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Telescope (from Greek tele, "distant", and skopein, "to behold"): a tool for collecting electromagnetic waves in the light, x-ray, infrared, or radio range in order to observe distant objects by magnifying their images. Telescopes are thus the most important tools for exploring the universe (see also ►mirror).

At this very moment thousands of telescopes are directed toward outer space. Here are the most important ones.

The Arecibo telescope, the largest radio telescope* on Earth. Located in a valley in Puerto Rico, it has a mirror 305 meters in diameter. This telescope has already played an important part in our search for ►extraterrestrials and is the only telescope that has been used to send out a message into space. Film buffs will be familiar with the Arecibo telescope from a James Bond movie as well as from the film Contact, which was based on a novel by Carl Sagan.

The Large Binocular Telescope (LBT), located on top of a 3200-meter-high mountain in Arizona and containing two 8.4-meter mirrors. It is currently the largest optical telescope on Earth. Much like a pair of binoculars, its two mirrors face the same direction, collect twice as much light as a single mirror would, and provide much higher resolution. The LBT began operation in October 2005; the USA, Germany, and Italy contributed to its construction.

The Hubble Space Telescope, which was released into space by a space shuttle in April 1990 and has been orbiting the earth ever since. Its main mirror has a diameter of 2.4 meters. The advantage of a space telescope is that the optical attenuation and refraction of the light caused by the atmosphere are eliminated. Shortly after releasing the telescope, however, scientists realized that the manufacturer had cut the main mirror incorrectly, causing it to generate out-of-focus images. Thus, further space shuttle missions became necessary in order to mount additional mirrors. Since then the telescope has been fully functional; it continues to generate spectacular images of the most distant regions in space, regions inaccessible to earthbound telescopes.

The furthest glance into space to date. Collage of protogalaxies
at a distance of more than 40 giga-lightyears (NASA).

Due to technical problems and NASA's massive budget problems, however, the continued operation of the Hubble telescope is at risk. Although in 2004 the US President announced a proposal to renew manned flights to the moon and even to land a manned spacecraft on Mars, he did not bother to specify whence the US space agency would receive the money needed for such ambitious if grandiose undertakings. If NASA does not continue to receive funding for needed maintenance flights, the Hubble telescope will sooner or later cease its operation. (As of this writing, the next maintenance flight — originally scheduled for October 2008 — is expected to take place in early 2009; if successful, it should extend the Hubble's operating life through 2013.) The expected time lag between the planned retirement of the current space shuttle and the activation of its successor is also a concern. But the Hubble telescope has weathered such threats before. A plan to call off all future shuttle flights to the Hubble for safety reasons, announced by NASA's top administrator in 2004 in the wake of the Space Shuttle Columbia disaster, met vociferous opposition from astronomers and the public and was withdrawn by that administrator's successor the next year — an indication of both the huge popularity of the telescope and the precariousness of its existence.

The Spitzer Space Telescope was launched into space in August 2003. It contains an 85-cm main mirror and registers the infrared energy radiated by cosmic objects, which would be largely filtered out by the Earth's atmosphere and thus blocked from earthbound telescopes. This is of value because infrared waves permeate the gas and dust clouds that obstruct our view of details at the center of galaxies; as a result, infrared radiation can provide us with information about the development of planetary systems and about processes that occurred in the early phase of the universe. An unexpected side effect is of a more esthetic nature: the Spitzer telescope generates images of delicate beauty, which cast new light even on very familiar objects.

* The largest telescope of all is the Pierre Auger Observatory, which began operations in November 2005. It does not, however, consist of a telescope mirror, but rather of 1600 water tanks set up in a desert in Argentina to collect rays of cosmic particles.