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Spherical mirrors, however, suffer from spherical aberration—parallel rays reflected from such mirrors do not focus to a single point. For parallel rays, such as those coming from a very distant object, a parabolic reflector can do a better job. Such a mirror can focus incoming parallel rays to a much smaller spot than a spherical mirror can.
A three-mirror anastigmat is an anastigmat telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations – spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.
A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary. The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary.
Spherical aberration makes the focus of telescopes and other instruments less than ideal. This is an important effect, because spherical shapes are much easier to produce than aspherical ones. In many cases, it is cheaper to use multiple spherical elements to compensate for spherical aberration than it is to use a single aspheric lens.
With two non-spherical mirrors, such as the Ritchey–Chrétien telescope, coma can be eliminated as well, by making the two mirrors' contribution to total coma cancel. This allows a larger useful field of view. However, such designs still suffer from astigmatism. The basic Ritchey–Chrétien two-surface design is free of third-order coma and ...
A system fulfilling this condition and free from spherical aberration is called aplanatic (Greek a-, privative, plann, a wandering). This word was first used by Robert Blair to characterize a superior achromatism, and, subsequently, by many writers to denote freedom from spherical aberration as well. [8]
Misaligned or malformed lenses and mirrors [ edit ] Grinding and polishing of precision optical parts, either by hand or machine, typically employs significant downward pressure, which in turn creates significant frictional side pressures during polishing strokes that can combine to locally flex and distort the parts.
The earliest reflecting telescopes used spherical mirrors which, although easy to fabricate and test, suffer from spherical aberration; a long focal length was needed to reduce spherical aberration to a tolerable level. Making the primary mirror parabolic removes spherical aberration on-axis, but the field of view is then limited by off-axis coma.