A Guide to Barlow Lenses for Astronomy
Â
Figure 1 – An array of Barlow lenses. Image credit and copyright Agena AstroProducts.
1. Overview
A focal extender, sometimes called Barlow lens, increases the effective focal length of a telescope and therefore increases the magnification of a given eyepiece used with that telescope. As a diverging lens, a focal extender works by reducing the angle of convergence of light from the objective lens of a telescope before the light enters the eyepiece (see Figure 2), making it seem as if the light was coming from an objective with a longer focal length and slower focal ratio. For visual observation, this longer effective focal length increases the magnification achieved by a given eyepiece and it means that each