Ground-based telescopes

In the age of the James Webb Space Telescope and other space-based observatories, ground-based telescopes are often perceived as antiquated. However, ground-based telescopes are still playing key roles in the search for extrasolar planets and other fields of cosmological research, and larger telescopes are always under construction or development.

Overview

Ground-based telescopes have a long history of advancing astronomy research. 1960s, large ground-based telescopes, situated in advantageous locations on the Earth’s surface and having apertures larger than 250 centimeters, were used to explore and further study objects in our solar system. For example, in the 1950s, the Martian atmosphere was believed to be thin, and, based on the intensity and polarization of Martian reflected light, Martian surface pressure was determined to be only 5 to 10 percent of that on Earth. Designers of preliminary Martian landers used this value when deciding whether a descent to the Martian surface should be affected by a balloon, glider, or downward-pointing rocket engine. Also, the presence of this much atmosphere suggested that Martian life might exist, as there would be enough air to breathe (if free oxygen were present) and enough protection from ultraviolet light and extreme temperature changes.

Hyron Spinard, a University of California scientist, was able to obtain good high-dispersion spectra using the Lick Observatory’s 3-meter telescope in the early 1960s. Spinrad concluded that the true value for the Martian surface pressure must be only about 5 millibars. This is only 0.5 percent of the Earth’s atmospheric pressure—a very small value. New designs had to be considered for Martian spacecraft, and at that time it seemed less likely that anything remotely resembling terrestrial life could exist under such harsh conditions.

The oldest observatory in the United States is Mount Wilson, which overlooks Pasadena, California. George Hale founded the observatory in 1904 and used funding from the Carnegie Institute of Washington, D.C., to create a solar research laboratory. The Snow Solar Telescope was moved to Mount Wilson in 1904 from Yerkes Observatory in Wisconsin. Long-term studies of the Sun began in 1905. Hale ordered the mirror for his 100-inch telescope in 1906. Two years later, the 60-foot solar telescope became operational. Hale used it when he detected the Sun’s magnetic field. In May 1912, a solar telescope was mounted atop a 60-foot-tall tower, and work began on the foundation for Hale’s 100-inch mirror telescope. The latter telescope achieved first light on November 2, 1917, to much fanfare. Several discoveries of great importance in the history of astronomy were made using the telescopes at Mount Wilson. These include detecting the first Cepheid variable star and measuring the distance to M31 (the Andromeda nebula, now called the Andromeda galaxy). This proved the Milky Way was but one of many galaxies in the greater universe;. The telescope also helped determine extragalactic distances and provided evidence that the universe is expanding. It also detected remnants of the 1604 supernova and discovered four additional satellites of Jupiter.

Mount Wilson remains one of the top observatories conducting astronomical research. In 1995, it became home to Georgia State’s CHARA stellar Interferometer array. The 150-foot-tall tower telescope is operated by the Astronomy and Astrophysics Division of the University of California at Los Angeles. It is used to study solar magnetic activity and look for long-term changes that may provide an understanding of the Sun’s variability and its implications for the Solar radiation received by the Earth and its biosphere. The solar telescope is part of a worldwide network monitoring the Sun’s surface (its photosphere). The 60-inch telescope, which became operational in 1908, is also the largest in the world available for approved use by the general public.

One of the oldest large optical telescopes in the United States is the Palomar Observatory’s 5-meter Hale reflector. Completed in 1948, for thirty years it was the world’s largest. Located in the mountains northeast of San Diego, California, Palomar was used primarily for Deep spaceCosmology and stellar studies. With the advent of the space program and the accompanying renaissance of planetary astronomy, the Palomar telescope came to be used occasionally for solar-system work. It was used, for example, to analyze the Martian atmospheric gases and, in the 1950s, to examine the Martian surface for signs of chlorophyll to see if greenish surface regions might indicate plant life; they did not. With infrared detectors, the telescope was used to make some of the first temperature maps of the observed disk of Jupiter and to sample the temperatures of the surfaces of the other major planets.

