TeleVue - 5x POWERMATE

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 Anyone telling you otherwise is using outdated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. 

	

Includes Brass clamp ring, filter threads and captive screw. Accepts PTR-1250