Editing Reflecting telescope (section)

==Use in astronomical research==
[[File:James Webb Space Telescope Revealed (26832090085).jpg|thumb|Main mirror of [[James Webb Space Telescope]] assembled at [[Goddard Spaceflight Center|Goddard Space Flight Center]], May 2016.]]
Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:
* Reflectors work in a wider [[spectrum of light]] since certain wavelengths are absorbed when passing through glass elements like those found in a [[refractor]] or in a [[catadioptric telescope]].
* In a [[lens (optics)|lens]] the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
* Light of different [[wavelength]]s travels through a medium other than [[vacuum]] at different speeds. This causes [[chromatic aberration]]. Reducing this to acceptable levels usually involves a combination of two or three aperture sized lenses (see [[achromat]] and [[apochromat]] for more details). The cost of such systems therefore scales significantly with aperture size. An image obtained from a mirror does not suffer from chromatic aberration to begin with, and the cost of the mirror scales much more modestly with its size.
* There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to [[gravity]], distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter.<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/515 515]}}</ref> In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.