If you need a high quality, medium high power Barlow for 1.25” eyepieces, look no further than this TeleVue. It’s a 2-lens achromatic design with superb sharpness to match that of TeleVue 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 Barlow in place should the focuser thumbscrew accidentally loosen. A non-marring brass clamp ring holds eyepieces securely in place. A captive thumbscrew (that can't be lost in the field) controls the clamp ring tension.
Its 5" length is marginal for use in star diagonals, however, as its length (combined with a physically long eyepiece) can put your eye nearly 7” above the diagonal when observing. This poses no optical problems, but some inexpensive diagonals have rather loose tolerances. The weight of the very long Barlow/eyepiece combination may cause the diagonal housing to loosen where it fits into the visual back and pivot unexpectedly.
This TeleVue 3x 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/Barlow combination. It is optically corrected down to f/4 for use with fast f/ratio reflectors, but will provide excellent performance with any scope type. Due to the length of the Barlow, the accessory container recommended below is substantially larger in diameter than is needed for a snug side-to-side fit, even though the container length is correct to hold the Barlow without rattling.
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