Editing Refracting telescope (section)

== Technical considerations ==
[[File:Yerkes Observatory Astro4p7.jpg|thumb|right|The {{convert|102|cm|in}} refractor, at [[Yerkes Observatory]], the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)]]

Refractors suffer from residual [[chromatic aberration|chromatic]] and [[spherical aberration]]. This affects shorter [[F-number#Focal ratio|focal ratio]]s more than longer ones. An {{f/|6|link=yes}} achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects); an {{f/}}16 achromat has much less color fringing.

In very large apertures, there is also a problem of [[lens sag]]ging, a result of [[gravity]] deforming [[glass]]. Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around {{convert|1|m|in|abbr=out|sp=us}}.<ref>{{cite book|author=Stan Gibilisco|title=Physics Demystified|url=https://archive.org/details/physicsdemystifi0000gibi|url-access=registration|year=2002|publisher=Mcgraw-hill|isbn=978-0-07-138201-4|page=[https://archive.org/details/physicsdemystifi0000gibi/page/532 532]}}</ref>

There is a further problem of glass defects, striae or small [[air bubble]]s trapped within the glass. In addition, glass is [[Opacity (optics)|opaque]] to certain [[wavelength]]s, and even [[optical spectrum|visible light]] is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in [[reflecting telescope]]s, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.

The ISS-WAC on the [[Voyager 1]]/[[Voyager 2|2]] used a {{convert|6|cm|in}} lens, launched into space in the late 1970s, an example of the use of refractors in space.<ref name=astrov>{{cite web|url=http://www.astronautix.com/v/voyager.html|archive-url=https://web.archive.org/web/20160911011643/http://www.astronautix.com/v/voyager.html|url-status=dead|archive-date=11 September 2016|title=Voyager|work=astronautix.com}}</ref>