Also found in: Wikipedia.
Barlow lens(bar -loh) An achromatic diverging lens (see achromatic lens) placed behind the eyepiece of a telescope, just inside the primary focal plane, in order to increase the effective focal length of the objective or primary mirror. This increases the magnification of the telescope so that a long-focus eyepiece may be used to give the higher powers needed to separate optical doubles, or for planetary observation under the best seeing conditions.
Barlow lens[′bär‚lō ‚lenz]
A lens with one plane surface and one concave surface that is placed between the objective and eyepiece of a telescope to decrease the convergence of the beam from the objective and thereby increase the effective focal length.