Barlow lens.

About Barlow lens

The Barlow lens, named after the English physicist and mathematician Peter Barlow (1776–1862), is an optical tube with diverging lens elements that, used in series with other optics in an optical system, increase the effective focal length of an optical system as perceived by all components that are after it in the system. The practical result is that inserting a Barlow lens magnifies the image. A real Barlow lens is usually not a single glass element, because that would generate chromatic aberration, and spherical aberration if the lens is not aspheric. More common configurations use three or more elements for achromatic correction or apochromatic correction and higher image quality.