Space Mountain

High in the Chilean Andes, the southern hemisphere's biggest radio observatory sets up shop

Sitting on a high, arid plateau in the Chilean Andes, a new telescope known as the Atacama Pathfinder Experiment (APEX)--the largest submillimeter radio telescope now operating in the southern hemisphere--officially opened for business in late September [see photo, " "]. IEEE Spectrum was in the neighborhood and dropped in for the inaugural ceremony.

The Chajnantor plain, located at a height of 5100 meters in Chile's Atacama Desert, said to be the driest place on Earth, is about as high and dry as you can get. The 14-kilometer dirt road leading up to the site demands skillful four-wheel maneuvering--in rain or snow or at night, it's downright treacherous. The oxygen-poor air at the summit leaves many a visitor dizzy and disoriented, including this reporter. "Some people have no problem, some people have to use oxygen, and a few get sick and have to be taken down to lower altitudes quickly," notes Lars-Ake Nyman, APEX's station manager. The staff keeps a supply of portable oxygen tanks on hand at all times for the faint of heart.

The telescope is designed to peer through interstellar clouds. These clouds of cosmic dust and gas are known to be the birthplace of stars, but at optical wavelengths and even at infrared wavelengths, these regions are totally opaque, notes Karl Menten, director of the group for millimeter and submillimeter astronomy at the Max Planck Institute for Radioastronomy, in Bonn, Germany, and the principal investigator for APEX. "The millimeter and submillimeter ranges are the only ranges where you can study the initial conditions of star formation," he says.

A joint project of the Max Planck Institute, the European Southern Observatory, in Garching, Germany, and Sweden's Onsala Space Observatory, APEX took about four years to complete and cost ¤11 million (about US $13 million, including infrastructure). Choosing the proper site was critical, Menten says. "At submillimeter wavelengths, your biggest enemy is Earth's atmosphere. Even at dry sites, it has a lot of water in it, and water absorbs the radiation from astronomical sources. So the higher you go, the less water content in the atmosphere, the drier it is."

It's hard to imagine much of anything being constructed on this rocky plain, let alone a huge, exquisitely sensitive radio antenna. APEX's 12-meter-diameter antenna consists of 264 polished aluminum panels, forming a near-perfect parabolic dish. Getting those panels to line up just right was a feat in itself, says Nyman. Each panel is adjusted by five screws--one in each corner and one in the center, he explains. "We transmitted a reference signal from the summit of Cerra Chajnantor to the dish, and from that, we created a holographic map of the dish. That told us how much to adjust each screw, by how many degrees." Over the course of two days, in freezing temperatures and high winds, APEX engineers painstakingly adjusted each screw with a screwdriver. All of the instrumentation must be cooled to less than 4 degrees above absolute zero. Otherwise, the heat from the instruments hinders their performance or prevents them from operating altogether.

"At submillimeter wavelengths, your biggest enemy is Earth's atmosphere"
Eventually, the telescope will be run remotely from the APEX base camp in San Pedro de Atacama, 50 km downhill from the observatory. A microwave link allows downloading of data and uploading of instructions for the telescope. At the moment, though, some of the instruments aren't equipped for remote operation, so a small crew of astronomers and engineers must be on site during observations.

Submillimeter observations are a relatively new domain for astronomers; the first submillimeter observatories were built in the 1990s. But APEX is paving the way for an even more ambitious undertaking on Chajnantor: the Atacama Large Millimeter Array, known as ALMA, which will consist of 50 millimeter-band antennas. Jointly funded by the United States, the European Union, and Japan, ALMA will use 12-meter antennas very similar in design to APEX's. ALMA's antennas will be mobile, so that the whole array can be reconfigured, and when completed, it will be the largest and most sensitive millimeter instrument in the world.

The first of its antennas will arrive in Chile next year, and operations are slated to begin in 2007, Menten says.

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