achromat

An achromat, in optical instrumentation, is a type of lens assembly designed to minimize chromatic aberration by combining two or more elements with different dispersive properties. The primary purpose is to bring two wavelengths, typically red and blue, to the same focal plane while maintaining acceptable sharpness.

In a common design, an achromat doublet consists of a positive lens of relatively low-dispersion crown glass bonded to a negative lens of high-dispersion flint glass. The combination reduces axial chromatic aberration across a portion of the visible spectrum, though residual color fringing remains at wavelengths outside the design. An achromat is typically used as a telescope objective or as a camera or microscope lens.

Historically, the concept originated in the early 18th century with Chester Moor Hall, who proposed using glasses of different dispersion. It was refined and popularized in the 1750s by John Dollond, making achromatic lenses practical for refracting telescopes and other instruments. Before the advent of low-dispersion glass and anti-reflective coatings, achromats were crucial for improving color fidelity in optical instruments.

Today, achromatic designs are common in amateur refracting telescopes and many consumer camera lenses, though many high-end systems have moved toward apochromats, which reduce chromatic aberration for three wavelengths or more and offer superior color correction at the expense of cost and complexity.