First telescope with active optics.

First telescope with active optics 

The world's first telescope for general astronomical use with "active" computer-controlled optics is about to be installed at the European Southern Observatory (ESO) at Cerro La Silla, Chile. Unlike traditional telescope mirrors, the 3.58-meter mirror of this New Technology Telescope (NTT) does not depend on the thickness of glass to keep its shape. It is only 24 centimeters thick, and is thus quite flexible. Therefore, its shape will be maintained by active supports pressing against its back under the control of a computer. ESO astronomers hope to begin observations with the telescope by the end of this year.

A similar principle of active support is used on the 10-meter Keck telescope being built on Mauna Kea in Hawaii by the California Institute of Technology and the University of California, but this much larger project will take a few more years to complete. The NTT is a forerunner and testing ground for design ideas for ESO's Very Large Telescope, a planned array of four 8-meter mirrors that will be able to work separately or together.

The building for the NTT--featuring an unusual octagonal design that was determined by wind-tunnel tests at the Technical University of Aachen, West Germany-was constructed by a consortium of Italian companies (Mecnafer of the Mestre suburb of Venice, Zollet of Belluno and Ansaldo Componenti of Genoa). Shipped from Europe inknocked-down form, it arrived at Valparaiso, Chile, early in February, and was trucked to La Silla. ESO estimates it will take about six months to assemble it on the mountain.

Traditionally, an optical telescope sits in a round building with a rotating dome on top. In the dome is a slit, the cove of which is pulled back to expose the telescope to the sky. For the NTT, the whole building will revolve, and it is designed to expose the telescope to the external environment as much as possible but to protect it from strong winds and dust. The floor of the building will be actively cooled to keep the interior temperature the same as that of the exterior. The reason for all this exposure and cooling is to minimize air turbulence around the telescope. Temperature differences are a major cause of such turbulence. With minimal turbulence, the NTT's images should be significantly sharper than those of telescopes of comparable size.

A rotating 350-ton building requires some large bearings. One for this project, manufactured by the RKS company of France, is a roller bearing 7 meters in diameter. (The first telescope with a rotating building seems to have been the Multiple Mirror Telescope on Mt. Hopkins in Arizona, completed about a decade ago.)

The mechanical structure of the telescope itself, built by the INNSE company of Brescia, Italy, will be disassembled and shipped to Chile shortly. It, too, involves a number of innovations that exemplify recent trends in telescope design.

Usually optical telescopes have been hung in what is called an equatorial mounting, with one rotation axis parallel to the plane of the celestial equator and the other parallel to theline between the celestial north and south poles. When pointing was done mainly by eye and moving from target to target was often by hand, this arrangement made pointing and moving the telescope easier, but it complicated the stresses the structure, and particularly the mirror surface, had to bear.

The trend among modern telescopes, including the NTT, is to use an altazimuth mount, in which one rotation axis is horizontal, the other vertical. This simplifies the stresses but makes pointing and tracking very complicated. However, the mechanical structure of the NTT is also computer controlled, and the computer has no problem doing the necessary calculations quickly enough. ESO astronomers expect to be able to point the telescope with an accuracy of 1 second of arc, which they say is "a figure unsurpassed by any other existing telescope of this size." The computer system may even permit remote control of the telescope from ESO headquarters in Garching, West Germany.

The NTT project began in 1982 when Switzerland and Italy joined ESO. ESO decided to use the entrance fees from these two countries for the project -- which will cost an estimated 25 million German marks, or $15 million, and is intended to be the world's finest telescope of the 4-meter class at one-third the cost of a conventional project.