The Palomar telescope has played an important role in space research with its exploration of objects that were first identified by the Infrared Astronomical Satellite (IRAS) as being anomalous infrared sources. Many of these objects are obscured stars and interstellar clouds within the Galaxy. These are not easily detected and studied at optical wavelengths, but Palomar observers have used infrared detectors, working at short enough wavelengths that are transmitted through the Earth’s atmosphere. There is still a considerable amount of research to be done before the place of infrared-emitting galaxies in the general scheme of the extragalactic universe is understood thoroughly.

The largest telescope at the Lick Observatory—located at Mount Hamilton, east of San Jose, California—is the Shane 3-meter telescope, completed in 1959. It has been used by University of California astronomers to make numerous discoveries related to space research. An exotic example of such discoveries has to do with the planet Mercury. In 1961, there were reports from the Soviet Union that Mercury appeared to have an atmosphere; according to one Soviet astronomer, it was composed of hydrogen, but another report claimed that it was carbon dioxide. Space scientists were concerned about these reports. Because Mercury’s surface was known to have high temperatures, astronomers had deduced that the planet should not be able to retain any appreciable atmosphere, because gases would be so hot that they would escape into space in a relatively short time. In 1962, to explore the question further, the Shane telescope was used to obtain high-dispersion spectra of Mercury. Because Mercury is always quite close to the Sun (never more than 28° from it in the sky), the telescope was used during the daytime for these observations. The results, after analysis, were clear. The better data provided no sign whatsoever of an atmosphere. Years later, in 1979, Mariner 10 confirmed this conclusion by returning data that showed that, because of its high temperature, Mercury does not have an appreciable atmosphere. It only retains temporarily a minute and tenuous envelope of hydrogen gas captured from the solar wind.

In 1979, the National Aeronautics and Space Administration (NASA) built a 3-meter infrared telescope to be used especially for space-related research. It was put on Mauna Kea in Hawaii to take advantage of the mountain’s high altitude (4,200 meters) and lack of water vapor. It has been employed successfully in a large number of infrared projects, including studies of asteroids, comets, planetary satellites, the galactic nucleus, and infrared-bright and active galaxies.

The 2.7-meter telescope of the McDonald Observatory, near Austin, Texas, was built in 1968 with NASA sponsorship and with the intent that it be used largely for space-related research. A particularly interesting example of this has been its lunar-ranging measurements. A powerful laser at its focal point is used to beam visible light to the lunar surface, and 2.5 seconds later the telescope detects the reflected light. The light’s travel time can be measured so accurately that the distance to the Moon could almost instantaneously be determined to an accuracy of about 2.5 centimeters.

In the mid-1980s some astronomers felt that ground-based astronomy had reached its limit. However, advances in computers and technology would prove them wrong. The first telescope using a new technique known as active optics was the New Technology Telescope (NTT) at the European Southern Observatory (ESO); it was installed in 1988. NTT has a 3.58-meter mirror that is only 24 centimeters thick and is therefore flexible. Traditional telescopes hold their shape by the thickness of the mirror. This thickness increases the mirror’s weight and limits its size. The shape of NTT’s mirror is maintained by a series of supports known as actuators, which are computer-controlled. Success with NTT design led to the construction of ESO’s Very Large Telescope (VLT), which is an array of four 8-meter mirrors.

Two of the most famous telescopes using active optics are located at the Keck Observatory in Hawaii near the summit of the dormant Mauna Kea volcano. These telescopes (Keck I and Keck II, completed in 1993 and 1996) each have mirrors measuring 10 meters in diameter. The two can also be linked together to form an even larger interferometer. Keck I and II are used for investigations of brown dwarfs, globular clusters, black holes, Jupiter’s atmosphere, and distant young galaxies, for example.

Active optics help minimize the limiting effects of the telescope itself, increasing light collection and image quality. Adaptive optics, on the other hand, reduces or eliminates the effects of the Earth’s atmosphere. Telescopes using adaptive optics (such as Keck) can achieve image resolutions of 30 to 60 milli-arc seconds at infrared wavelengths. Without adaptive optics, they would be able to produce resolutions of only one arc second. One method of adaptive optics uses a guide or reference star. The telescopes are equipped with a wavefront sensor that measures the atmospheric distortion of light. Information is sent to a computer, which alters the telescope’s deformable mirror to correct the image. When astronomers study very distant objects, the target is often too faint for this process. Instead, astronomers use a brighter reference star that is located near the target. Light from the guide star is then used to determine how to adjust the telescope to negate the effects of atmospheric distortion of the target object. A second method for adaptive optics is using a laser beam in place of a reference star. The laser guide star (LGS) is directed toward the upper atmosphere and often is pulsed. Reflected light is then detected as it travels back down through the Earth’s atmosphere, where it is used as an “artificial” reference star. LGS has advantages for scientists, because a reference star of sufficient brightness is not always found in all parts of the sky.

