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solar telescopeA reflecting (or refracting) telescope plus associated instruments used in studying the Sun. The Sun is the only star whose disk can be resolved and studied by a telescope. The diameter of the solar image is given by 0.0093f, where f is the focal length of the telescope primary mirror or objective. Long-focus telescopes must therefore be used in order for a large image to be formed. Such telescopes are usually fixed in position, with the sunlight being reflected on to the mirror or lens by a heliostat. This light is then directed in a fixed direction into a spectrograph or some other measuring instrument.
The intense heat of solar radiation requires the measuring instruments to be cooled. In some solar telescopes the entire building is cooled. In most cases the primary mirror and instruments are underground to facilitate this. The heat problem can be reduced by evacuating the air from the rooms housing the instruments and in some cases from the whole telescope housing.
Many large solar telescopes have been built as tower telescopes (or solar towers), in which the sunlight is directed down a vertical path inside a solid or girder construction and reflected into underground rooms containing the measuring instruments. In a vacuum tower telescope, the entire optical path is virtually air-free. The US National Solar Observatory's vacuum tower telescope at Sacramento Peak, New Mexico, is 41 meters high with a further 67 meters underground; it has a 76-cm diameter mirror, focal ratio f/72. The 16-meter high Swedish solar tower on La Palma in the Canaries is patterned after the Sacramento Peak telescope, but has a more compact and optically simple system (50 cm, f/45 lens). Both telescopes give high-resolution images.
The McMath–Pierce Solar Telescope at the Kitt Peak National Observatory, in Arizona, has a somewhat different design (see illustration). Sunlight is reflected from a 2.1-meter heliostat, which is 30 meters above the ground. It traverses 150 meters along the polar axis and is focused by a 1.6-meter concave mirror and a second flat mirror on to the various measuring instruments.
The best solar seeing is found above large lakes; in addition, a high altitude is required in order to reduce the disturbing effects of the Earth's atmosphere. The site of the Big Bear Solar Observatory, in California, has both these features.
a telescope for observation of the sun.
Solar telescopes with small-diameter objectives and relatively short focal distances usually have equatorial mountings. Such telescopes include the following three important types: corona-graphs, which are used to observe the solar corona outside eclipse; telescopes for the photographic observation of the photosphere; and telescopes for the photographic observation of the chromosphere. The third type of instrument is usually equipped with a biréfringent filter, which permits the sun to be observed in the light of the hydrogen line Ha.
In the case of large solar telescopes, the telescope proper is fixed. A coelostat, or system of moving flat mirrors, is provided to direct the sun’s light into the telescope. Such telescopes are also equipped with various devices for studying the sun—for example, photographic cameras, photoelectric light detectors, spectrographs, and solar magnetographs. According to the direction of the optical axis, a distinction is made between horizontal telescopes and vertical, or tower, telescopes. Solar telescopes are also constructed with an inclined axis.