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Giant eyes of the future : If you think today's monster telescopes are big . . . look what's coming over the horizon.

Since the 40-inch objective of the Yerkes refractor and the 200-inch mirror of the Palomar reflector have apparently reached the practical construction limits for telescopes of their respective types, it is extremely doubtful if a greater light-gathering eye of either kind will ever again be built.

-- A. Frederick Collins The Greatest Eye in the World, 1942

THAT GLIB ASSESSMENT remained the accepted wisdom about big telescopes for more than a generation after the Hale 200-inch (5.1-meter) first opened its eye on the universe in 1948. It wasn't just that the 200-inch stretched the engineering of its day to the limit. Astronomers declared that the Earth's unsteady atmosphere would negate the value of any larger aperture or any better optics. A touch of excuse-making for the 200-inch's somewhat mediocre optical quality (not a secret, but not much talked about in public) probably helped such pronouncements to become entrenched. After the Soviet 6-meter (236-inch) reflector opened in 1975, its low productivity and reports of serious problems (including a main mirror that had to be remade) only reinforced the idea that ground-based telescopes had come up against their fundamental limits. Such thinking led to the start of the Hubble Space Telescope project in the late 1960s and early 1970s.

What a difference new technology makes. The biggest ground-based optical telescope on the drawing boards today will have a 100-meter aperture. That's 4,000 inches, the length of a football field. Its designers think it could work just fine.

So what's in store for ground-based optical telescopes in the next couple of decades? More than you may have imagined.

The Near Term

First of all, we'll see new 10-meter-class telescopes matching or slightly surpassing the twin 9.8-m Keck reflectors now on Mauna Kea, Hawaii. At the Roque de los Muchachos Observatory on La Palma in the Canary Islands, the Spanish Instituto de Astrofisica de Canarias (IAC) is building the 10.4-m Gran Telescopio Canarias (GTC), affectionately known as Grantecan. When it achieves first light in late 2002, Grantecan will be by far the largest optical telescope on European territory.

Like all new great telescopes, Grantecan will be equipped with an adaptive-optics system that can reduce or eliminate the Earth's atmospheric blurring under certain circumstances. It will come with a panoply of optical and infrared cameras and spectroscopes. Its hexagonal mirror segments, produced by the French company REOSC, are expected to have significantly higher optical quality than the Keck hexagons. The total project cost is estimated at only $75 million.

The Large Binocular Telescope (LBT) will consist of twin 8.4-m telescopes on Arizona's Mount Graham. They will ride on a single mounting and be linked by interferometry to yield the resolution of a 23-m telescope in one direction. As the sky turns during the night this direction will rotate, allowing an image to be built up that is supersharp in two dimensions. The $84 million project, a collaboration between Italian, U.S., and German institutions, is under construction and should be completed in 2004.

Another large telescope based on an innovative design is the Southern African Large Telescope (SALT), to be built at the South African Astronomical Observatory over the next five years (see the June issue, page 20). SALT will be the Southern Hemisphere's clone of the Hobby-Eberly Telescope at McDonald Observatory in Texas; its segmented mirror, 11 m wide, will have a clear aperture of 9.1 m. Like Hobby-Eberly, SALT will be a specialized instrument designed only for spectroscopy. Its construction is expected to cost a mere $20 million. Poland is a major partner in the program; overseas contributions have also been received from Rutgers University in New Jersey and the University of Gottingen in Germany, and other institutions are interested.

Survey Scopes

Some other important new telescopes will have smaller diameters but much larger fields of view. A 10-m telescope is great when you want to study a particular galaxy in detail, but what you gain in sensitivity you lose in wide-field discovery capability. SALT, for instance, will have a maximum field of just 4 arcminutes, while many instruments on large telescopes have even narrower vision. To scan for new objects such as edge-of-the-universe galaxies and quasars, brown dwarfs, faint supernovae, Kuiper Belt objects, and gravitational microlenses - and to bring the power of massive statistics to bear on questions of all kinds - large sky surveys are indispensable.

