Six views of the Crab Nebula at different wavelengths of light Since then, a large variety of complex astronomical instruments have been developed. The first purpose-built radio telescope went into operation in 1937.

The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The largest reflecting telescopes currently have objectives larger than 10 meters (33 feet), and work is underway on several 30-40m designs. The maximum physical size limit for refracting telescopes is about 1 meter (39 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing speculum metal mirrors employed during the 18th and early 19th century-a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932. The invention of the achromatic lens in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter, more functional refracting telescopes. In 1668, Isaac Newton built the first practical reflecting telescope, of a design which now bears his name, the Newtonian reflector. The potential advantages of using parabolic mirrors-reduction of spherical aberration and no chromatic aberration-led to many proposed designs and several attempts to build reflecting telescopes.

The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. Galileo heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects. The actual inventor is unknown but word of it spread through Europe. The earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lipperhey for a refracting telescope. 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. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. They were used for both terrestrial applications and astronomy. The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century. The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.Ī telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.

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