Technological advances in both active and adaptive optics have led to an ever-increasing number of larger ground-based telescopes. The largest optical telescope in the United States is the Large Binocular Telescope (LBT) located near Safford, Arizona. The telescopes are a joint project with the Italian and German astronomical communities, the University of Arizona, Arizona State University, Northern Arizona University, Ohio State University, Research Corporation of Tucson, and the University of Notre Dame. The LBT consists of two 8.4-meter mirrors with a common mount. Therefore the two telescopes together have the equivalent power of an 11.8-meter telescope. The first telescope was built in Italy and was shipped to Arizona in 2002. In March, 2008, LBT produced its first binocular images, making it the most powerful optical telescope in the world. The first images produced were false-color pictures of NGC 2770, a spiral galaxy 102 million light-years away. One image shows hot young stars concentrated in the spiral arms; the second shows older stars that are more evenly dispersed. The third image is a composite photograph that shows the full range of stars from cool to hot. An instrument is being developed to work with the telescopes at Near-infrared wavelengths that would produce images ten times sharper than ones taken by the Hubble Space Telescope. A high-resolution spectrograph is also planned, which would help astronomers study the magnetic field of the Sun and other stars by looking at Zeeman splitting of spectral lines.

Several other extremely large telescopes (ELTs) include the 25-meter Giant Magellan Telescope (GMT). This is a joint endeavor of the Carnegie Institute of Washington, Harvard University, the Massachusetts Institute of Technology, the University of Arizona, the University of Michigan, the Smithsonian Institution, Australian National University, the University of Texas at Austin, and Texas A&M University. Located in Las Campanas, Chile, GMT is equipped with six mirrors surrounding a centrally located mirror. Each mirror will measure 8.4 meters, giving GMT a diameter of 25.2 meters. Scientists use the GMT to study the origins and evolutions of planetary systems, the formation of black holes, and dark matter.

The Thirty Meter Telescope (TMT) is a planned extremely large telescope envisioned to host 492 hexagonal mirrors, each measuring about 1.44 meters in diameter. The 30-meter telescope will use six laser guide stars to study distant star systems and galaxies. Mauna Kea Observatory in Hawaii is the nominal host site. Construction of the TMT began in 2014 but was halted the following year due to local opposition. Construction has not resumed and as of 2023 alternative sites are now being considered.

The Large Aperture Mirror Array (LAMA) is another conceptual telescope envisioned to combine a series of 66 individual 6.15 meter telescopes. The mirrors of the LAMA telescope are planned to be coated with a very thin layer of liquid mercury approximately 2-3 millimeters thick. The mirrors will spin constantly. After the mirrors are closed, a layer of oxide will form on top that will seal the surface. This will allow extra mercury to be skimmed off and achieve a liquid mercury layer about 1 millimeter thick. Liquid mirrors are favored over glass because on average they cost 95 percent less. Another benefit of liquid mirrors is that the primary mirror does not tilt. This means that the support structure does not need to be as massive, thus also reducing cost. Together the mirrors that form the array will have the power of a 42-meter telescope. Sites are being considered in New Mexicoand Chile. Scientists plan to use LAMA to study distant galaxies, stars, and extrasolar planets.

The most ambitious telescope planned for development was the European Southern Observatory’s 100-meter Overwhelmingly Large Telescope (OWL). The primary mirror was envisioned as 3,042 segments, each 1.6 meters in diameter. The projected costs to construct the OWL became excessive and in 2017 the project was cancelled.

