics 600EGTO.
Word of the TEC 140 APO spread quickly through the office the morning the scope arrived at Sky & Telescope, and a small group gathered in the shipping room even before we unpacked the instrument. When we snapped open the latches and lifted the cover of the ScopeGuard case, we couldn't decide which end of the 34-inch-long telescope to examine first. Of course there was the objective, but there was also the impressive-looking and massive Feather Touch focuser.
Optics won, and we unscrewed the machined lens cap to reveal the aplanatic, three-element objective, which has a central element made of ED glass. Because the thin spaces between the lens elements are filled with oil, there isn't a hint of reflection from the internal glass surfaces, and the objective appears to the eye as if it were a monolithic piece of glass. Furthermore, both of the outside air-to-glass surfaces have a seven-layer coating that is said to reduce reflections at each surface to an average of 0.25 percent in the visual range between wavelengths of 400 and 700 nanometers. Under some angles of lighting, the glass seems to almost disappear. Very impressive.
The scope we tested has three internal baffles to keep light scattered by the tube's inside walls from reaching the eyepiece, where it could compromise image contrast. While the images were indeed free of scattered light, future versions of the scope will have a fourth baffle. With a wry sense of humor percolating through his soft-spoken Russian accent, Petrunin notes that "three baffles satisfies the [optical] design, but four satisfies better people who like to argue about telescope quality based on numbers of baffles."
Starlight Instruments collaborated with TEC in designing the Feather Touch 3545 focuser, which is truly a work of art. As with other Feather Touch models, it has a two-speed focusing system. Two large knobs provide coarse-focus motion. Turning either one moves the drawtube about 22 millimeters for each full turn. A fine-focus knob protruding from the right-hand coarse-focus knob has a 9:1 planetary-reduction system, which moves the drawtube just 2.3 millimeters for each full turn. The arrangement can be configured to have the fine-focus knob on the left, but TEC recommends that this be done by its technicians or those at Starlight Instruments. The drawtube is driven by helical rack-and-pinion gears and moves on three stainless-steel "wear strips." Its 4 1/2 inches of travel is as smooth and precise as that of the finest focusers I've tried.
The focuser accepts 2-inch eyepieces and accessories, and holds them in place with a locking collet rather than thumbscrews. The collet keeps equipment precisely aligned with the scope's optical axis. While there are no plans at present to make a large-aperture photographic field flattener for the scope, the end cap on the focuser's drawtube could easily be replaced to accept one.
In addition to the locking collet, another unusual feature of the Feather Touch focuser is a quick-release ring that allows the whole unit to be rotated without shifting the image or changing focus. This makes it easy to keep the focus knobs logically positioned parallel to the ground regardless of where the scope is pointed. Because the finder bracket is attached to the focuser's rotating body, it too moves to stay in a comfortable position. It's little things such as this that make the scope especially nice to use.
A Stellar View
If for no other reason than the telescope's outward appearance of quality, I was expecting first-class images in the eyepiece. I wasn't disappointed. Star testing demonstrated that the scope has excellent optics. The common optical defects of spherical aberration and astigmatism were as well corrected in this telescope as in any I've ever tested. This finding was further supported by Ronchi tests made while I viewed bright stars and also during bench testing done with a double-pass autocollimation setup using a green-laser light source.
Saturn, Jupiter, and the Moon were my most frequent targets. The views were superb. Images snapped into focus, with crisp detail visible during moments of good astronomical seeing. Particularly noteworthy was the visibility of low-contrast features on the ball of Saturn and within the broad equatorial belts of Jupiter. At times I felt as if I could almost become lost within the shadows cast by lunar features along the Moon's terminator. I can imagine lunar fanatics spending a lifetime observing with this telescope and never wanting more.
The lunar views were so spectacular that I nearly forgot about my real mission --testing the telescope. But even when reality got the upper hand and I looked critically at the images, there was no sign of scattered light or color fringes at the high-contrast boundaries of shadows or the lunar limb. Indeed, I'd rate the TEC 140 APO's images as free of false color for all typical observing. But, for the record, when examining the brilliant blue-white star Vega at 330x, I could see a small, faint, purple halo around the star's in-focus image. The halo, however, was not visible around fainters stars, or even brilliant Arcturus, which has an intrinsically yellow tint. Simply stated, I know of no 5 1/2-inch f/7 refractor that can claim better color correction than the TEC 140 APO.
As with many observers, two of my mid-year deep-sky favorites are the Ring Nebula in Lyra and the globular cluster M13 in Hercules. The nebula appeared pretty much as I expected in a telescope of this aperture under my modestly light-polluted sky. The smoke-ring structure was instantly visible, and there was even a hint of detail on the brighter, northwest rim. The globular, on the other hand, was a bit of a surprise. Rather than being an amorphous glow with a sprinkling of individually resolved stars, the cluster was literally peppered with sparkling suns, a testament to the scope's razor-sharp resolution. This image quality also made for particularly memorable views of other open and globular star clusters.
Refractor or Reflector?
There are many reasons why amateurs ultimately settle on buying one type of telescope over another. I never question anyone's choice, especially since experience has taught me that often the selection process is based on highly personal criteria. But I've also been asked countless times by people new to astronomy why someone would fork over thousands of dollars for a modest-aperture refractor when a fraction of that amount can buy a reflector at least twice as big. It's a reasonable question, especially since there's no shortage of quality information telling beginners that aperture matters most when selecting a telescope.
