The Great Telescope Race

Instead, they've made the world's biggest glass honeycomb, with its front curved to focus starlight. The empty honeycomb structure behind keeps the mirror rigid enough to stay in shape as the telescope tilts, without complicated support structures. At the same time, it is much lighter in weight than a solid mirror of the same proportions.

These honeycomb mirrors are made in a giant furnace in a basement in Tucson. Glass is melted in a mould that contains ceramic blocks where the holes are required: when the mirror has cooled, the ceramic will be washed out with a high-pressure hose to leave the empty spaces in the honeycomb. Before that, the mould is heated to melt the glass; the whole red-hot mass is then spun around hundreds of times a minute, so the top surface naturally forms into a shallow bowl shape - by a fortunate coincidence, just the right shape to focus starlight.

These 'ten-metre' class telescopes are giving the Hubble Space Telescope a good run for its money. Although Hubble has the sharpest views of the distant Universe, it is puny compared to these giants: its mirror is outranked by some 30 telescopes on the ground. Each of the huge Keck Telescopes in Hawaii, for example, collects so much light that it can "see" stars 10 times fainter than Hubble can perceive. Astronomers routinely search out strange distant galaxies with Hubble's sharp eye, then analyse their light with the vast collecting power of Keck.

The Next Generation Space Telescope will combine Hubble's sharp views with the power of today's large telescopes on Earth. And so, some astronomers are moving on to dream up even bigger instruments that could be built on terra firma. The most audacious plan of all the OverWhelmingly Large Telescope.

The OverWhelmingly Large Telescope - OWL for short - would have a mirror 100 metres across. That's as big as a radio telescope like Jodrell Bank or the Green Bank radio dish in West Virginia. In everyday terms, we're thinking not so much of a tennis court but of a football pitch.

The OWL concept has been hatched by astronomers from the European Southern Observatory, which has built the Very Large Telescope in Chile. Inspired by the segmented mirrors of Keck and the Hobby-Eberly telescope in Texas, they propose fitting together some 1600 separate hexagonal mirrors, to create a mirror surface 100 metres across. It will be housed in a vast metal framework that can swivel to point to any part of the sky.

If OWL is built, it will have the power to pick out individual stars in galaxies halfway across the Universe, and to see planets in orbit around other stars. It may take us on the final step to answering the ultimate questions about the beginning and end of the Universe, and the existence of life elsewhere.