Notes from Under Sky

The Tele Vue 2.5x Powermate

In which I review this accessory

One of the first accessories that beginning amateurs are advised to buy is a quality Barlow lens. The Barlow slides into a telescope's focusing tube, just like a regular eyepiece, but you don't look directly into it. Instead, you insert an eyepiece into the other end of the Barlow, and look into that. Optically speaking, a Barlow contains a negative lens group--usually an achromatic doublet--that stretches out the light cone of the telescope, giving an eyepiece double the magnification or more. It sounds like the perfect solution.

But Barlows have a checkered reputation. They're versatile, but they are a compromise in many people's minds. First of all, they throw extra glass into the light path, and that can work against contrast. Secondly, and less obviously, their optical design is not conducive to uniform illumination across the field of view.

Enter the Tele Vue Powermate, another innovation by designer and company head Al Nagler. The Powermate consists of four elements in two groups: a negative doublet followed by a positive doublet. In a brief conversation, Tele Vue representative Steve White explained that the negative doublet at the front intercepts the light rays just like a Barlow lens. However, instead of converging to a point at the field stop of the eyepiece, the light rays become almost parallel before being intercepted once more by the larger positive doublet, and then finally passing on to the eyepiece.

One advantage conferred by this design is that the eyepiece "sees" the incoming light from a much wider source (the positive doublet) than in a normal Barlow (a smaller negative doublet). The Powermate is thus designed to avoid the vignetting that is the result when certain eyepieces are used in traditional Barlows. The groups are also specially designed to minimize the effect of various edge aberrations in eyepieces. The design is sufficiently different, in fact, for Nagler to call it a "compact amplifying lens system" instead of a Barlow.

The Powermate comes in three flavors: two 1-1/4-inch models, with 2.5x and 5x amplification, and a 2-inch model that amplifies 4x. I bought the 2.5x model for use with my Celestron C5+, a 5-inch SCT, and Tele Vue's own Ranger, a 70 mm achromatic refractor. At about $168 U.S., it falls halfway in between the company's series of Plossls and its premium eyepieces in price.

A Look Under the Hood

A brief examination of the Powermate revealed good baffling and also confirmed the basic configuration of the elements as Tele Vue representative Steve White described it (see main article), but I wanted to know more about the optical design. How strong, relatively, were the lens groups, and how far were they separated? I discovered these in an unexpected way.

The normal way to use the Powermate, as with a Barlow, is to slide the narrow end into the focuser, and put the eyepiece in the wider end. Instead, I used the Powermate all by itself, backwards. This revealed that the Powermate is, at its essence, a Galilean refractor. Whereas an ordinary or Keplerian refractor consists of a positive objective and a positive eyepiece, a Galilean refractor consists of a positive objective and a negative eyepiece--precisely the Powermate's optical configuration when used backwards.

It turns out that used in this unorthodox fashion, the Televue 2.5x Powermate yields…2.5x! Now, on a Galilean refractor, as with a regular refractor, the magnification is the ratio between the focal lengths of the two lens groups, but the separation between the groups is the difference between the focal lengths, not the sum. On this unit, that separation is 40 mm.

To complicate matters a little, the "refractor" only reached focus for objects at a distance of about 80 cm, so separation is actually a little greater than the difference between focal lengths, but even allowing for this, simple algebra yields focal lengths of about –25 mm for the negative group, and about 60 mm for the positive group. These are ballpark figures, of course, arrived at without any actual testing equipment.

The 2.5x Powermate is about the size of an ordinary Barlow, about 105 mm in length. Like many of the Tele Vue eyepieces, its barrel has a recessed groove about 5 mm across, where the focuser's set screw goes, in order to prevent the Powermate from falling out should that screw loosen a little. In turn, the Powermate's set screw is captive, so that you can't overloosen it and have it fall out. The Powermate feels much heftier and smoother than an ordinary Barlow--this is one solid piece of optical machinery. There is obvious attention in general to quality and ease of use. Unlike the 5x Powermate, however, the barrel of the 2.5x model is not threaded to accept filters. You will have to thread them onto the eyepieces directly.

