by Nigel Henbest

High above the Earth, a gargantuan telescope peers into space. It dwarfs the Hubble Telescope, the twentieth-century's greatest scientific instrument. With a mirror half the size of a tennis court, the Next Generation Space Telescope will reveal the edges of the observable Universe.

The Next Generation Space Telescope is already on the drawing board. Within 10 years, this ultimate telescope will be flying in space. Instead of orbiting the Earth, the Next Generation Space Telescope will follow its own orbit around the Sun. But its designers will place it at a special balance point, outside Earth's own orbit. At this location, the so-called Lagrangian-2 point, our planet's gravity still ensnares the telescope, so it travels round the Sun always a million miles from the Earth.

Huge solar panels will provide the telescope with power, and - just as important - shield it from the Sun's heat. Cooled down to the temperature of deep space, the Next Generation Space Telescope can observe faint heat signals from the farthest reaches of the Universe. It will peer beyond where even Hubble can see, to the very edges of the observable Universe.

Because light takes billions of years to travel this far, the Next Generation Space Telescope will be seeing these distant regions as they were billions of years ago - just after the Big Bang in which everything began.

This ultimate telescope combines the clear views that Hubble enjoys with the mammoth size of the biggest telescopes currently being built on the ground.

In the dry deserts of Chile, astronomers are this year completing the most powerful telescope ever constructed on Earth. The Very Large Telescope in fact comprises four telescopes, on the peak of a mountain that has been blasted flat to form a platform for the cosmic quartet. Each of the telescopes has a mirror 8 metres (26 feet) across. Linked together, the four instruments will collect as much light as a single telescope over 50 feet in size.

NASA The Next Generation Space Telescope will travel round the sun 1 million miles from the Earth

Three of the four telescopes are already working, and astronomers have been astounded by the quality of their views of the cosmos - of glowing nebulae, dying stars and distant galaxies.

To construct such a vast telescope, its builders have pushed engineering to the limit. In particular, the telescope must tilt to turn to different galaxies, giving gravity a purchase on its huge - but thin - mirror. If gravity bends the mirror by an amount smaller than the width of a human hair, the telescope's view of giant galaxies is blurred to uselessness. So the designers have attached the back of the mirror to hundreds of moving supports, computer-controlled to push the mirror back into shape - a millionth of an inch at a time - as the telescope tilts.

While telescopes in Chile are placed to view the southern sky, the best place on Earth for observing the northern heavens is the 14,000-foot high peak of Mauna Kea, Hawaii. Here, astronomers have constructed the world's biggest astronomical observatory. Put together, the total area of the mirrors in these telescopes would outstrip even the Very Large Telescope. But these instruments are competitors, each seeking to be the most powerful telescope observing the northern part of the sky.

Gemini North is the latest, starting work this year. The surface of its mirror is so smooth that if you enlarged it to the size of the Earth, the largest bump would be only a foot high. Next door is the Japanese telescope Subaru, with the biggest single mirror in the world - almost 8.3 metres (27 feet) across. Like the Very Large Telescope, it has hundreds of supports, actively pushing the mirror into the precisely correct shape.

But the jewels in the crown of Mauna Kea are the pair of Keck Telescopes. Each has a mirror 10 metres (33 feet) across. No-one could manufacture a single piece of glass that size, so the Keck mirror is made of 36 pieces, each just six feet across, fitted together like bathroom tiles. A perfect fit between the tiles is ensured by a computer-activated system, pushing on the back of each segment to ensure the tiled mirror forms a single perfectly smooth surface. To spread the forces evenly, the support system is based on a design used to even out the force of horses pulling a Wild West covered wagon!

The first Keck telescope has been observing for eight years, and it has revolutionised astronomy, from discovering tiny ice planets beyond Pluto, to tracking planets of other stars - and pinning down the distances to the farthest known galaxies, over 10 billion light years away.

Texas has an even bigger panelled telescope, the Hobby-Eberly Telescope, some 12 metres (almost 40 feet) in diameter. Its design has brought it in at only one-tenth the cost of Keck, with the payoff that it cannot focus quite as accurately. Its strength, instead, will come from splitting up the light from distant objects in more detail than any other telescope.

On Mount Graham, in Arizona, the world's 'biggest binoculars' are about to peer at the sky. The Large Binocular Telescope has two giant mirrors - each 8.4 metres (27 feet) across - mounted on the same frame. But astronomers here have decided against the complication of using hundreds of computer-controlled supports to keep the mirror in shape.

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.

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