Cerro Paranal. In the north of Chile, on a plateau of stones, thefourmetal cylinders that form theVery Large Telescope (VLT)-theworld’s most advanced optical observatory-, stand guard 2,635 meters above the Pacific Ocean. It is difficult to conceive of a more moisture-free landscape. Protected within eight-story structures, the telescopes command respect.
Each has a one-piece head mirror of8.2 meters in diameterthat collects the weak light coming from the most distant objects. The 18-centimeter-thick mirrors are very flexible and use a technology calledactive opticsto adapt its curvature to compensate for the effect of gravity. In addition, its adaptive optics correct for atmospheric turbulence and changes in temperature. Each mirror is mounted on a 430-ton steel frame that can be moved with one finger. His sensitivity is so exquisite that the light from a cigarette affects his measurements and the headlight of a car damages his delicate pupil.
Three of the four telescopes –Antu, Kueyen y Melipal, which mean the Sun, the Moon and the Southern Cross in the Mapuche language – they have gone through the same process: installation of the structure, placement of the primary and secondary mirrors, instrument calibration and, finally, the intense moment of the first light. During the night of my visit, in March 2000, it was the turn of Yepún (Venus).
I remember that in the control room the tension was increasing. The murmurs mixed with the typing of the computers. The screens revealed color charts and coordinate lists that began the mechanical giant’s ballet. Above, the building that houses Yepún spun on its base and faced the Pacific with a metallic screech. Seconds later, the telescope mount tilted. When the main mirror was perpendicular to the floor, the ceiling opened and let in a breeze accompanied by the red light from the setting. In the living room, a finger pressed a key that opened the upper windows and the telescope frame tilted to look up at the sky.
Night fell on Paranal. The first stars appeared and minutes later, the milky spot of theMilky Way.The specialists were crowding around the computers. The primary mirror was filled with photons that produced a distorted light on the monitor and, little by little, the stars emerged like sugar on black velvet. Yepún’s infrared instruments penetrated a nearby cloud of stellar gas and focused on a yellow image that filled the screen. Diffuse at first, it was gathering resolution until it became a clear luminous focus. That light had departed from its star about 8,000 years ago, before the formation of the Earth. Yepún had just opened his eye to the past. A chorus of cheers and applause erupted in the control room: the technological marvel was working!
The VLT is much more than four independent eyes. Thisdesigned to work as a single telescope, uniting the light collected by the four individuals with aresolving power equivalent to that of a 200 meter mirror. The secret weapon that must unite the signals of Antu, Kueyen, Melipal and Yepún is called an interferometer and is hidden in a labyrinth of tunnels under the esplanade of the telescopes. Inside that gallery there are rails on which 17 mirrors come and go in silence. The complication is that light from celestial objects reaches each telescope with a slight time difference, but the signals from all four must arrive simultaneously at the focal point; otherwise, the image is blurred. Several mirrors are needed to bounce the light from one side to the other and delay its arrival until all the beams are synchronized.
In addition, the tunnels must be perfectly flat. The level of the water that runs through nearby channels is a reference to draw the lines of the rails and correct the very small deviation caused by the curvature of the Earth. Even subtle air turbulence inside tunnels is capable of affecting readings. So far, theinterferómetro it is capable of combining the light from three of its four telescopes.
The three united eyes of Paranal have contributed to the discovery ofnew planetary systemsvery different from the solar system, with giant planets rotating in very close orbits. Other studies show how the disks that make up planets around young stars interact with the particle wind. The detail in the images is extraordinary. Among other objects, individual telescopes have captured theblack holethat hides in the center of the Milky Way and thebrown dwarfyounger within a star system. That image had a resolution of 0.18 arc seconds. One arc second is equal to the diameter of a coin seen from a distance of two kilometers.
The VLT is a truetime Machinewhatseek the primordial light of the first moments of the universe. One more soldier in that army of mega-scopes, whose ambition grows with the thirst for knowledge of its astronomers, eternally hungry for light.
Angela Posada-Swafford
