ULTRAVIOLET TELESCOPES
The ultraviolet portion of the electromagnetic spectrum has been divided by astronomers into three segments, more or less derived from the time in which serious research into them began. First there is the ground-based ultraviolet, from 4000 angstroms to the atmospheric cutoff at 3000 angstroms; next the far ultraviolet from 3000 to 1000 angstroms; and last the extreme ultraviolet from 1000 to 100 angstroms.
Ultraviolet observations began after World War II, in October 1946, when a captured German V-2 rocket carried a small ultraviolet spectrograph to a height of 100 kilometers. During the ascent it recorded the ultraviolet portion of the solar spectrum down to 2200 angstroms. Telescopes, analyzing instruments, and radiation detectors for ultraviolet research are basically the same kinds of instrument used in visible and infrared observations. The principle difference is that a number of types of glass are not transparent to ultraviolet photons but are highly absorbing. Therefore, special materials must be used for lenses and entrance windows into the instrument. The principles of operation are the same as those for visible radiation. Since ultraviolet-sensitive film cannot be retrieved from an orbiting satellite, photoelectric devices have been the primary radiation detectors, so that data could be radioed back to ground stations.
Between 1962 and 1975 eight Orbiting Solar Observatories (050-1 through 050-8) were launched for study of the sun in ultraviolet wavelengths arid X-ray and gamma-ray radiation. In December 1968 the first Orbiting Astronomical Observatory (OAO-2) began sampling the ultraviolet and far-ultraviolet radiation. By the time OAO-2 ended its useful life in February 1973, it had carried out photometry on more than 1000 objects from planets to galaxies. Its successor, illustrated in Figure 5.18, OAO-Copernicus, launched in August 1972, carried an 0.8-meter ultraviolet telescope and three small X-ray telescopes, and was even more active than OAO-2.
In january 1978 the International Ultraviolet Explorer, an orbiting observatory, shown in Figure 5.18, was launched by NASA. This was a joint undertaking by NASA and several western European countries. Its facilities have been used for studies of planets, stars, galaxies, and the interstellar medium in the wavelength range from 1150 to 3200 angstroms. Astronomers conduct their experiments from an elaborate console of controls located at the Goddard Space Flight Center.
