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The Schmidt is an optical system used exclusively for imaging ap- plications such as sky surveys (such as those that have discovered dwarf planets and other objects in our solar system beyond Pluto), monitoring of galaxies for supernova explosions, and studies of comet tails. The primary mirror of a Schmidt camera has a spherical shape, and therefore suffers from spherical aberration. To correct for this problem, the light passes through a thin corrector plate as it enters the tube. Schmidt camera correcting plates are among the largest lenses made for astronomical applications. The special features of this system combine to produce good images over a far larger angular field than can be obtained in a Cassegrain telescope. Solar telescopes Solar instrumentation differs from that designed to study other ce- lestial objects, because the Sun emits great amounts of light ener- gy and could overheat conventional telescopes or detectors. In one refracting telescope design (Fig. 10), a large heliostat incorporating a lens also seals in a vacuum. This design reflects sunlight down a fixed telescope tube, where it is reimaged through another lens, which is as large as any telescope lens in use, to an imaging bench or spectrograph. This system is evacuated to avoid problems that would be created by hot air currents. The largest solar telescope in the world, the 4.24 m (13.9 ft) Daniel K. Inouye Solar Telescope on Haleakala, Maui, Hawaii, uses a reflecting telescope design. Radio telescopes Radio telescopes use mirrors of very large size compared with the wavelengths being observed. For long wavelengths, a reflecting "dish" may consist of only an open wire mesh while for shorter wavelengths, down to about 1 mm, the surfaces are solid and more accurately formed in absolute units. Because of the limitation by diffraction, arrays of large reflectors have been constructed. The output of each mirror of the array is combined in a process called aperture synthesis to yield a resolution roughly equivalent to that provided by a telescope the size of the array. Infrared telescopes In infrared telescopes, the secondary mirror is caused to oscillate rotationally about an axis through a diameter. This motion causes an infrared detector to see alternately the sky and the sky plus the desired object. The signals received at these two mirror positions are subtracted and, as a consequence, the large background radia- tion received both from the atmosphere and from the telescope is canceled. However, because of the random nature of thermal radia- tion, the fluctuations of the background emission are not canceled. Thus, infrared telescopes are additionally designed to reduce the telescope background radiation and its fluctuations. Notable infra- red telescopes include the 3.8 m (12.5 ft) United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, NASA's Spitzer Space Telescope, and the European Space Agency's (ESA's) Herschel Space Observa- tory, the largest telescope ever put in space and which operated from 2009 to 2013. Telescope (continued) + ward ' s science + ward ' s science Fig. 10 Solar telescope configuration. (a) Swedish 1-m (40-in.) Solar Telescope of the Royal Academy of Sciences, located on La Palma, Canary Islands. (b) Dia- gram of the optical elements in the telescope. The 1-m lens is the largest in use.