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Role of radio isotopes in dating the past

In addition, image data from several telescopes focused on the same object can be merged optically and through computer processing to produce images having angular resolutions much greater than that from any single component.The atmosphere does not transmit radiation of all wavelengths equally well.Telescopes for infrared, X-ray, and gamma-ray observations have been carried to altitudes of more than 30 km (100,000 feet) by balloons.Higher altitudes can be attained during short-duration rocket flights for ultraviolet observations.This restricts astronomy on Earth’s surface to the near ultraviolet, visible, and radio regions of the electromagnetic spectrum and to some relatively narrow “windows” in the nearer infrared.Longer infrared wavelengths are strongly absorbed by atmospheric water vapour and carbon dioxide.

Although the human eye remains an important astronomical tool, detectors capable of greater sensitivity and more rapid response are needed to observe at visible wavelengths and, especially, to extend observations beyond that region of the electromagnetic spectrum. However, photography still provides a useful archival record.The largest modern telescopes are all reflectors, the very largest composed of many segmented components and having overall diameters of about 10 metres (33 feet).Reflectors are not subject to the chromatic problems of refractors, can be better supported mechanically, and can be housed in smaller domes because they are more compact than the long-tube refractors.Telescopes for all wavelengths from infrared to gamma rays have been carried by robotic spacecraft observatories such as the Hubble Space Telescope and the Wilkinson Microwave Anisotropy Probe, while cosmic rays have been studied from space by the Advanced Composition Explorer.Very Large Array (VLA), operated near Socorro, New Mexico, by the National Radio Astronomy Observatory, 27 movable radio dishes are set out along tracks that extend for nearly 21 km.Infrared telescopes have been located atop Mauna Kea in Hawaii, in the Atacama Desert in Chile, and in the Canary Islands, where atmospheric humidity is very low.Airborne telescopes designed mainly for infrared observations—such as on the Stratospheric Observatory for Infrared Astronomy (SOFIA), a jet aircraft fitted with astronomical instruments—operate at an altitude of about 12 km (40,000 feet) with flight durations limited to a few hours.Having apertures much larger than the pupil of the human eye, telescopes permit the study of faint and distant objects.In addition, sufficient radiant energy can be collected in short time intervals to permit rapid fluctuations in intensity to be detected.Also, major photographic surveys, such as those of the National Geographic Society and the Palomar Observatory, can provide a historical base for long-term studies.Photographic film converted only a few percent of the incident photons into images, whereas CCDs have efficiencies of nearly 100 percent.

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