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A telescope is an optical instrument that uses lenses, curved mirrors, or a combination of these to observe distant objects, or various devices used to observe distant objects by emitting, absorbing or reflecting electromagnetic radiation. The first known practical telescopes were refractive telescopes, invented in the Netherlands in the early 17th century using glass lenses. They have been used for both terrestrial applications and astronomy.

A reflective telescope, which uses mirrors to collect and focus light, was invented over the course of several decades as the first refractive telescope. Many new types of telescopes were invented in the 20th century, including radio telescopes in the 1930s and infrared telescopes in the 1960s. The word “telescope” now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases, other types of detectors.

The earliest telescope in existence was a 1608 patent filed with the government by Middelburg-based eyewear manufacturer Hans Lippershey for a refractive telescope. The real inventor is unknown, but rumors about him spread throughout Europe. Galileo heard about this and in 1609 built his own version and conducted telescopic observations of celestial objects.

The idea that the objective or light-collecting element could be a mirror rather than a lens was explored shortly after the invention of the refractive telescope. The potential benefits of using parabolic mirrors — reduced spherical aberration and no chromatic aberration — have led to many proposed projects and several attempts to build reflective telescopes. In 1668, Isaac Newton built the first practical reflective telescope, the design of which now bears his name – Newtonian reflector.

The invention of the achromatic lens in 1733 partially corrected the color aberrations present in simple lenses and made it possible to create shorter and more functional refractive telescopes. Reflecting telescopes, while not limited to the color problems seen in refractors, were made difficult by the use of fast tarnishing speculum metal mirrors used in the 18th and early 19th centuries, and the problem was solved with the advent of silver coated glass mirrors in 1857 and illuminated mirrors in 1932. The maximum physical size limit for refractive telescopes is about 1 meter (40 inches), which suggests that most large optical telescopes built in the late 20th century have reflectors. Currently, the largest reflector telescopes have targets over 10 meters (33 feet), and several projects are under development at 30-40 meters.

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