Barlow lens
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The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, effectively increases the focal ratio of an optical system as perceived by all components after it in the system.
In its astronomical use, a Barlow lens may be placed immediately before an eyepiece to effectively decrease the eyepiece's focal length by the amount of the Barlow's divergence. Since the magnification provided by a telescope and eyepiece is equal to the telescope's focal length divided by the eyepiece's focal length, this has the effect of increasing the magnification of the image. For this reason, astronomical Barlow lenses are named for the amount of magnification they induce. Most commonly, Barlow lenses are 2x, but 3x Barlows and adjustable Barlows are available.
Barlow lenses have an unfortunate reputation in some quarters due to poor results given by the Barlows included with some department-store telescopes. These results, however, are perhaps better attributed to poor optics and shaky mounts permitting less practical magnification in these telescopes than to an inherent failing of Barlow lenses.
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