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used with a 2x Barlow lens would have the magnifying power of a 9mm eyepiece. It's like having two eyepieces in one.

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by doubling the magnification of any eyepiece in use.The Celestron Ultima series Barlow is an air-spaced, 3-element apochromatic design with a 27mm clear aperture.  This lens has full multicoatings and is quite small and lightweight[weighs 0.31 lb (141 g) with length of 2.7" (7.0 cm)].

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Celestron carries an assortment of these highly useful lenses. A Barlow lens doubles the magnifying power of your eyepiece by doubling its effective focal length. For example, an 18mm eyepiece used with a 2x Barlow lens would have the magnifying power of a 9mm eyepiece. It's like having two eyepieces in one!If you're looking for an affordable way to achieve high-power viewing of the moon and planets, double stars and even close-up views of terrestrial subjects, a Barlow lens is the answer.Barlow lenses mount into either your telescope's focuser or visual back.The .96" Economy model is a good choice when you want to expand your power ranges at a very economical price. Celestron's premium quality 1¼" achromatic Barlow lenses (#93507) are compact and lightweight. It is under three inches long, weighing in at 4 ounces.The Ultima Series Barlow (2x - 1¼") is an air-spaced, three element apochromatic design with a 27mm clear aperture. This lens features a very high quality, fully multicoated design. Amazingly compact and lightweight, it's just 5 ounces and 2.75" in length.