- Photography-only telescope used mainly for wide-angle sky surveys. Schmidt cameras use a thin non-spherical lens, the correcting plate, at the skyward end of the telescope to capture light from a wide sky area faithfully. The principal technical problem with Schmidt optics is the correcting plate, which is designed to match the optical properties of the telescope mirror and which is complex to grind exactly. Schmidt telescopes are also long for their light-gathering power because the lens needs to be at the centre of curvature of the mirror, while most large refractors are only half this length, as the focal length of a mirror is only half of its radius of curvature.
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