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Pwermate VS big barlow

Started by ref68, 08/07/2010 04:23AM
Posted 08/07/2010 04:23AM Opening Post
any differance for the price we pay?
Thanks Refet.
Posted 08/11/2010 07:55PM | Edited 08/11/2010 07:58PM #1
The key difference is that Powermates are telecentric lenses. For Solar System imaging this is an important feature.

Televue excerpt:

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 Anyone telling you otherwise is using outdated information.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 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.

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™).

* 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.

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