Glossary term: Galilean Telescope

Description: In a refracting telescope, light first encounters a convex lens (converging 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. This is the task of an additional optical element: the eyepiece, which is where you put your eye if you observe through a telescope.

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 concave lens (diverging lens) as the eyepiece.

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. Compared to a Keplerian telescope, a Galilean telescope provides a visual image that is upright (not inverted), but it has a much narrower field of view than a Keplerian telescope.

The wider field of view is why nearly all modern refracting telescopes used by amateur astronomers are Keplerian in design – in the case of particularly high-quality telescopes, Keplerian with additional lenses providing improved image quality. For professional astronomy, the distinction between Keplerian and Galilean is largely irrelevant: professional observations use cameras instead of eye pieces, and most professional telescopes are reflective (mirror) telescopes, not refracting telescopes.

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