Tele Vue Powermates.

Concept: While Barlows are powerful tools, the negative element defining them also limits
their ability. 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. My 4-element form 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, I hope you'll agree Powermate™ raises the standard for image
amplification.
- Al Nagler


Al Nagler’s new 4-element concept, continues the tradition of full-field sharpness with
optimum
aberration control. Like long eye-relief? Retain the eye-relief of long focal length eyepieces,
while at high power. Enjoy, for example, the sharpness and comfort of the 32mm Plössl at up
to 5 times the power!

Powermate™ vs. Barlow
To understand the Powermate, we must first understand the Barlow lens. Barlows amplify 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 Anyone telling you otherwise is using outdated information.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
Tele
Vue’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.

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.

Unique Benefits and Applications for 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 Hydrogen-alpha filter use such as with Daystar models.
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.

If, for example, you own a 24mm Panoptic, you’ll have the power of a 12mm focal length
eyepiece with the 2.0x Powermate, 9.6mm with the 2.5x Powermate, a 6mm with the 4x
Powermate, or a 4.8mm with the 5x Powermate!

Imaging the Solar System with 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.

Prices:
PMT-2000 - 2.0x 2" Powermate - £194.00
PMT-2513 - 2.5x 1.25" Powermate - £130.00
PMT-4201 - 4.0x 2" Powermate - £203.00
PMT-5126 - 5.0x 1.25" Powermate - £130.00

The Astronomy Shop

1 Tone Hill. Wellington

Somerset TA21 0AU

United Kingdom

Telephone: +44 (0)1823 665510