Barlow vs Powermate

I've read the televue website explaining the differences between a Powermate and a barlow. My question is there really a big enough difference to justify the costs? Is a a Powermate really better than a barlow? If the Powermate is so good then why does televue still make a barlow?

Why does televue make this freakin' odd 2.5x powermate that screws up the eyepiece set built on a 2x barlow? (I know they make a 2" 2x Powermate but I'm talking 1.25".)

There's a huge difference when used with long focal length eyepieces since vignetting and pupil throw-out are eliminated. Also, you get approximate parfocality in 1.25" star diagonals. Frankly, I prefer the Barlow with shorter focal lengths. As to why 2.5x? No clue. Maybe it had something to do with the fact that the 5x 1.25" Powermate came first. Also the old 1.25" Tele Vue Barlows were 2.5x and 1.8x. I don't think Tele Vue has any special appreciation for building a set of eyepieces around specifically a 2x Barlow.
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Mike

TeleVue Powermates are "Telecentric" devices, their Barlows aren't.

Edmund Optics has a good primer about telecentricity, which helps to explain the potential bonuses of a system such as is used by the PowerMates:

http://makeashorterlink.com/?L25562512

Al has ingeniously designed the PMs such that the extenders are removable from the chromed optics barrels, with customized T-adapters screwing on in their place. This allows close placement of a camera or CCD (using a T-ring), to reduce or eliminate vignetting, and NO extra couplers, thumbscrews, or other geegaws are needed! A real boon to those who may someday want to use these for gathering images in addition to visual use.

Paul

--- http://www.astro-nut.com/ ---

Actally, the 2.5x was a big selling point for me - 2x isn't really high enough if you want comfortable eye relief at high powers. I was re-organizing my ep collection anyway, so it worked out great.

For most visual observing, I doubt the Powermate is noticeably different from a high-quality barlow, but I don't think the price is really that high in the scheme of premium eyepieces, and the 2.5x might save you a little - less need to extend your ep collection to shorter focal lengths.