If you need a very high quality 2x 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.
This new model is nearly parfocal (unlike the 2” Big Barlow/Interface Lens combination), so it can be effectively used in TeleVue 2" diagonals. Its performance is excellent with every 1.25" or 2" TeleVue eyepiece made, even the 31mm Nagler Type 5, so you won’t have to do any major refocusing as you change from an eyepiece alone to an eyepiece/Powermate combination.
It is fully multicoated, with blackened lens edges and internal antireflection threading for excellent contrast. 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 on the Powermate (that can't be lost in the field) operates its clamp ring. There's an undercut safety groove in the barrel of the 1.25" adapter that helps hold it and the eyepiece in place should the Powermate thumbscrew accidentally loosen. The Powermate 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.
An optional T-ring adapter, #9122T, is available to allow the 2x Powermate to be used for lunar and terrestrial photography with short focal length refractors.
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