Ritchey-Chrétien telescope
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Ritchey-Chrétien is a type of astronomical telescope. It is a specialized Cassegrain reflector with the advantage that it is coma free. This is why almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910's.

The original article can be found at: http://en.wikipedia.org/wiki/Ritchey-Chrétien_telescope.
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