telescope

A telescope is an instrument to observe objects and phenomena at a distance from the instrument, presenting a magnified image of the EMR as received from the object itself, gaining some of the advantages of closer observation. Older/traditional usage of the term refers to an optical instrument, but with the development of the ability to sense other EMR bands, the term is used in a wider sense, often qualified, e.g., "radio telescope".

Telescopes are used in and outside astronomy, and within astronomy, both by the generally-interested and by researchers. Such research telescopes come in all sizes but include the larger units with larger diameters (apertures), which give them the ability to resolve more detail and to observe fainter (e.g., more distant) objects. The largest optical telescopes are on the order of 10 meters in diameter with telescopes in the range of 25 to 40 meters in current development (extremely large telescopes). The largest radio telescopes include up to 100-meter diameter units that can be aimed and units larger than 300 meters with limited pointing capability. The atmosphere limits the abilities of telescopes (e.g., atmospheric windows and seeing) which has motivated the development and use of space telescopes.

See telescope types and list of telescopes.