Apochromat

[1] The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length.

Focused and guided properly during the exposure, these apochromatic objectives are capable of producing the sharpest wide-field astrophotographs optically possible for the given aperture sizes.

Graphic arts process (copy) cameras generally use apochromatic lenses for sharpest possible imagery as well.

Classically designed apochromatic process camera lenses generally have a maximum aperture limited to about f/9.

Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings.

Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.
Apochromatic lens brings three colors to a common focal plane. Notice that this lens is designed for astronomy, not viewing, since one of the wavelengths (~0.780 μm ) is in the near infrared , outside the visible spectrum.
Apochromat lens.svg
The Apochromatic lens usually comprises three elements that bring light of three distinct colors to a common focus
Focus error for four types of lens, over the visible and near infrared spectrum.