Extremely large telescope

The European Southern Observatory’s governing council announced on 11 December that ESO would go forward with a final-design study for the European Extremely Large Telescope. With a primary-mirror diameter of 42 m, the EELT would have about 20 times the light-gathering capacity of today’s largest optical–infrared telescopes. The ESO council’s decision follows a yearlong conceptual-design effort.

The council’s approval of a €57 million ($74 million) budget for the final-design study makes it possible for construction to begin in about three years, assuming that the 12-nation ESO accepts the final design. The Czech Republic became ESO’s 12th full member in January. The telescope’s site should be chosen by 2008.

The EELT could be completed by 2017 at an estimated cost of €800 million. “Such an extraordinarily ambitious instrument requires a complete rethinking of how we make telescopes,” says Catherine Cesarsky, ESO’s director general. The primary mirror is envisaged as a mosaic of 906 hexagonal segments. Adaptive optics to compensate for atmospheric turbulence will be incorporated into the EELT’s design from the start. The telescope’s 4-m tertiary mirror will relay light to the adaptive-optics system’s two 2.5-m mirrors, one of which will be backed by thousands of actuators that can distort its shape at kilohertz rates in response to turbulence.

The resulting image sharpness and sensitivity, at optical and infrared wavelengths, is expected to “revolutionize ground-based astronomy at all distance scales,” says Cesarsky. Nearby, for example, it will permit detailed studies of planets orbiting stars in our vicinity, and at cosmological distances it should let astronomers analyze the spectra of the very first generation of stars.