If you need a very high quality, very high power 4x Barlow for 2" eyepieces, look no further than this TeleVue 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. No additional "interface lens" is needed, since the Powermate already has it built-in.
The Powermate is fully multicoated, with blackened lens edges and internal antireflection threading for excellent contrast. There's an undercut safety groove in the barrel that helps hold the Powermate in place should the focuser thumbscrew accidentally loosen. The Powermate is threaded to hold 48mm filters. It is 5.25" in length, but the chrome barrel is only 1.5" long, allowing use of the Powermate in star diagonals. Both the 2" eyepiece holder and the supplied low-profile 2" to 1.25" eyepiece adapter use brass clamp rings for secure, non-marring operation. A captive thumbscrew in the Powermate (that can't be lost in the field) operates its clamp ring.
It is optically corrected down to f/4 for use with fast f/ratio reflectors, but does not correct the coma inherent in such scopes. If used with the 2" Paracorr in fast focal ratio reflectors, the Powermate should go into the focuser first, with the Paracorr being inserted into the Powermate body.
An optional T-ring adapter, #9140T, is available to allow the 2x Powermate to be used for lunar and terrestrial photography with short focal length refractors.