Radio telescopes form another category of ground-based telescopes. These telescopes, used to receive and analyze extraterrestrial radiation at radio wavelengths, have been of great importance to the development of modern astronomy and space science. Most large radio telescopes have been one of three distinct types: large single dishes, interferometric arrays of dishes, and millimeter telescopes.

Centimeter-wavelength radio waves can best be detected by means of a very large parabolic reflecting surface (a “dish”) that can move in position to follow the celestial object as it moves across the sky. The largest of these actually does not move, however, but instead has a moving receiver suspended above the dish. The 300-meter Radio telescope of the Arecibo Ionospheric Radio Observatory was made by smoothing out a depression in the hills of Puerto Rico and lining it with a parabolic metal surface suspended a few feet above the ground. The Arecibo Telescope was, however, decommissioned in 2020.

A different type of single-dish antenna is the 100-meter Effelsberg radio telescope near Bonn, West Germany. Its parabolic metal dish is fully movable, can turn to any place in the sky and can follow celestial sources at the diurnal rate. Although is smaller than the Arecibo telescope, its flexibility has made it useful for many projects that would have been impossible with a large, fixed dish.

Similar, though smaller, single dishes that have been important are a 76-meter telescope at Jodrell Bank, England, a 64-meter telescope at Parkes, Australia, and a 43-meter one at Green Bank, West Virginia. A 90-meter transit-type radio telescope at Green Bank collapsed suddenly in 1988. It was later replaced by the Robert C. Byrd Green Bank Telescope (GBT), which is the world’s largest fully steerable single-dish radio telescope. The 100-meter telescope became operational in 2000.

High resolution at radio wavelengths is hard to achieve. Resolution is proportional to wavelength; to achieve the same resolution afforded by an optical telescope, a radio telescope working at a long Wavelength such as the 21-centimeter neutral hydrogen line must be many kilometers in diameter. This size would be impractical for a single-dish design, so radio astronomers have constructed giant arrays of radio dishes, connected electronically so that the signals received are blended and analyzed as if from an immense single telescope. These instruments are called “interferometric arrays” and range in size from 1 Kilometer or so for the pioneer instrument in Cambridge, England, to intercontinental arrays.

The largest interferometers that are located in physical proximity are the Very Large Array (VLA) near Socorro, New Mexico, and the Westerbork Array in the Netherlands. The former consists of twenty-seven 25-meter dishes spread out in a Y-shaped pattern on the dry lake bed of the St. Augustine plain; the latter consists of a linear array of telescopes. All radio interferometers of this type have at least some of their antennae on wheels and tracks so that the spacing can be adjusted according to the needs of different observing projects.

The Australia Telescope array was designed with some of the properties of both the VLA and Westerbork; a continental array of telescopes planned to span North America would permit extremely high resolution. Ad hoc interferometers, made up of existing single dishes that are coordinated to simulate an array, have utilized even larger baselines (for example, from Australia to Canada), achieving radio images that are more detailed than even optical telescopes can produce, using normal detectors. Spaceborne radio telescopes can achieve even wider separations, as large as the solar system.

Millimeter-wavelength radio telescopes tend to have characteristics midway between those of standard radio telescopes and those of optical telescopes. Most are housed in domes of some sort and are fully steerable. For example, the 9-meter telescope on Kitt Peak in Arizona has a dome-shaped housing with a large slit that can be opened for observing. Other large millimeter telescopes are located in Hawaii (on Mauna Kea), in Japan, and in Massachusetts. They are especially powerful for detecting and analyzing emissions from cool molecular gas in star-forming regions.

The world’s largest millimeter telescope is located on the Sierra Negra volcano, 350 kilometers southeast of Mexico City. The Large Millimeter Telescope (LMT) has a 50-meter dish. LMT is a joint effort between the National Institute of Astrophysics, Optics, and Electronics (of Mexico) and the University of Massachusetts. Construction was completed in 2006, after which followed a two-year testing period before initial research began. Scientists plan to use the large telescope to study extrasolar planetary atmospheres, compositions of comets, and the origins of the universe.

