TeleVue Barlows and Powermate Image Amplifiers
Barlows and Powermates are are used to amplify or enlarge images in both visual and imaging applications. Barlows are powerful tools for extending an eyepiece collection at minimal cost, however; there are certain practical limitations to their application. The simple negative element Barlow increases eyepiece eye-relief. With short focal length eyepieces the effect is negligible. However, on long focal length eyepieces the exit pupil position moves well beyond the designer's intended position, resulting in vignetting with many eyepieces. This is why "Shorty" Barlows in particular, with their strong negative element often vignette and degrade long focus eyepieces.
The 4-element TeleVue Powermate image amplifier picks up where the Barlow concept can go no further. Powermate is a technically universal solution, using a positive field lens to redirect field rays. The result is an exit pupil that stays where the eyepiece designer intended. With freedom from aberrations, greater magnification potential, and compact size, the 4-element TeleVue Powermates raise the bar for image amplification.
Powermate vs. Barlow
To understand the Powermate, we must first understand the Barlow lens. A Barlow lens amplifies the power of a telescope. They can be considered "focal reducers" for eyepieces, or "focal extenders" for objectives. Terence Dickenson, in his Barlow test report in Sky and Telescope, July 1997, says: "...Technology has erased the old objections. A modern Barlow will not degrade your telescope's optics. Anyone telling you otherwise is using outdated information. Moreover, the highly regarded Nagler eyepieces and their clones have built-in Barlows - ample evidence that the lens is not some detrimental intruder. Thanks, Terence, for laying the myth of the degrading Barlow to rest.
A high quality Barlow must be properly designed and manufactured in order to avoid compromising a telescope’s color and spherical aberration corrections. The invisibility of TeleVue’s 2-element Barlows has been noted in test reports in the astronomy magazines. However, by the nature of its negative power lens, a Barlow will do more than just increase magnification, regardless of the number of elements.
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The diverging rays leaving the Barlow result in moving the exit pupil further out, thereby extending the eye relief. In short to medium focal length eyepieces the change is not noticeable. However, in the case of long focal length eyepieces, the increase can be significant and not without performance consequences.
Vignetting can occur due to the altered ray path, when the eyepiece’s lenses are not large enough in diameter to allow all the rays to make it through. Shorter Barlows, or ones with too much magnification, only exacerbate the problem because the ray path entering the eyepiece is steeper.
We had to go beyond the Barlow concept to achieve the goal of a compact, high power, fully corrected image amplifier. The Powermate™ consists of a negative doublet plus a positive “pupil-correcting” doublet. This 4-element system provides the magnifying function of a Barlow without its limitations by restoring the field rays back to their original direction, as if the Powermate™ were not there. The result is a pure magnification increase.
Applications and Unique Benefits of Powermates
- Vignetting, edge field aberrations and pupil movement - all introduced when using Barlows with long focal length eyepieces, are minimized.
- Telecentric operation (field rays leave parallel to optical axis, unlike Barlow lenses which diverge rays). This is ideal for rear mounted solar filters, such as with Daystar filters, which require
- High performance for image amplification, CCD or film (some of the finest Earth-based planetary imaging is being done with Powermates™ -- see below).
- Flexible for visual and imaging with all types of telescopes and eyepieces. Essentially Parfocal, and with nearly constant magnification regardless of image distance behind top surface (except 5x model which increases 1x for every 35mm of image distance increase).
- Higher magnifications are possible with high optical performance compared to “doubling up” Barlows.
- All will reach parfocal point in star diagonals, except for the 2”, 2x which comes very close.
In addition to full multi-coating, Powermates™ include other Tele Vue special touches such as safety undercuts where practical on the chrome barrel and captive lock screws that can’t fall out, and filter threads. Our 100% QC program at f/4 insures that Powermates™ will add dramatic power (and NOTHING ELSE) to your favorite eyepieces.
Solar System Imaging using TeleVue Powermates
Some of the finest Solar System images have been made with Powermates. Used singly or stacked, Powermates™ help any scope achieve the long focal-length required to image details on Solar System bodies. The increased image scale allows for imaging without an intervening eyepiece -- like in prime focus photography.
Powermate Magnification Chart
For astro-imagers, the chart below indicates the magnification for each Powermate (2x, 2.5x, 4x & 5x) as a function of distance from the Powermate top surface to the imaging surface (CCD chip or film plane).
