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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.
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.
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 ...
They suffer from spherical aberration and coma, but do not suffer from astigmatism like a spherical mirror when used in an off-axis geometry, provided the angle of incidence is matched to the design angle. Because they are easier to manufacture, they are much cheaper than ellipsoidal or paraboloidal mirrors for the same surface quality.
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.
She began experiencing agoraphobia, an anxiety disorder in which a person avoids places and situations in which they feel helpless or fearful. "It was scary," the singer, born Jewel Kilcher, says ...