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37211_Ward's World+MGH Telescope

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Telescope (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this. Ultraviolet telescopes Ultraviolet telescopes have special mirror coatings with high ultraviolet reflectivity. Since the atmosphere is not transparent below a wavelength of 300 nanometers, ultraviolet telescopes are usually flown above the atmosphere either in rockets or in orbiting spacecraft. An early example was the International Ultraviolet Explorer, which functioned from 1978 to 1996. The Hubble Space Telescope, primarily a visible light instrument, has ultraviolet sensitivity and is the largest ultraviolet telescope in space (Fig. 11). X-ray telescopes X-ray telescopes must be used above the atmosphere and are flown in rockets or satellites. The focusing type uses an unusual optical design in which the reflection from the surfaces occurs at nearly grazing incidence. This is the only way of achieving reflective optics for x-rays. Launched in 1999, NASA's Chandra X-ray Observatory gives fine resolution matching that typical of ground-based optical telescopes. ESA's XMM (X-ray Multi- Mirror)-Newton, also launched in 1999, has a larger collecting area though less fine focusing. The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in 2012, is the first focusing high-energy x-ray mission. Gamma-ray telescopes Gamma-ray telescopes use coincidence and anticoincidence circuits with scintillation or semiconductor detectors to obtain directional discrimination. With coincidence counting, two or more detectors in a line must give a simultaneous detection for a gamma ray to be counted. Other detectors are often used to surround the telescope to reduce the unwanted background arising from undesired particles. A simultaneous count received in one or more of these shielding detectors nullifies (by anticoin- cidence) the detection otherwise registered in the coincidence detectors. Thus gamma rays that trigger only the coincidence circuits are detected. Since many gamma rays are produced within the atmosphere by other particles, the telescopes are usually flown in balloons, rockets, or satellites. However, at very high energies (more than 100 GeV), ground-based techniques have been used either to detect the Cerenkov light from the shower of electrons produced when a gamma ray hits the atmosphere or to detect directly the particles that penetrate to the ground. The largest gamma-ray telescope in space is NASA's Fermi Gamma-ray Space Telescope, launched in 2008. It con- tains a large-area telescope and a gamma-ray burst monitor. Cosmic-ray telescopes These telescopes detect cosmic rays, which are usually protons or heavier-element nuclei, or the products produced when these particles interact with the atmosphere in so-called air showers. In its simplest form, a cosmic-ray telescope may consist of nuclear track emulsions borne aloft in balloons or spacecraft. Other telescopes, like the Pierre Auger Observatory in Argentina, use Cerenkov detectors on the ground in combi- nation with atmospheric ultraviolet fluorescence detectors. Neutrino telescopes As with cosmic rays, although neutrinos are not electromag- netic radiation, devices to collect astrophysically generated neutrinos are, by analogy, often called neutrino telescopes. Cerenkov radiation is again used as a detection method to infer the properties of incident neutrinos. Neutrino emissions from Supernova 1987A were the first confirmed neutrinos from a distant astrophysical source. High-energy neutrinos detected by the IceCube Neutrino Observatory, completed at the South Pole in 2010 and currently the world's largest neutrino tele- scope, were first traced back to a source—an active galaxy known as a blazar—in 2017. Fig. 11 The Hubble Space Telescope. (Credit: NASA)

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