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Tele Vue Powermate - 2.5X Barlow-like



There was a time when the negative lens system know as the Barlow lens was looked down upon by many amateur astronomers. Although they have been improved a great deal since the time of the old tiny Edmund Barlows, the Barlow still has a bit of a stigma attached to it. With newer and more sophisticated designs of eyepieces, barlows sometimes seemed a bit redundant. However, the short focal length eyepieces used to get high power with the newer short f/ratio instruments still lacked a little eye relief, with some still seeming to give the "looking through a pinhole" effect. Hence, a good Barlow on a longer-focal length eyepiece seemed to give better eye relief and good overall performance, while extending the power range of the amateur's eyepiece collection. Now, optical wizard Al Nagler has turned the Barlow concept inside out, and has once again worked his magic to produce a true innovation in telescope optics; the Tele Vue powermate 2.5X "Image Amplifier".

To be upfront, calling the Powermate a Barlow would be insulting to the Powermate! Most Barlows consist of an achromatic or apochromatic negative lens set which magnifies the light cone from the telescope to give a longer effective focal length, and thus a higher magnification with a given eyepiece. Barlows will often introduce their own set of optical problems into the telescope/eyepiece mix, often resulting in something of a compromise in performance. Since the negative lens of Barlows diverges the light, they also change the light angles of the light cone, resulting in only a rough simulation of a longer f/ratio system. The lens set of the common Barlow is often located down a tube some distance away from the eyepiece, allowing at least some light from the narrowly-converging light cone to scatter off the sides and reduce the contrast unless the tube is well baffled. By contrast, the Powermate isn't really a Barlow at all. It is a set of 4 lenses which, in effect, faithfully re-creates the characteristics of the light cone from a telescope with 2.5 times its original focal length. While this might not seem to be much of a conceptual difference, in performance, this idea results in a significant improvement in the view over many Barlow lens systems.

The Powermate is similar in size to a regular 2x barlow, being 4 1/8" long and 1 5/8" wide at the top end with the top end flared slightly to accomodate 1.25" eyepieces without vignetting. Everything about the Powermate is machined to Tele Vue's usual high standard of quality. The upper half has a smooth black finish, while the lower half is bright chrome, with a wide machined groove in the 1.25" lower section above the lens section to help hold the set screw of the focuser. While some might be annoyed at this wide groove, those who use big heavy eyepieces on large scopes with tubes which allow the eyepiece to point downward slightly will know that this slot prevents some big heavy optics from ending up on the ground! Similarly, the set screw of the Powermate itself is captive, so it can't become lost.

The lens at the front of the Powermate is only 7/8ths of an inch wide, but up inside, the back lens set looks to be close to 1.2 inches in width, again showing that this optical invention is not a Barlow! In fact, unlike many Barlows, the field lens of most eyepieces ends up being only an inch or so behind the larger back lens of the Powermate, greatly reducing the chance that light will scatter off the inside of the Powermate's tube. The Powermate is used pretty much like you would a Barlow with the eyepiece in its usual spot. Unlike many Barlows which require some in-focusing, the Powermate needed only between ¼" and ½" of outward focus from the nominal eyepiece position. Once the proper focus is found with any one eyepiece/Powermate combination, using another eyepiece in the Powermate required only a slight re-focusing to again get a good focus. My smaller 1.25" eyepieces initially were slightly troublesome to insert into the upper end of the Powermate due to a slightly wider machined lip which holds the set screw. However, once I got the hang of the little "wiggle" I had to do, they went in without much trouble. This lip did not seem to bother my 14mm Meade Ultrawide eyepiece, so those of you who like the big "glass hand grenades" shouldn't have much trouble getting your eyepieces into the Powermate.

I tested the Powermate on my ten inch f/5.6 Newtonian with a variety of eyepieces. I had one of those "nights in a hundred", where the seeing is rock steady, so I could push the power on the ten as high as I wanted to go. At the telescope, the difference in performance between the Powermate and my older 3-element air-spaced Meade Model 140 "Tele-negative Amplifier", was nothing short of remarkable. The contrast in the Powermate was noticably higher, with no evidence of scattered light, edge-of-field aberrations, or lateral color. With my 10mm Ultrascopic on Jupiter, the disk showed both better contrast and a bit more detail (352x) than with the Meade Barlow. With Saturn, I sometimes had trouble with the fainter moons using the old Barlow, but they popped out nicely with the Powermate. The detail on both planets was amazing that night, so I did the usual silly thing of pushing into the realm of "empty magnification", using my 6.4mm "Super Plossel" and the Powermate. When I had tried this trick with the Meade Barlow (440x), the image was stable, but slightly dim and not absolutely sharp, with some weak color excess. It was almost like what you get when you are viewing a planet with a regular Achromatic objective and seeing a little hint of violet or bluish tint in the overall image. By contrast, using the Powermate, there was no hint of this color, and the image was a bit better overall even at 550x than it was at 440x with the Meade 2x Barlow. The Encke gap was visible most of the way around the outer portion of the A-ring, although it was not all that easy. The detail in the B-ring was easy, with a "stair-step" brightness change being noticable. The C-ring was not visible at 440x using the Meade Barlow, but was easily seen at both 352x and 550x using the Powermate. At the really high end of the power scale, the Meade Barlow was definitely being outperformed by the Powermate! On Gamma Andromedae, I had not seen the tiny close (0.46 arc sec.) companion to Gamma-B in at least 5 years, but that night, the Powermate and the 6.4mm showed its tiny Airy disk overlapping Gamma-B, with clear notching. Indeed, a small reddish star next to Jupiter showed its wonderful tiny and perfect diffraction pattern next to the comparatively-huge disks of the Galilean satellites.

On wider field eyepieces, the Powermate gave me wonderfully sharp views of the moon, giving me much more of a feel of flying over the surface. It helped the slight lateral color of my Meade 14mm Ultrawide only slightly, but it did give the breathtaking feeling of "looking out a spaceship window", at 252x rather than the usual high power "down a little tube" view. The picture was razor-sharp from edge to edge, and with its nearly 20 minute field of view, the Powermate/14mm Ultrawide combination is now one of my favorites for lunar sight-seeing. My 24mm Koenig was also better using the Powermate than it was in the old Meade Barlow, as the image showed a bit more contrast and less glare from bright objects at or beyond the field edges. Similarly, my 30mm Ultrascopic performed very well on Jupiter and various smaller deep-sky objects like big globulars and smaller open star clusters. As a final test, a friend of mine had suggested that I try the Powermate as a focal extender for my Daystar T-scanner H-alpha solar filter. I had been using a barlow at the front of the T-scanner to get the required f/32 light cone, but this meant that only a portion of the solar disk was within the passband. My friend had indicated that the Powermate might be "telecentric", which meant that might establish the field angles of an intrinsic long f/ratio system. Sure enough, when I used the Powermate, the entire solar disk was now showing chromospheric detail, so the barlow will soon be replaced by the Powermate!

In summary, the Tele Vue powermate 2.5X Image Amplifier is an outstanding new optical invention which may eventually replace the standard Barlow in the arsenal of many amateur astronomers. If you have been considering a new Barlow and if your budget allows, you might want to get a powermate instead!
David Knisely




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