Guillaume Cassegrain
fl. 1672
French physicist who invented the reflecting telescope that bears his name.
Cassegrain reflectors employ a concave primary mirror to focus and reflect light onto a convex secondary mirror. The light is then reflected back through a hole in the primary mirror to a lens. Cassegrain's design increased angular magnification over other types of reflectors. A more advantageous feature, not recognized for another century, is the elimination of spherical aberration inherent in other two-mirror reflectors.
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