Describing a lens or optical system with very little chromatic aberration. In an apochromatic lens (also known as an apochromat) the chromatic aberration is completely cancelled out at three or more wavelengths, compared with two for a typical achromatic lens. Apochromats usually have at least three optical elements, made from different types of glass. Fluorite has become a popular material for apochromats, giving virtually perfect colour correction with only two elements, but at considerable cost.
Subjects: Astronomy and Astrophysics.