Definitions for "Achromat"
The most common type of objective lens found in a refractor telescope. An achromat has two elements made of two different types of glass with different refractive indicies. A properly designed achromat will bring at least two different wavelengths (colors) of light to a common focus. In practice, achromats are designed to minimize errors across all wavelengths.
Traditional refracting (lens-based) telescope with two glass elements. False color ( chromatic aberration) is reduced to acceptable levels for smaller scopes. Also see apochromat, refractor, chromatic aberration.
A lens made of two pieces of glass (or, occasionally, other materials) with differing optical properties, specifically chosen to minimize chromatic aberration (which see).