If you must wear glasses while observing, and you're looking for an ultra high power Barlow to get high magnifications from your favorite long focal length/long eye relief eyepieces, look no further than this 5x Powermate. While not technically a Barlow, this unique new 4-lens image amplifier performs the same functions, but at a higher level of optical correction.
Barlow lenses have a negative power that diverges field rays, lengthening the effective focal length of the telescope and increasing the magnification. If Barlows are made too short or with too much magnification, the diverging rays can cause severe vignetting and image degradation in eyepieces due to mismatched pupils.
The Powermate is a 4-element parfocal compact amplifying lens system that consists of a negative achromatic doublet (a Barlow) plus a positive "pupil-correcting" doublet. While it looks and functions like a Barlow, your eyepieces will operate with pupil locations where the eyepiece designer intended, avoiding vignetting and mismatch problems with wide field eyepieces. The Powermate optimally mates its power to your eyepieces.
It is fully multicoated, with blackened lens edges and internal antireflection threading for excellent contrast. Its barrel is threaded for standard 1.25" filters. There's an undercut safety groove in the barrel that helps hold the Powermate in place should the focuser thumbscrew accidentally loosen. A non-marring brass clamp ring (with a captive thumbscrew that can't be lost in the field) holds your eyepieces securely in place. The Powermate has a 3.8" length and a short chrome barrel that allows it to be used in star diagonals.
This TeleVue is parfocal with TeleVue Plössls below 32mm in focal length, so you won't have to do any major refocusing as you change from an eyepiece alone to an eyepiece/Powermate combination. It is optically corrected down to f/4 for use with fast f/ratio reflectors, but will provide excellent performance with any telescope type or focal ratio. It is parfocal with the 1.25" diagonal of the TeleVue Ranger for quick power changes during terrestrial observing.
The chrome barrel containing the Powermate's optics can be removed from the black eyepiece holder section and used for lunar and terrestrial photography with close coupling cameras or CCD imagers. It is particularly useful for increasing the photographic magnification with short focal length refractors. An optional T-ring adapter, 81PMT, is needed to connect the chrome optical portion of the Powermate to a camera or CCD imager.
