A diverging lens, usually a doublet, placed in front of the focal point of the telescope to multiply the magnification of an eyepiece. A typical Barlow lens effectively doubles the focal length of the telescope and hence doubles the power of any eyepiece used with it. The value can be varied somewhat by altering the distance between the Barlow and the eyepiece. It was invented in 1834 by the English physicist and mathematician Peter Barlow (1776–1862).
Subjects: Astronomy and Astrophysics.