It's true that if your goal is hunting faint deep-sky objects under a dark sky, then any decent 10-inch reflector will outperform even the finest 5-inch refractor. Furthermore, a properly optimized 8-inch reflector will certainly give any 5-inch refractor a run for its money when it comes to observing detail on the Moon and planets or separating close double stars--the very areas where conventional wisdom says refractors excel. So, is there some reason apart from personal preferences that gives the smaller refractor an edge? After dwelling on this question while using the TEC 140 APO from my driveway several dozen nights over a three-month period, I'm convinced there is.
My suburban skies don't offer a very good venue for deep-sky observing. Anything I can see with a 10-inch or even 12-inch reflector from this location would almost certainly be visible in the TEC 140 APO under a rural sky, and it wouldn't require a Herculean effort to transport the refractor to such a location, especially on short notice. So, in a practical sense, a large reflector doesn't offer me a significant advantage over the refractor (except, perhaps, in affordability).
More noteworthy, in my opinion, is that the refractor will essentially resolve all the detail on the Moon and planets that is visible given my observing conditions. It is only on the rarest of nights that the astronomical seeing would give an edge to a larger aperture, regardless of whether it be a reflector or a refractor. Most of the time there would be little benefit in hassling with a scope bigger than the TEC 140 APO unless I was looking for the most subtle shades of color in planetary images or hunting very faint double stars.
I found it particularly appealing that I could carry the scope outside with two short trips from the garage to the driveway, one for the mount and the other for the scope, and be set up for observing within minutes. Unlike a reflector, the refractor gave essentially perfect images immediately--there was no need to wait while the optics acclimated to the nighttime temperatures. Furthermore, I never had to mess with finicky collimation issues, since the refractor's optical alignment is permanently fixed.
It took just minutes to go from thinking about observing to sitting at the eyepiece enjoying the view. No muss, no fuss, and perfect images from the get-go, which revealed virtually all the detail on the Moon and planets that any telescope would typically show from my driveway location. And all that with a superbly engineered telescope that's easy to store and transport. I found it very appealing.
An Extraordinary Opportunity
Along with the 140-millimeter refractor, TEC loaned us a prototype 160-mm (6.3-inch) f/8 apochromat, which is scheduled for production in 2004 as the TEC 160 APO, with an estimated cost around $8,000. As fate would have it, the two refractors overlapped with Software Bisque's Paramount ME in the parade of equipment that regularly marches through Sky & Telescope's offices (the Paramount ME was reviewed in last May's issue, page 50). Recognizing that such an embarrassment of riches offered an unusual observing opportunity, I spent one Sunday afternoon mounting and aligning the two refractors on the rock-solid Paramount along with my own planetary telescope, a Russian-made 6-inch f/6 Maksutov-Newtonian similar to the one reviewed by Alan Dyer in the April 1999 issue (page 65). It was the perfect setup for making side-by-side comparisons of the three scopes.
Because the focal lengths varied between 914 mm for the Mak-Newt and 1,280 mm for the larger refractor, I had to do some eyepiece juggling in order to use each scope at approximately the same magnification, though not always with eyepieces of the same optical design. Nevertheless, the test was still fascinating. Low altitude and a nearby stand of pine trees eliminated Saturn from the observing roster, so I concentrated on Jupiter and the Moon just two days past first quarter. Star tests confirmed that the larger refractor was every bit as excellent as its smaller sibling. They also confirmed that my Mak-Newt is very good optically, though still slightly inferior to the refractors. Furthermore, the Mak-Newt rendered a slight yellowish hue to the Moon, compared to the neutral white view in the refractors.
More interesting to me, however, was that try as I might, I never could identify detail on Jupiter or the Moon that was visible in only one telescope. At some point or another each scope revealed some tiny feature that I hadn't seen before. But in every case careful examination would show the same feature in the other two scopes. Maybe it was just coincidence, but the one that most often showed the subtlest features first was the TEC 140 APO--the smallest of the trio.
I wasn't expecting there to be a profound difference between the eyepiece views with the three scopes, since the maximum difference in aperture was only 20 mm, and that was between the two refractors. But I also wasn't expecting the difference to be so subtle that I'd actually have to remind myself which refractor I was looking through (especially since the focusers are identical on both). When I mentioned this to TEC's president, Yuri Petrunin, he didn't seem surprised. "The difference," he said, "will be on deep-sky objects." Then, pausing for a moment, he added, "and on the invoice."
TEC 140 APO Refractor
What we liked:
* First-class optics and mechanics
* Superb, feature-packed Feather Touch focuser
What we disliked:
* Having to return the scope after our tests
TEC 140 APO refractor: 5.5-inch f/7 apochromatic refractor with Feather Touch 3545 focuser.
US Price: $4,500 (optical tube assembly only)
Available from the manufacturer and its exclusive US dealer, Anacortes Telescope & Wild Bird
<ADD> Telescope Engineering Company 15730 W. 6th Ave. Golden, CO 80401 Phone: 303-273-9322 www.telescopengineering.com </ADD>
S&T Measurements Effective aperture 5.5 inches (140 mm) Focal length 979 mm f/7.0 Weight 18.8 lb (8.5 kg)
While the family considers all pets equal in the animal farm at senior editor DENNIS DI CICCO'S home, the dog clearly thinks he's more equal than the others.
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|Author:||Cicco, Dennis di|
|Publication:||Sky & Telescope|
|Date:||Dec 1, 2003|
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