More Magnification for Observing

Readers are certainly familiar with the recent dramatic growth in telescopes with short focal ratios, both apochromatic refractors and Newtonian reflectors. To achieve a magnification of 30x to 50x per inch of aperture, considered ideal for detailed lunar and planetary observing or for resolving double stars, these instruments require either an impracticably short-focus eyepiece or a Barlow lens. The Barlow is often preferred since, for a given magnification, the eye relief will be longer, making the viewing more comfortable.

Because they work by increasing a telescope's effective focal ratio, Barlows also improve the performance of older eyepieces designed in the days when f/8 and longer focal ratios were the norm. Barlows typically consisted of a two-element negative lens that amplified 2x to 3x . Nevertheless, as such they would still require a rather short-focus eyepiece to achieve a high magnification with today's short-focus telescopes. Enter the Tele Vue 5x Powermate.

The 5x Powermate features a four-element design with two doublets: a negative pair to increase the effective focal length of the telescope and a positive pair to minimize vignetting and aberrations that typically arise when one tries to squeeze too much magnification out of a traditional Barlow. Indeed, Tele Vue's design is enough of a departure from a traditional Barlow that company founder Al Nagler skirts the issue of calling the Powermate a Barlow at all, terming it instead "a compact amplifying lens system."

Like all Tele Vue products, the 5x Powermate is solidly built and well finished. It has a chromed 1.25-inch barrel and black-anodized drawtube with a captive thumbscrew to secure eyepieces. The barrel is threaded to accept standard 11Ž4-inch filters, and the 5x Powermate's interior, including the edges of the lenses, is blackened to reduce scattered light.

I tested the 5x Powermate on my 6-inch f/7 apochromatic refractor with a 20-mm Plössl eyepiece. This combination yielded 45x per inch of aperture and comfortable eye relief. With a double star centered in the field, sharply defined, color-free Airy diffraction disks were readily seen, indicating good optics. As the stars were shifted back and forth across the field, their appearance did not change. The images remained sharp right to the edge of the view. Next, I placed the limb of the almost-full Moon near the edge of the field. Although this is a severe test for lateral color, the limb appeared sharp and free of color. I tried several eyepieces; all did well.

Since the 5x Powermate slides into an eyepiece holder only as far as a typical 1.25-inch eyepiece, it works well with star diagonals. Another commendable convenience is that it is parfocal. I checked this by focusing the telescope with an eyepiece in the 5x Powermate, then removing the Powermate and using the eyepiece alone. Without my touching the focus, the images were sharp. This worked with all the Tele Vue eyepieces I tried, but not with some eyepieces by other manufacturers.

While many observers know that a Barlow maintains a given eyepiece's eye relief, it is less common knowledge that it actually increases the eye relief of long-focal-length eyepieces. Thus, it can be difficult to position your eye properly when a Barlow is coupled to such eyepieces. The problem is compounded when you are viewing a bright object like the Moon ‹ the eye's pupil contracts. I discovered this phenomenon when I tried lunar observing with a 32-mm eyepiece and the 5x Powermate. In practice, the magnification of this combination (28x per inch of aperture) doesn't require the extreme amplification of the 5x Powermate; a shorter-focal-length eyepiece coupled with a traditional-power Barlow would be a better combination.

The 5x Powermate is an ideal approach to achieving high magnification with today's short-focal-length telescopes. It offers magnification and good eye relief with no noticeable image degradation.

Telescope maker, astrophotographer, and long-time amateur astronomer George East often reviews products for Sky & Telescope.