Editing Refracting telescope (section)

===Apochromatic refractors===
{{Main|Apochromat}}
[[File:Apochromat.svg|alt=Apochromat lens.svg|thumb|right|The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus]]
'''Apochromatic refractors''' have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens.{{citation needed|date=July 2024}} Such telescopes contain elements of [[fluorite]] or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration.<ref>{{cite web |url=http://starizona.com/acb/ccd/equipbasicsref.aspx |title=Starizona's Guide to CCD Imaging |publisher=Starizona.com |access-date=17 October 2013 |archive-date=17 October 2013 |archive-url=https://web.archive.org/web/20131017204031/http://starizona.com/acb/ccd/equipbasicsref.aspx |url-status=dead }}</ref> Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes.<ref name=":4"/>

One of the famous triplet objectives is the [[Cooke triplet]], noted for being able to correct the Seidal aberrations.<ref>{{Cite book|url=https://books.google.com/books?id=mberzJtkU4MC&q=%22Cooke+triplet%22+-wikipedia&pg=PA199|title=Fundamental Optical Design|last=Kidger|first=Michael J.|date=2002|publisher=SPIE Press|isbn=9780819439154}}</ref> It is recognized as one of the most important [[Objective (optics)|objective]] designs in the field of photography.<ref>{{Cite book|url=https://books.google.com/books?id=czjUBwAAQBAJ&q=%22Cooke+triplet%22+-wikipedia&pg=PA187|title=Classical and Evolutionary Algorithms in the Optimization of Optical Systems|last=Vasiljevic|first=Darko|date=2012-12-06|publisher=Springer Science & Business Media|isbn=9781461510512}}</ref><ref name=":1">{{Citation|last=Vasiljević|first=Darko|chapter=The Cooke triplet optimizations|date=2002|pages=187–211|editor-last=Vasiljević|editor-first=Darko|publisher=Springer US|doi=10.1007/978-1-4615-1051-2_13|isbn=9781461510512|title=Classical and Evolutionary Algorithms in the Optimization of Optical Systems}}</ref> The Cooke triplet can correct, with only three elements, for one wavelength, [[spherical aberration]], [[Coma (optics)|coma]], [[Astigmatism (optical systems)|astigmatism]], [[field curvature]], and [[Distortion (optics)|distortion]].<ref name=":1" />