Glossary term: Galilean Telescope

Description: In a refracting telescope, light first encounters a convex lens, called the objective lens, which serves to bundle infalling parallel light rays. Such almost perfectly parallel light rays correspond to light we receive from a distant object, such as a star. In order to produce an image that can be observed by eye, those converging rays must be made parallel again. In a Galilean telescope, named after the model of telescope built by Galileo Galilei in 1609 and used for some of the first systematic astronomical telescope observations, this is achieved by inserting a convex lens. In contrast, in a Keplerian telescope, invented by Johannes Kepler in 1611, the converging light rays are allowed to cross, and the resulting divergent light rays are then made parallel using a second convex lens.

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