As astronomers mull over the merits of large versus smaller ground-based telescopes, another — equally contentious — debate is taking shape. Which will provide the best value for money: orbiting successors to the Hubble Space Telescope, or ground-based titans with mirrors of up to 100 metres in diameter? The answer may depend on whether adaptive optics, which eliminates the distortion caused by atmospheric turbulence (see main story), can be applied to the huge ground-based telescopes that are now on the drawing-board. “Adaptive optics is the single critical-path technology that has to be proven. And without high-quality wide-field adaptive optics, it is probable that, ultimately, there is no future for astronomy on the ground,” says Gerry Gilmore, an astronomer at the University of Cambridge.
Applying the technology to monsters such as the European Southern Observatory's proposed 100-metre OverWhelmingly Large (OWL) telescope poses a formidable challenge, not least because it will require supercomputers hundreds of times faster than current models. But if the technical obstacles can be overcome, OWL could provide images of individual stars within galaxies so far away that the light started on its journey to Earth when the Universe was a quarter of its current age. It could also observe planets orbiting around other stars in our Galaxy. Such performance would exceed by an order of magnitude that of the 8-metre Next Generation Space Telescope (NGST, right), which NASA, in collaboration with the European Space Agency and Canada, plans to launch around 2009, for about the same projected cost of US$1 billion.
Indeed, the huge costs of getting large mirrors into space — not to mention the technical difficulties — mean that space telescopes will be unable to compete at wavelengths that can be studied from the ground, if the issue of adaptive optics can be cracked. Space telescopes also have the disadvantage that their instruments cannot readily be updated. “You are always flying old instrumentation,” says Piero Benvenuti, who is coordinating European planning for the NGST.