The current big optical and infrared surveys - the Deep Near-Infrared Survey of the southern sky (DENIS), the 2-Micron All-Sky Survey (2MASS), and the Sloan Digital Sky Survey (SDSS) - use telescopes with apertures of 1.0, 1.3, and 2.5 m, respectively. They will be superseded by the British Visible and Infrared Survey Telescope for Astronomy (VISTA), due to achieve first light in 2004 at Cerro Paranal in northern Chile. VISTA will have a 4-m primary mirror and a field of view 1.7[degrees] wide, unprecedented for such a large scope, taking in nine times the apparent surface area of the full Moon at a shot. An initiative of 18 British universities, VISTA will cost around $40 million. Construction at Paranal, already home to the European Very Large Telescope, begins in 2002.

A survey telescope dedicated to spectroscopy will be the Large Sky Area Multi-Object Spectroscopic Telescope (LAMOST), an unusual Chinese telescope that will be built at the Xinglong Station of the Beijing Astronomical Observatory in northern China close to the Great Wall. The $30 million LAMOST will have a stationary 4-m primary mirror and a field of view 5[degrees] wide. A fully steerable mirror will direct light from the sky to the fixed primary. When completed in 2004, LAMOST will be the most productive spectral survey tool ever: it will be able to take spectra of up to 4,000 objects at once, using fiber optics, robotic fiber positioners, and 20 separate spectrographs.

More impressive still are plans for the Large-aperture Synoptic Survey Telescope (LSST), which would have a 6.5-m primary and a very wide field a few degrees across. The LSST is one of the major new ground-based telescopes recommended in May by the U.S. Decadal Review of Astronomy committee - a group of distinguished astronomers that is assembled every 10 years to recommend the best U.S. projects and investments for the next decade. Co-chair Christopher McKee (University of California at Berkeley) says that the LSST would focus on variable phenomena such as faint supernovae and asteroids. "It would scan the sky once a week down to the 24th magnitude," he says. "The telescope is expected to discover 90 percent of all near-Earth asteroids larger than 300 meters, as well as many thousands of Kuiper Belt objects."

The LSST is a slightly trimmed-down version of the Dark Matter Telescope (DMT) designed by Roger Angel of the University of Arizona. With an 8.4-m primary mirror and a 3[degrees] field of view, the DMT would be about 1,000 times as effective in surveying the sky as the venerable 1.2-meter (48-inch) Schmidt camera on Palomar Mountain. Its potential for studying the distribution of dark matter in the universe (by investigating the distortions of distant galaxy fields through weak gravitational lensing) gave the Dark Matter Telescope its name.

According to McKee, the smaller LSST would also be capable of mapping weak gravitational lensing, albeit at a lower sensitivity. If the recommendations of the Decadal Review committee are followed (and they usually are), McKee says the U.S. government would spend $170 million on the LSST, including instrumentation, grants, and five years of operation.

The Supergiants

The future instruments discussed so far are comparable in size to today's largest telescopes and those under construction. What about the next big leap in mirror size? How soon will the Keck telescopes be dwarfed by a 30- or 50-m behemoth?

Probably fairly soon. At a recent technology conference in Munich, Jerry E. Nelson and Terry Mast of the University of California's Lick Observatory described their plans for a 30-m California Extremely Large Telescope (CELT). At the time of writing the necessary $400 million was not yet secured, though rumors circulate that a major California chip manufacturer is interested in becoming a sponsor.

CELT could be described as a mega-Keck, in several ways. Its paraboloidal primary would consist of hundreds of computer-controlled hexagonal segments; it would be situated on Mauna Kea (if space allows); and it would belong to the University of California and Caltech, the latter of which has a world's-largest-telescope tradition dating back to the Hale 5.1-m.

On the other hand, a 30-meter Giant Segmented Mirror Telescope (GSMT) could become a U.S. national facility, according to the Decadal Review report. If half the construction cost came from corporate donors, the other half, as well as data-collection instruments and operations, could be paid for by the National Science Foundation, says McKee.

Nelson, who designed the Kecks, regards a 30-meter-class telescope as a prototype of still larger telescopes for the distant future. "I don't know where the limit is," he said at a workshop in Sweden last year.

Right now it looks as if adaptive optics may be the limiting factor. It would be useless to build a 30-m telescope without adaptive optics - the naysayers of the 1940s did have a point - but analyzing such huge wavefronts and then altering the positions and shapes of hundreds of corrector elements every hundredth of a second or less is beyond today's computing power. It's no coincidence that a chip maker is said to be involved in the funding talks.

