Britain's part in funding the huge telescopes capable of viewing the surfaces of stars boosts our scientific prestige, argues John Meaburn

The Hubble Space Telescope is arguably the most ambitious astronomical project ever conceived. Yet the launch of the precious telescope - just 2.4 metres in diameter - was blighted by misfortune and plain bungling. When it finally made it into the sky five years ago, the initial euphoria quickly evaporated with the discovery that the primary light-collecting mirror had been made to the wrong shape. Horribly blurred images were returned to earth. It seemed as though a 20-year investment of $2,000 million (Pounds 1,290 million) had been wasted. More than this, whole careers were on the line, as scientists who had worked on the project since its inception in the early 1970s confronted the possibility that all their efforts had been in vain. So it was with more than relief that two years ago the first repair mission - with a spectacular show of televised space gymnastics thrown in for good measure - managed to fix every deficiency. Happiness returned to the world community of astronomers, not only in the United States but also in the United Kingdom, which has a fractional involvement through the 15 per cent stake of the European Space Agency.

It is the straightforward imagery of the heavens in visible light that gives the HST its universal appeal. The impact of pictures of astronomical objects, with an improvement in clarity of about 100 times compared to the best achieved from earth, can be appreciated by scientist and layman alike. For the astronomer, there is the prospect of a seemingly unending flow of truly spectacular results. For instance, there are the myriad of "Einstein arcs" found around images of nearby galaxies. Here, light from ultra-luminous objects far out into the universe is being focused by strong gravitational fields. Closer to home, within our galaxy, is the nursery of young, low mass stars imaged in great detail in the core of the Orion nebula, whose greenish glow can be seen by anyone with a pair of binoculars on a winter's night. Even images of the planets are unique - not least those that recorded the impacts of the fragments of comet Shoemaker-Levy, with Jupiter last year. The Hubble archive, produced from the accumulation over the decade-and-more lifetime of the project, will remain a repository of unsurpassable knowledge of the universe well into the next century. After all, practically any research astronomer has access to it because - with breathtaking high-mindedness - the teams working closely with Hubble have preferential access to the subsequent results for only one year.

So where does this leave ground-based, near infra-red and optical, observational astronomy - the sort in which the UK has an even more significant stake? Curiously, the success of the HST has stimulated rather than diminished interest in the construction of the new generation of giant telescopes on the best high altitude sites, primarily for follow-up and complementary observations. These include the two ten-metre diameter telescopes to be located on Mauna Kea in Hawaii and privately funded for the University of California by the Keck Foundation. These have huge primary mirrors which are composed of "honeycombs" of separate segments individually computer-controlled to form the optimum overall shapes for the composite mirrors.

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The designers of all the other giant telescopes have opted for so-called "monolith" mirrors which are thin and flexible, but whose shapes are also computer-controlled. There are the two eight-metre diameter "Gemini" telescopes, one set for Mauna Kea, the other for Cerro Tollolo in Chile, which are being funded by the UK, US, Canada and Chile. There is the Japanese-funded telescope, also intended for Mauna Kea and also eight metres in diameter. And finally there is the set of four 8.2 metre diameter telescopes - dubbed VLTs for "Very Large Telescope" - to be sited at Paranal in the Atacan desert in Chile and funded by the European Southern Observatory consortium of Germany, France, Italy, the Netherlands, Belgium, Switzerland, Denmark and Sweden.

All these giant telescopes will act individually as huge "light buckets" which are capable of pouring far more radiation into the slits of spectrometers than the much smaller HST. For one thing, they will probe luminous objects much further out into the universe. Their working lifetimes, with frequent instrumental upgrades, are likely to be several decades. On this timescale, the ingenuity of many thousands of astronomers, distributed around the world's academic institutions and served by such powerful instruments, is bound to generate as yet unimaginable discoveries. A large effort is being expended in the development of active and adaptive optics to permit these telescopic giants to overcome atmospheric turbulence and approach, and even surpass, the angular resolutions attainable with the HST.

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The concept of the VLT array even includes the future combination of the four telescopes in the array, each employing adaptive optics, to achieve a resolution equal to a single telescope of an aperture equal to their separation (ie equivalent to an aper- ture hundreds of metres in diameter). If ever this far-reaching aim of imagery with angular resolutions of better than 0.001 arcsecond is achieved, it will only be applicable to a handful of the brightest sources over microscopic fields of view. But detailed views of the surfaces of nearby stars and even the nearest quasars are promised.

Alas, the technical difficulties of implementing adaptive optics, even on one telescope, are very severe. The current technique is to flex a small intermediate mirror in the telescope's output beam to counter the atmospheric distortions. This has to be performed in real time every 0.02 of a second or so after monitoring the atmospheric distortion of the light from a reference star, or laser beam bounced off the high atmosphere, very near to the direction of the astronomical target. After much early promise, progress has been painfully slow and the technique should only start to make a significant impact in the easier longer wavelength, domain of the near infra-red within the next few years. Correction for targets anywhere in the sky, in the more difficult visible domain and for fields of view comparable with those of the HST, are unlikely to be achieved within even the next 20 years unless very large resources are thrown at the problem or as yet unforeseen technical breakthroughs occur.

Of course, ground-based telescopes will never observe ultraviolet light, which cannot pass through the earth's atmosphere and here the HST data will remain supreme until an even larger telescope is placed in orbit or on the moon. The importance to the United Kingdom of active involvement in these projects must be restated for it is being questioned at many levels. I remember the open day we held a few years ago when citizens, mainly from the Manchester area, trooped through in reassuringly large numbers to see the research being carried out in the university. I was taken aback by the significant fraction who looked down my viewer at the beautiful images of clusters of galaxies to be found on UK Schmidt telescope plates and said "very interesting but what use are they?" They seemed quite happy with my reply that for one thing they kept me in a job. Would I dare to be so cavalier in the present climate where the virtues of "near-market" university research are extolled over those areas designated as "pure", "blue sky" or "big science"? Their sentiments echo those of the US congressmen who cite the funding of research into gnats' kneecaps, or similarly uncommercial sounding projects, as examples of waste and the repetition by various people of the hoary old put-down "that the only useful thing to come out of the Apollo missions to the moon has been the non-stick frying pan". Yet, being taxpayers, all these sceptics deserve a sensible answer even though the capital cost per nation spent on pure science programmes is minute compared with other areas of national expenditure.

For a start, involvement in these stimulating projects, at any level, drags a share of the cleverest young people into science and technology and, through teaching, high-level knowledge, available for exploitation, percolates down to a large number of the community, including the business community. The failure of the UK to make a postwar fortune out of high-technology, non-military, products is primarily the consequence of the failure of industry to invest at a competitive level in long-term research and development programmes since the 1950s. And this failure was compounded by the "brain drain" of many of the best scientists and technologists to US industry in the 1960s which was stimulated in part by the vigour and excitement, never mind high salaries and prestige, of their space programmes. The real worth of projects such as the HST, and the creation of the world's giant optical and near-infrared telescopes, is also the associated prestige of being in the "first world". What is less easily quantifiable is the powerful need to satisfy mankind's unquenchable curiosity about the universe. There will be no final answer but the journey is compulsory.

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John Meaburn is professor of astronomy, department of physics and astronomy, University of Manchester.