Light and Magic
On a clear night--with this telescope--you can see forever.
- By Eric Adams
- Air & Space magazine, July 2000
(Page 3 of 4)
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.
Once light reflects off the actively controlled mirrors, it is further enhanced at an adaptive optics filter, which also employs a deformable mirror and works to counter errors, such as high-altitude thermal changes and atmospheric turbulence, that occur more rapidly than those the active optics can correct. These conditions are monitored by focusing on a single guide star; in response, the computer orders changes in the mirror's shape 100 times per second. Though several observatories around the world, most notably the Subaru Telescope atop Mauna Kea, Hawaii, are applying or developing active and adaptive optics, the VLT is by far its largest application, and the ESO was one of the first organizations to develop the technology. "When we were developing the active optics system in the 1980s, nobody was sure that this would work at this scale, and many people actually opposed the idea," Tarenghi recalls.
The other remarkable achievement taking shape on Paranal is the VLT interferometer. This setup, common in radio astronomy, capitalizes on the fact that if a telescope has a larger aperture, it can collect more of the light from objects in space. Interferometry enables the four telescopes to focus on the same object and gather its light as if the group were a single telescope as big as the combined distances between the individual telescopes-in the VLT's case, a distance of 426 feet.
Light beams collected by the four telescopes are deflected by mirrors into underground tunnels, where they are gathered at a single sensor. The sensor generates an image that is a cumulative product of the four beams. The trick is getting the light waves to meet at the sensor at the same time: As objects are tracked, the telescopes' relative positions change. Consequently, in the tunnel, the light is bounced off several retro-reflectors sitting on small, precisely positioned rail carts that move on 200-foot tracks to compensate for these changes. In addition, three small auxiliary telescopes, also moveable, on the surface will fill in the spaces between the four UTs to further punch up the resolution.
Final processing will prove that the VLT is far greater than the sum of its parts, with an angular resolution of 0.001 arcsecond. (The "celestial sphere" around Earth is 360 degrees; the full moon has an apparent size of 0.5 degree; a degree has 60 arcminutes; an arcminute has 60 arcseconds.) The Hubble, which sits above the atmosphere but has only a 2.4-meter mirror, can resolve to 0.1 arcsecond. The VLT's resolution is fine enough to capture detailed images of distant galaxies, clues about the chemical and biological composition of extrasolar planets-and snapshots of lunar rovers.