The board of the TMT Observatory Corp., based in Pasadena, Calif. (U.S.A.), announced today that it chose Mauna Kea to build the 30-m-wide segmented-mirror telescope over Cerro Armazones in Chile’s Atacama Desert. The TMT team had narrowed its list of potential sites from five to those two in May 2008.

With a summit of 4,205 m above sea level, Mauna Kea already hosts some of the world’s finest observatories, including the twin 10-m Keck telescopes, which are connected with a fiber link for interferometric observations; the Japanese Subaru telescope; and one of the twin multinational Gemini telescopes (the other is in Chile).

TMT board chair Henry Yang, chancellor of the University of California at Santa Barbara (U.S.A.), said both site finalists could provide high-quality astronomical observations. However, Mauna Kea edged out the Chilean plateau because of its higher altitude, lower humidity, smaller temperature variation and greater atmospheric steadiness.

The Keck telescopes, with their primary mirrors composed of 36 hexagonal segments in computer-controlled alignment, were considered revolutionary when they opened for business in the mid-1990s. TMT is the same concept on steroids: its 492 1.66-m hexagonal segments will stretch a total of 30 m across. Astrophysicist Jerry Nelson, who spearheaded the Keck design, serves as TMT’s project scientist.

Thus, TMT will look a little different from the subject of OPN’s July/August cover story, the Giant Magellan Telescope (GMT), which will feature seven of the largest mirror segments that can be made. Both, however, are part of the next generation of 25- to 40-m telescopes that likely will make the big astronomical discoveries of the third decade of the 21^st century. The European Space Organization is planning a highly segmented Keck-style 40-m instrument called the European Extremely Large Telescope (E-ELT).

When completed in 2018, the TMT will sit on a plateau a bit below the main Mauna Kea summit area on the western side of the mountain. Its fast f/1.0 primary will allow its enclosure to be as compact as possible.

Assuming the Hawaiian site receives its final land-use permits, site construction could begin in 2011, according to Yang.

TMT is mostly a North American project; its partners are Caltech, the University of California and the Association of Canadian Universities for Research in Astronomy. The National Astronomical Observatory of Japan is listed as a collaborating institution. Several public and private organizations in the United States and Canada have provided funding for the design stage.

I’ve actually visited the summit of Mauna Kea in preparation for my September 2007 OPN feature on ground-based telescopes. It is an awesome place, literally above most of the clouds, with a sky deeper blue than at sea level. I look forward to many exciting astronomical discoveries in the future.

Today is the 40^th anniversary of man’s first walk on the Moon. Thus, it’s also the 40^th birthday of an experiment that keeps on providing crucial data for tests of fundamental physical theories.

During their excursion onto the lunar surface, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin deployed the Laser Ranging Retroreflector (LRRR), an array of corner-cube reflectors inside a metal box angled to face toward Earth. Two weeks after the moonwalk, scientists started firing laser pulses at the reflector box to get a more precise measure of the distance between the two celestial bodies.

As Science reported in January 1970, some six months after the historic space mission, the LRRR project arose from discussions among Robert H. Dicke and other members of the experimental gravitational research group at Princeton University (U.S.A.). Not only did these scientists want to study the basic physics of the Earth-Moon system, but they also wanted to test Einstein’s theory of general relativity.

Three months after Apollo 11, using an electronic receiver with an accuracy of a few nanoseconds, the LRRR team measured the Earth-Moon distance to ± 30 cm from the McDonald Observatory in Texas (U.S.A.). Three years later – after two other Apollo crews had deployed retroreflectors at their lunar landing sites -- scientists had improved the uncertainty to 15 cm and gained new insights into the complexity of the Moon’s mass distribution and rotation.

By 1994, a quarter-century after Apollo 11, and with advances in both laser and detector technology, researchers had improved the Earth-Moon distance accuracy to 2 or 3 cm and had also used lunar laser ranging to measure the Earth’s precession and the Moon’s tidal dissipation. Team members also reported “unprecedented accuracy” in verifying the equivalence principle for massive bodies, one of the tests of relativity. Not bad, considering that the reflected signal received back on Earth represented only 10^–21 of the outgoing beam.

New Scientist recently reviewed the LRRR’s accomplishments over the past four decades. The equivalence principle has held up to one part in 10¹³ and the movement of the Moon can be tracked to within a few millimeters. Thanks to 40 years’ worth of data, researchers have shown that Newton’s gravitational constant changes less than one part in 10¹² per year.

The laser ranging using the three Apollo-era LRRR users continues on clear nights at Apache Point Observatory in New Mexico (U.S.A.). Who knows how much more precision scientists will gain by the year 2019 – or even 2049?

(Incidentally, in honor of the Apollo 11 anniversary, Science opened up the online version of its January 30, 1970, themed issue on lunar science to all with free registration. If you hear someone complaining that no science came out of the first Moon landing, point him or her to this issue.)

The Gran Telescopio Canarias (GTC) – for now, the largest single optical telescope on Earth – will host its own inaugural party next week.

The primary mirror of the GTC – named for its location in the Canary Islands, the Spanish archipelago off the coast of Morocco – consists of 36 hexagonal segments, just like the twin Keck telescopes in Hawaii. However, the GTC’s primary measures 10.4 m across, 40 cm bigger than the 10-m Kecks, and it has 75.7 square meters of light-gathering area.

Presumably, the GTC also takes advantage of the latest advances in active optics and adaptive optics since the Keck telescopes saw first light in the mid-1990s. “Active optics” is the system that keeps the primary mirror segments and the secondary mirror aligned despite changes in air temperature, the pull of gravity due to the telescope’s orientation and so forth. “Adaptive optics” lessens the optical aberrations caused by the atmosphere above the telescopes.

Next year the CanariCam, featuring innovative mid-infrared optics (wavelengths of 7.8 to 24.5 µm), will go into operation on the GTC.

The GTC won’t be the world’s largest single telescope for very long. As recounted in OPN’s July/August 2009 cover story, the Giant Magellan Telescope, now under construction, will feature seven primary mirror segments for a total aperture 25 m across. And even that instrument may be eclipsed by other huge telescopes that are now in the planning stages, such as the Thirty Meter Telescope.