Light and Magic

On a clear night—with this telescope—you can see forever.

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The base camp below is a clean, orderly village that is mostly made up of bright white ship cargo containers that have been converted into surprisingly nice offices and dorm rooms. On one side of the camp sit a helicopter pad and a soccer field; on the other, a parking lot filled with white four-wheel-drive trucks bearing ESO logos on the doors. Scattered throughout are a two-story telescope service building, a platform with eight 20-foot-tall water tanks that get replenished twice daily by trucks from Antofagasta, a power station, and a dormitory being built for staff and visitors.

Beyond this, there is nothing. As workplaces go, Paranal has little appeal. Though serene and beautiful, it is also hot and dry, and far from any diversions. "Personally, I consider Paranal to be one of the better places on the Earth to read books," says VLT staff astronomer Gianni Marconi, a friendly 39-year-old Italian who spends his nights on the mountaintop operating the telescopes for visiting astronomers. "I'm used to walking far from the base camp to where human-produced noise disappears and I am disturbed only by the wind." Visitors aren't encouraged to take such walks, though: Two who wandered off last year quickly became disoriented in the featureless hills and ended up lost for two days.

But for astronomers, the lonely desert site has several advantages: the dry air, which makes for clearer skies and a low risk of condensation collecting on telescope mirrors; the distance from any sources of the urban light pollution that plagues much of the world; and the roughly equatorial positioning, which gives it access to objects in both northern and southern skies.

When the sun goes down and the night sky emerges, any doubts about why someone would travel so far for this are squelched. Scrolling up from the east, the Milky Way shines steadily against the deep-black sky, with dozens of fuzzy nebulas and star clusters visible to the naked eye. Two galaxies, consuming startlingly large swaths of sky, hover in the south: The Large Magellanic Cloud spans the width of about 14 full moons; the Small Magellanic Cloud, six. On this mountain, you feel as if you are staring out into space, rather than merely up at the stars.

On the telescope platform at dusk, the four 100-foot-square silver enclosures, each containing a five-story telescope, await instructions to open their doors and commence their night's work, either observation or, for the unfinished scopes, calibration and testing. All are cast in a shimmery reddish yellow from the sunset. Massimo Tarenghi, the VLT's project manager, stands amid a somewhat treacherous network of half-finished concrete channels and open pits that will soon contain the interferometry hardware. He wonders if the project will end up spoiling his colleagues: "Will we ever be able to work on a telescope that isn't at least as big as this?" he asks.

Though it isn't quite dark yet, the doors to Unit Telescope 1 slowly crank open and the dome rotates around to face south. This process helps ensure that the temperature of the air inside of the dome equals that of the air outside; inequalities would produce turbulence, which would distort the observed images.

Tarenghi notes that every major component of the telescope had a team of engineers dedicated to it. One group designed the 8.2-meter primary mirrors. Another focused the active optics system, which compensates for changes in the thermal conditions around the telescope and for tilt-induced weight changes that would alter the mirror's performance. Still another team mastered the $20 million secondary mirror system, which collects light focused by the larger primary mirror and directs it into the telescope's instruments. This system consists of a five-foot-long cylinder with a beryllium mirror on the bottom, and within it hundreds of pounds of electronics that constantly adjust the mirror's angle-another part of the telescope's active optics system. The secondary mirror hovers 40 feet above the primary and resembles a satellite in structure.

In each of the telescopes (UT1 through UT4), all of this hardware rests on a two-axis alt-azimuth mount that itself floats in a narrow track on a layer of oil only 0.05 millimeter thick. This makes for exceedingly smooth action-the whole 470-ton telescope can be moved with only a nudge of a hand. Of course, the motion of the telescopes is actually controlled by computers, from initial positioning to tracking of celestial objects during prolonged imaging and study. In the control building, a separate cluster of computers operates each telescope and provides continuously updated visibility conditions and tracking information. A fifth cluster of computers will call the shots when the interferometric mode becomes functional.

Though there is much for the VLT's engineers to brag about, several technological triumphs stand out. The first are the mirrors and their control mechanisms. The 8.2-meter Zerodur glass disks are among the largest single-piece telescope mirrors in the world. Manufactured in Europe, they were brought separately to South America on ships and then sent on a painstaking journey from Antofagasta to Paranal. Preceded by several grading trucks that smoothed out the dirt road, the trucks bearing the fragile mirrors drove a steady 3 mph. The trip took three days.

The 540-square-foot mirrors were aluminized in the ESO's mirror maintenance facility at Paranal, then inserted into basket-like cells and driven the final two miles up to the enclosures. These cells contain the 150 active optics actuators, small hydraulic pistons that flex the seven-inch-thick mirror in sub-millimeter increments.

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