telescope in which a spherical primary mirror receives light that has passed through a thin aspherical lens, called a correcting plate, that compensates for the image distortions—namely, spherical aberrations (aberration)—produced by the mirror. The Schmidt telescope is thus a catadioptric telescope; i.e., its optics involve both the reflection and refraction of light. Because the Schmidt telescope uses a spherical collecting mirror instead of a paraboloidal one (as conventional reflecting telescopes do), it is free from astigmatism and so has a wide field of view. The Schmidt instrument can, in effect, provide a sharper image of a larger area of the celestial sphere than ordinary reflectors and is thus ideal for star surveys.