Editing Reflecting telescope (section)

===Optical errors===
Reflecting telescopes, just like any other optical system, do not produce "perfect" images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design.

[[File:Sirius A and B Hubble photo.jpg|left|thumb|180px|An image of [[Sirius A]] and [[Sirius B]] by the [[Hubble Space Telescope]], showing [[diffraction spike]]s and concentric [[Airy disk|diffraction rings]].]]

Because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a [[secondary mirror]], film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to [[diffraction]] effects of the obstruction as well as [[diffraction spike]]s caused by most secondary support structures.<ref>[http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/c-o%27s.html Rodger W. Gordon, "Central Obstructions and their effect on image contrast" brayebrookobservatory.org]</ref><ref>[http://spider.seds.org/scopes/obstruct.html "Obstruction" in optical instruments]</ref>

The use of mirrors avoids [[chromatic aberration]] but they produce other types of [[Aberration in optical systems|aberrations]]. A simple [[spherical mirror]] cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer the center of the mirror, a defect called [[spherical aberration]]. To avoid this problem most reflecting telescopes use [[parabolic reflector|parabolic shaped mirrors]], a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror's [[optical axis]]), but towards the edge of that same field of view they suffer from off axis aberrations:<ref>[http://farside.ph.utexas.edu/teaching/316/lectures/node136.html Richard Fitzpatrick, Spherical Mirrors, farside.ph.utexas.edu]</ref><ref>{{Cite web |url=http://www.vikdhillon.staff.shef.ac.uk/teaching/phy105/telescopes/phy105_tel_reflectors.html |title=Vik Dhillon, reflectors, vikdhillon.staff.shef.ac.uk |access-date=2010-04-06 |archive-date=2010-05-05 |archive-url=https://web.archive.org/web/20100505062511/http://www.vikdhillon.staff.shef.ac.uk/teaching/phy105/telescopes/phy105_tel_reflectors.html |url-status=dead }}</ref>

* [[coma (optics)|Coma]] – an aberration where point sources (stars) at the center of the image are focused to a point but typically appears as "comet-like" radial smudges that get worse towards the edges of the image.
* [[Field curvature]] – The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field. It is sometimes corrected by a field flattening lens.
* [[Astigmatism (optical systems)|Astigmatism]] – an [[azimuth]]al variation of focus around the aperture causing point source images off-axis to appear elliptical. Astigmatism is not usually a problem in a narrow [[field of view]], but in a wide field image it gets rapidly worse and varies quadratically with field angle.
* [[Distortion (optics)|Distortion]] – Distortion does not affect image quality (sharpness) but does affect object shapes. It is sometimes corrected by image processing.

There are reflecting telescope designs that use modified mirror surfaces (such as the [[Ritchey–Chrétien telescope]]) or some form of correcting lens (such as [[Catadioptric system#Catadioptric telescopes|catadioptric telescopes]]) that correct some of these aberrations.