A telescope, designed to aid the observation of remote objects, collects some form of electromagnetic radiation (such as visible light). The Netherlands developed the first known practical telescope in the 17th century. The term “telescope” was termed in order to describe Galileo’s instruments in 1611. However, Galileo was not the inventor of the telescope. It was Hans Lippershey, Zacharias Janssen, and Jacob Metius who are credited with the creation of the telescope. In 1668, Isaac Newton built the first reflecting telescope.

The achromatic lens, created in 1733, corrected color aberrations in the simple lens allowing for a more functional refracting telescope. The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. In 1937 the first purpose built radio telescope was built and since then a large variety of astronomical instruments have been developed.

Optical telescopes focus light mostly from the visible spectrum. They increase the angular size of distant objects as well as their brightness. The lenses or mirrors gather light and bring it to a focal point in order for the image to be observed. There are three main types of optical telescopes: the refracting telescope, the reflecting telescope, and the catadioptric telescope. The refracting uses lenses to form an image. The reflecting uses mirrors to form an image and the catadioptric uses both to form images.

Radio telescopes, used for radio astronomy, are directional radio antennas. They are also used to collect microwave radiation. SETI and Arecibo Observatory use radio telescopes to search for exterrestrial life.

X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth’s atmosphere is opaque to this part of the electromagnetic spectrum.

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