Properly designed Barlows do wonderful things. First, they amplify power while retaining the eyepiece's eye-relief. Second, they slow the telescope's f/#, which improves eyepiece sharpness. Third, the designer can introduce compensations for eyepiece aberrations.
Tele Vue's 2" 2x BIG Barlow is envisioned as a magnification window. Multi-coated high index glasses assure optimum aberration correction, exceptional contrast and virtually no light loss. Tele Vue Barlows achieve their design goals even when tested by the fastest scopes. They even improve our competitors' eyepieces and telescopes!
For eyeglass wearers, the Big Barlow is perfect for doubling the power while retaining the eye-relief of 2" Tele Vue eyepieces such as the 55mm Plossl, 35mm and 27mm Panoptics (requires use of Panoptics Barlow Interface) and 31mm and 22mm Naglers.
Tele Vue's design approach includes mechanical conveniences. The Barlow is parfocal with our eyepieces and the BIG Barlow includes a parfocal 2" - 1 1/4" adapter. The BIG Barlow is threaded for 48mm filters and has a captive lock-screw.
Why compromise any optical link? Tele Vue product design and execution deliver consummate satisfaction. Every Tele Vue Barlow must pass the same rigorous f/4, full field testing as all their eyepieces before leaving their hands for yours.