Barlow Lenses and Focal Reducers

Simply, it's a concave (negative) lens placed between a telescope objective and eyepiece to increase the magnification, usually by two or three times. The negative lens reduces the angle of convergence of the light cone, effectively making it appear to the eyepiece that the primary has a longer focal length. It was invented in the early1800s by English physicist Peter Barlow (1776–1872)

Eyepieces for Telescope

What is the Eyepiece

The eyepiece is the lens assembly at the focal point of the telescope that forms and magnifies the image. Most telescopes, even department store types, come with at least one for initial use. Sadly today many of the major manufacturers deliver their telescopes with just that one eyepiece. Today’s Celestar 8 and Nexstar 8 telescopes,the current editions of the legendary Celestron C8, come with only a single 25-mm Plossl eyepiece (though Celestron does offer a nice observing kit that adds five more eyepieces and a filter set for under $100).