First conceived in 1995, the Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint effort of organizations from North America, Europe, Japan, and Chile. ALMA is located in the Atacama Desert of the Andes Mountains in Chile. Completed in 2013, the array includes eighty radio antennas operating at 0.3 to 9.6 millimeters. The dishes range in size from 7 to 12 meters in diameter. ALMA should produce resolutions about ten times better than the Hubble Space Telescope. Radio astronomers use ALMA to study galactic nuclei, quasars, distant stellar compositions, and galactic, stellar and planetary formation.

The largest radio telescope presently in development is the Square Kilometer Array (SKA). When finished, this international facility will be fifty times more sensitive than any existing radio telescope. SKA will consist of thousands of radio dishes measuring 10 to 15 meters in diameter. Sites in South Africa and Australia are being considered. If built in South Africa, the Karoo Array Telescope (MeerKat) will play a central role, with other dishes spiraling outward across Africa.

Knowledge Gained

In the past, ground-based telescopes have helped scientists learn more about our solar system. Among the most important ways are the following: gathering of basic data on planetary celestial mechanics; discovery and orbit determination of comets and asteroids; mapping of galactic X-ray and infrared sources; discovery of quasars and radio galaxies; detection of cosmic background radiation; and the search for extrasolar planets.

In order to determine the continuously changing orbits of the planets well enough to make interplanetary spaceflight possible, it is important to map planetary positions very accurately. Laser ranging of the Moon, for example, allows astronomers with large ground-based telescopes to measure its distance to within a few centimeters. Radar-ranging measurements of the planets, especially Venus, Mercury, and Mars, have led to very precise determinations of their positions.

Spacecraft exploration of comets depends on ground-based telescopes, to discover comets in the first place and then to monitor them in their somewhat unpredictable paths near the Sun and the Earth. Exploration of asteroids by Spacecraft similarly depends on ground-based telescopes for tactical support.

Most of the cosmic objects found at X-ray and infrared wavelengths by orbiting detectors would be unexplained were it not possible to study them at other wavelengths from the ground. For example, two of the objects thought to be black holes, LMC X-1 and Cygnus X-1, would merely be mysterious, unidentified X-ray sources if it had not been possible, using large ground-based telescopes, to discover that each is a binary star, with a normal star in orbit around a dark, massive object.

The Keck telescopes in Hawaii have been used to study seasonal variations on Uranus. In 2007, astronomers were able to photograph the change of seasons on Uranus for the first time. Still in its early stages, this study found significant changes in some cloud features that had previously appeared to be unchanging. Scientists were also able to calculate wind speeds on the planet more extensively, up to 901 kilometers an hour. That year astronomers were also able to use the Keck telescopes to look at Uranus’s ring system edge-on. They found that Uranus’s dusty rings have changed since Voyager 2 visited the system in 1986. This ring crossing was also observed using the Very Large Telescope (VLT) in Chile, and the Palomar Observatory in Southern California. Such observations would not be possible, or economically feasible, without ground-based instruments.

As ground-based telescopes increase in size, they also increase in power. The newer generations of VLTs (very large telescopes) and ELTs (extremely large telescopes) allow scientists to study objects at greater distances and therefore look back in time, since their light takes years (light-years) to reach us.

Context

Astronomy seeks a better understanding of cosmology. Ground-based telescopes have provided the basic list of objects and phenomena (including quasars, radio galaxies, and cosmic background radiation) that allow astronomers to penetrate to the edge of the universe and the beginning of time. New generations of ground-based telescopes are designed to push farther into deep space and to determine the true story of how the universe came about. They study distant galaxies, stars, and planetary systems to learn about the formation and evolution of our own solar system.

Despite the advent of space-based observatories, ground-based observations will continue to play a major role in astronomy so long as the risks and costs associated with space-based telescopes continue to be high. Ground-based observations will therefore continue to provide data and images complementary to space-based observations, which together will aid astronomers in understanding the nature and evolution of the physical universe.

The 2030s are projected to begin the era of the "Extremely Large" ground-based telescopes. This will commence with the completion of the Giant Magellan Telescope, which as of the early 2020s was under construction at the Las Campanas Observatory in Chile. The centerpiece of this installation will be a mirror that is 25.4 meters (80 feet) in diameter. It is projected to have four times the resolving power of the James Webb Space Telescope. One of its primary missions will be the location of exoplanets, or planets out side of Earth's solar system.

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