Describing a lens consisting of two or more optical components (elements), intended to correct for chromatic aberration. Commonly used as the objective of small refractors, the achromatic lens (or achromat) was invented in 1729 by the English optician Chester Moor Hall (1703–71) and first manufactured commercially by J. Dollond in 1758. It has one element of crown glass and another of flint glass. The dispersion (1) of the crown glass compensates for the chromatic error of the flint glass, while still leaving some refractive power. The two-element design is termed an achromatic doublet. It is practically impossible to correct all wavelengths of light, however, and most lenses adopt a compromise, bringing two particular wavelengths to a common focus, thus reducing the false colour. A lens that corrects for more than two wavelengths is termed an apochromatic lens.
Subjects: Astronomy and Astrophysics.