They can also be specialty monochromatic lenses used in modern research telescopes and other instruments.
Early telescope objective, such as those built by Johannes Hevelius and Christiaan Huygens and his brother Constantijn Huygens, Jr.,[1][2] utilized single small (2"-8") positive lenses with enormous focal lengths (up to 150 feet in length in tube telescopes and up to 600 feet in non-tube aerial telescopes).
This allowed the observer to use higher magnification while limiting the interfering rainbow halos caused by chromatic aberration (the uncorrected chromatic aberration fell within the large diffraction pattern at focus).
They are also used in astrographic telescopes where multiple single narrow wavelength images are used in stellar classification.
[citation needed] Non-achromatic objectives are also used in monochromatic laser applications such as collimators, beam expanders, and highly corrected pupil imaging for wavefront error sensors for adaptive optics.