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A simple type of reflecting telescope, designed by Isaac Newton (1642-1727), who demonstrated it to the Royal Society in London in 1671. The primary mirror is a paraboloid (or spherical for small apertures), and the secondary is a flat mirror positioned in the converging reflected beam at an angle of 45° to the optical axis to form an image just outside the main tube (see illustration). The design is widely used for small amateur instruments, but is not suitable for large telescopes.
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