About the TeleVue 2.5X Powermate - 1.25"...
To understand the TeleVue 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 Anyone telling you otherwise is using outdated information.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 TeleVue Telescope Eyepiece 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 lens to rest.
A high quality TeleVue Barlow Lens 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 lens will do more than just increase magnification, regardless of the number of elements.
The diverging rays leaving the Barlow lens 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 telescope eyepieces, the increase can be significant and not without performance consequences.
Vignetting can occur due to the altered ray path, when the telescope 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.
TeleVue had to go beyond the Barlow lens 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 lens without its limitations by restoring the field rays back to their original direction, as if the TeleVue Powermate were not there. The result is a pure magnification increase.
"While Barlow lenses 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 telescope 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 lens concept can go no further. TeleVue 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 TeleVue Powermate raises the standard for image amplification." --Al Nagler
Be sure to check out our line of TeleVue Telescopes, TeleVue Telescope Mounts and TeleVue Telescope Accessories. Don'''t forget TeleVue Telescope Eyepieces, TeleVue Dioptrx and TeleVue Eyepiece Filters, too!
TeleVue 2.5X Powermate - 1.25" Features...
- 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.
- 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.
TeleVue 2.5X Powermate - 1.25" Specifications...
Equipment Type: 2.5X Powermate
Barrel Size: 1.25"
Weight: 0.45 lbs.
Warranty: Limited LifetimeTeleVue Product Number: PMT-2513