Tele Vue Barlows: "An Optically Perfect Window" 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 1 1/4" 2x and 3x Barlows were envisioned as magnification windows. 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. The Tele Vue design approach includes mechanical conveniences. The Barlows are parfocal with Tele Vue eyepieces. The 2X Barlow is threaded for 1 1/4" filters.