Telescopes continued to evolve, but until the 20th century they were all some form of optical telescope, primarily reflector types. In , Karl Jansky, an American-born radio engineer, provided the idea for a radio telescope, which engineer Grote Reber built in During the 20th century, infrared telescopes also evolved, and due to necessity, some were launched into space, starting with the Infrared Astronomical Satellite IRAS , which was deployed in space in In , the Hubble Space Telescope, an optical, infrared and ultraviolet telescope, was launched into space, and is one of the most advanced space-based telescopes currently in operation.
Telescopes come in a variety of design types. The oldest type of telescope is the optical telescope, which is broken up into refractors and reflectors.
Refractor telescopes direct light through a series of lenses directly to an eyepiece at the back end, through which an observer can look. Reflector telescopes use mirrors to redirect and focus light. Reflector telescopes overcame refractors in popularity because they produced less serious optical distortions, and this made them much more valuable to astronomers.
In addition to optical telescopes, there are also infrared telescopes, which view the universe by observing infrared energy that is not visible to the naked eye.
Infrared telescopes depend on high altitude deployments to reduce absorption of infrared energy by the atmosphere. They produce the best results, however, when launched into space. Radio telescopes, built like giant satellite dishes, scan for radio signals from celestial bodies, which are not absorbed by the atmosphere.
They tend to be very large, with dishes up to 1, feet in diameter, such as the Aricebo Observatory. X-ray telescopes and ultraviolet telescopes both require high altitudes or deployment in space, and they scan for x-ray radiation and ultraviolet radiation, respectively. X-ray telescopes are good at scanning for black holes and neutron stars, while ultraviolet telescopes provide clues about the evolution of young and old stars, as well as galaxies.
Infrared telescopes are used to find celestial bodies such as stars and planets, while optical telescopes observe visible light thrown off by heavenly bodies and galaxies, etcetera. Radio telescopes observe radio wave energy coming from quasars and neutron stars, and are also instrumental in the search for alien life, such as the Search for Extraterrestrial Intelligence project, or SETI. One of the most celebrated telescopes in history is the Hubble Space Telescope HST , which was launched into space in This serviceability proved to be critically useful when it was discovered that its main mirror was defective, requiring service.
Another four servicing missions were carried out to replace, repair and upgrade parts in the HST. The HST originally was built with reel-based tape recording instruments, and was upgraded with solid state storage systems. Two times a day the HST transmits its data, a total of gigabytes every week, to a satellite which beams it down to Earth.
Usage of the telescope is technically open to anyone, regardless of their nationality, who can submit a proposal. Amateur astronomers were also invited to participate, but due to budget cuts, only one project was ever approved.
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