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Observational astronomy
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=== Important factors === For much of the history of observational astronomy, almost all observation was performed in the visual spectrum with [[optical telescope]]s. While the Earth's atmosphere is relatively transparent in this portion of the [[electromagnetic spectrum]], most telescope work is still dependent on [[astronomical seeing|seeing]] conditions and air transparency, and is generally restricted to the night time. The seeing conditions depend on the turbulence and thermal variations in the air. Locations that are frequently cloudy or suffer from atmospheric turbulence limit the resolution of observations. Likewise the presence of the full [[Moon]] can brighten up the sky with scattered light, hindering observation of faint objects. For observation purposes, the optimal location for an optical telescope is undoubtedly in [[outer space]]. There the telescope can make observations without being affected by the [[Earth's atmosphere|atmosphere]]. However, at present it remains costly to lift telescopes into [[orbit]]. Thus the next best locations are certain mountain peaks that have a high number of cloudless days and generally possess good atmospheric conditions (with good [[astronomical seeing|seeing]] conditions). The peaks of the islands of [[Mauna Kea Observatory|Mauna Kea, Hawaii]] and [[Roque de los Muchachos Observatory|La Palma]] possess these properties, as to a lesser extent do inland sites such as [[Llano de Chajnantor Observatory|Llano de Chajnantor]], [[Paranal Observatory|Paranal]], [[Cerro Tololo Inter-American Observatory|Cerro Tololo]] and [[La Silla Observatory|La Silla]] in [[Chile]]. These observatory locations have attracted an assemblage of powerful telescopes, totalling many billion US dollars of investment. The darkness of the night sky is an important factor in optical astronomy. With the size of cities and human populated areas ever expanding, the amount of artificial light at night has also increased. These artificial lights produce a diffuse background illumination that makes observation of faint astronomical features very difficult without special filters. In a few locations such as the state of [[Arizona]] and in the [[United Kingdom]], this has led to campaigns for the reduction of [[light pollution]]. The use of hoods around street lights not only improves the amount of light directed toward the ground, but also helps reduce the light directed toward the sky. Atmospheric effects ([[astronomical seeing]]) can severely hinder the [[Angular resolution|resolution]] of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, telescopes larger than about 15β20 cm in [[aperture]] can not achieve their theoretical resolution at visible wavelengths. As a result, the primary benefit of using very large telescopes has been the improved light-gathering capability, allowing very faint magnitudes to be observed. However the resolution handicap has begun to be overcome by [[adaptive optics]], [[speckle imaging]] and [[aperture synthesis|interferometric imaging]], as well as the use of [[space telescope]]s.
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