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11 December 2008 23:14
- an optical device, typically used just before the eyepiece in the light path of a telescope, that multiplies the magnification of the eyepiece
- usually referred to simply as a Barlow, often (incorrectly) barlow with a lower-case "B"
- Barlows are basically a tube with a lens at the bottom
- named for its creator, the English engineer Peter Barlow
- they come in 2x, 3x, 4x, and other variations, typically (but not always) whole numbers
- using a 2x Barlow is precisely like using an eyepiece of half the focal length, so experts often advise not to buy eyepieces of exact multiples of 2 - so buying an 18mm and a 9mm eyepiece of the same design is somewhat redundant
- using a Barlow increases the eye relief of an eyepiece, the effect is more noticable as the focal length of the eyepiece increases
- Televue's Powermates perform a function similar to a Barlow, but they consist of two doublet lenses that increase magnification without increasing the focal length of the eyepiece
- "stacking" barlows yields even higher magnifications (i.e., a 2x and 3x stack results in 6x magnification) - seldom useful for visual due to seeing limitations; planetary imagers often stack barlows to achieve insane focal lengths
- planetary imagers sometimes put extenders or diagonals after the barlow to further increase the magnification
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Orion Telescopes
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