I pitted the Powermate against an inexpensive 1-1/4-inch Barlow from Orion ($48 U.S. in their latest catalog). This Barlow is nominally 2x, and since errors and defects in the image become all the more noticeable at higher powers, the Powermate was at a disadvantage from the outset. Keeping up with the Orion unit would actually mean the Powermate was ahead in the game.

The targets for this comparison were Jupiter and Saturn. High in the sky, they could be observed with a minimum of atmospheric and local air turbulence. I began by using a 15 mm Tele Vue Plossl with both units and the C5+, yielding about 208x or over 40x per inch with the Powermate, and 167x or over 30x per inch with the Orion Barlow.

As the telescope began cooling down, the difference was difficult to see at first. One factor immediately revealed itself: the Powermate is, as advertised, nearly parfocal. Not exactly, but close enough to make refocusing a snap. In comparison, the Orion unit requires a lot of refocusing. In fact, eyepieces won't even come to focus with the Orion Barlow on the Ranger. This is a big ease of use consideration, especially with mounts that don't track, since you don't need to spend a lot of time finding the best focus, time that allows your target to run out of the field of view.

Another difference that became quickly apparent was vignetting. The Orion unit showed mild but noticeable vignetting at the edge of the field of view, while the Powermate revealed none at all. While not in and of itself an optical error, it made detail difficult to see at the edge of field, which can also be a more significant issue on non-tracking mounts. If your Barlow reduces the field over which you can see maximal detail, you'll have to guide the scope that much more often.

While turbulence was still present, there was little difference in contrast between the two configurations. Both the Powermate and the Orion unit suffered at this high power. After an hour of testing, though, both the atmosphere and the tube currents had settled down enough to enable detailed observation and comparison.

Somewhat surprisingly, on-axis contrast is very nearly a push between the two units. In excellent seeing, as there was on this night, you may detect a slight edge to the Powermate. The festoons on Jupiter showed just a bit more detail, the north temperate belt showed just a bit more unevenness, and on Saturn, the Cassini division was ever so slightly darker in the Powermate than in the Orion. But during those intervals of average or worse seeing, the two were virtual equals.

Off-axis, the difference was noticeably greater. The Powermate had a moderate but consistent edge over the Orion Barlow whenever the target was closer to the edge of field than to the center. As if to back up this observation, stars in the Orion unit showed a tad more flaring near the edge than they did in the Powermate. On extended objects such as the gas giants, this smearing leads to loss of detail. Again, when the seeing is bad, this difference may not be visible.

Just to demonstrate how good these units really are, I compared them directly to a Tele Vue 6 mm Radian, an eyepiece well-regarded for its optical quality. The Radian consistently beat out the two composite configurations, but the difference was never more than moderate.

When the units were tested on the Ranger, the difference between the units was less noticeable. Part of this is due to the short focal length--even a 15 mm Plossl in the Powermate gives you only 80x, and that is not enough to see much of the detail present on the planets, or for that matter the difference between the two amplifiers. For this test, the 6 mm Radian was used instead. Since the Ranger wouldn't come to focus with the Orion Barlow in the usual configuration, I put it between the tube and the diagonal, yielding a somewhat higher 2.5x amplification, much like the Powermate.

The two units performed much closer to each other in the Ranger than they did in the C5+. This may be because the Ranger is not as well color-corrected, by a large margin, and the color error leads to a subtle but noticeable loss of contrast already. This error cannot be corrected entirely, not even by the optical magic in the Powermate, and it partially hides the difference between the two.

Is the Powermate really 3-1/2 times better than the Orion Barlow? That depends, of course, at least in part on your own personal considerations. On its side, the Powermate gives you much greater ease of use, better illumination, and when it really matters, when the seeing is great, it gives you that last ounce of detail, where the other units peter out. In my opinion, that makes the Powermate well worth the extra price.

Copyright (c) 2001 Brian Tung