Despite the technological challenges, it appears that a 30- to 50-meter-class telescope "will go along no matter what happens," says Torben Andersen of Lund Observatory in Sweden. Andersen heads a team that has been studying a Swedish Extremely Large Telescope (XLT) since 1991. This would be a fully steerable 50-m telescope, comparable in design to CELT, but with a paraboloidal primary consisting of almost 600 segments. According to Andersen, XLT would cost some $750 million.

Similar concepts are being studied by the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona. NOAO's Maximum Aperture Telescope (MAXAT) would be a 30- to 50-m instrument, but there's no final design or formal proposal yet. One option would be to build a fast (f/1) segmented aspheric primary with a very small (2-m) secondary mirror, following a design by James Oschmann, the project manager of the Gemini Project. According to Gemini director Matt Mountain, a 50-m telescope built along these lines would cost a staggering $1 billion.

Another option for MAXAT is a scaled-up version of the Hobby-Eberly Telescope. This Extremely Large Telescope (ELT) would have a 30-m spherical primary consisting of 127 identical segments and would cost a "mere" $250 million. Says Frank Bash of McDonald Observatory, "The ELT design is, by far, the most affordable design for a 30-m telescope. It is clear, even at this stage, that there are no fundamental technical problems in the design, and the ELT's cost is known to a better level of accuracy than other competing designs." It wouldn't be fully steerable, it could see only 70 percent of the sky above the horizon, and it could track objects for only an hour or so, but Bash says these disadvantages are outweighed by the simple design.

NOAO hasn't decided on a location for MAXAT but is considering the mountains surrounding the 5,000-m (16,400-foot) high Chajnantor plain in the Chilean Andes, where American and European radio astronomers are planning the Atacama Large Millimeter Array (ALMA; see page 18).

Possible partners could include institutes in the U.S., Lund Observatory, and institutes in Spain and France. "Right now everybody is competing with each other," says Andersen, "but funding agencies will probably request international cooperation." The competition may end up being between the American Giant Segmented Mirror Telescope and the rest of the world.

Finally, technicians at the European Southern Observatory (ESO) are seriously considering a gargantuan 100-m telescope, dubbed the Overwhelmingly Large Telescope (OWL). According to ESO's Philippe Dierickx, OWL is in the conceptual design phase. "Although there is still quite a lot of work ahead to come to a 'finalized' design, I'd say that we feel we have already solved all major technical issues, apart from adaptive optics," he says. OWL project manager Roberto Gilmozzi believes that this supertelescope can be built within a budget of $1 billion, less than the 2.4-m Hubble cost more than a decade ago.

OWL's spherical primary mirror, as currently conceived, will consist of 2,000 mass-produced segments. Spherical aberration will be removed by a complicated, five-mirror corrector unit that by itself is larger than a Keck telescope. The structure will tower more than 100 m high and stand in the open air; the giant mirror will be protected by thermally controlled covers when not in use.

Dierickx thinks OWL could start science observations as soon as enough mirror segments are in place. "First light could occur within eight or nine years after project funding," he says. Completion of the entire aperture would take approximately 15 to 17 years.

Whether ESO will be able to fund OWL is unclear at the moment. ESO's Very Large Telescope is still under construction, and its next giant project, the ALMA array in Chile, will get higher priority. Nevertheless Dierickx is confident. "I guess the community would agree that ESO - together with the European industry - is far ahead of the pack," he says. The leading OWL design "includes all essential functions of the telescope, which its closest competitor does not."

Others, however, think OWL is too big and complex. In the past, the "world's largest telescope" has doubled in size about every 35 years. Jumping from 10 to 100 meters in one leap might be too much. Even former ESO director-general Lo Woltjer said at a recent symposium in Leiden, the Netherlands, "It's not extremely obvious that building a 100-meter telescope is the way to go."

One thing's for sure, though. In November 2009, when we're celebrating the 400th anniversary of Galileo's first telescopic drawings of the Moon, today's giant Keck, Magellan, Gemini, Subaru, and VLT telescopes will look puny compared to a new generation of giant eyes under construction. Ground-based astronomy has a bright future.

Govert Schilling, a Sky & Telescope contributing editor, reports on astronomical research worldwide from the Netherlands.

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Author:Schilling, Govert
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2000
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