Looking up, down the barrel

Of all human inventions, the telescope is the one which produces the most wonderful results with the least material. We now take it for granted, but could anyone ever have imagined that two pieces of glass could abolish space, and perform the miracle of bringing distant objects apparently within reach? And without the telescope, we would still be totally ignorant of our true place in the universe.

In principle, it might have been invented thousands, not merely hundreds, of years ago. It seems impossible that Archimedes - or some Chinese inventor even earlier - did not toy with the idea. Some imaginative historians claim that Ceylon's maverick King Kasyapa, who in the 5th century AD built the rock fortress of Sigiriya (which I included in my personal seven wonders of the world in a recent BBC programme), had a telescope to spy on his harem.

Patrick Moore's new book, written in celebration of the 40th anniversary of his television programme, The Sky at Night, is a slim but lavishly illustrated account of the telescope's history from Galileo's 1609 "optick tube" to the first, but certainly not the last, of the great observatories beyond the atmosphere - the Hubble Space Telescope.

Telescope making is an art as much as a science, and is also one of the very few branches of engineering in which amateurs can be as good as professionals. For several decades, telescope making was a popular hobby (especially in the United States), but with the advent of relatively cheap, commercial instruments, it went into a decline.

Now amateurs have an exciting new opportunity, thanks to the astonishing development of light-trapping devices dozens of times more sensitive than photographic plates. Coupled with the easy availability of powerful computers, these now allow backyard astronomers to outperform the most well-equipped observatories of the past. Even so, no more advanced instrumentation than a Mark 1 eyeball can still ensure instant fame and a degree of astronomical immortality, as Messrs Hale and Bopp recently demonstrated.

One of the fascinating themes in Eyes on the Universe is the long battle between lenses and mirrors - refractors and reflectors. (Too bad the printer's devil mixes them up on page 51.) That particular war now seems over: the mechanical and optical difficulties of making very large lenses are so great that they have been abandoned.

Yet there will always be a certain glamour about the old-type refractors with their gleaming brass tubes. There are not many scientific instruments which can still perform just as well as when they were built a hundred years ago - and may continue to do so for centuries to come.

Nobody would have guessed when, after decades of delay the 200-inch Hale telescope was dedicated in 1948, that the great age of telescope building still lay ahead. But as Moore makes clear, this is certainly the case, and instruments have been designed now with apertures which would have seemed like fantasy only a generation ago. The largest refractor ever made (Yerkes, 1897) had a lens just over a yard across. If all goes well, in the year 2000, a quartet of mirrors will give an equivalent aperture of more than 50 feet.

There would have been no point in constructing such huge instruments, even on high mountains where the air is thin and steady, if great advances had not been made in eliminating the atmospheric effects which have always been the bane of ground-based astronomers.

However, developments in "adaptive optics" - a spin-off from the notorious "Star Wars" programme - now allow such telescopes to perform almost as well as if they were in orbit, at least in the visible and infrared. For the shorter wavelengths (ultraviolet and extreme ultraviolet) the earth's atmosphere remains an impenetrable barrier. The Hubble telescope and its successors need fear no competition from sea level.

The effect of politics on astronomy (as in other matters) has often been baleful. I can still recall my incredulity in the early 1960s when I heard of plans to install a 100-inch telescope at the Royal Observatory, even though that had moved from Greenwich to the marginally better climate at Herstmonceux. I recall a sarcastic remark at a Royal Astronomical Society meeting that as the skies were so bad in England it was obviously necessary to have the best possible telescope to take advantage of the few moments of good visibility that did occur. Fortunately, the Isaac Newton telescope is now more sensibly installed in the Canary Islands, at an altitude of more than 7,000 feet.

Another unexpected development that has made such sites convenient is the global communications network. Many astronomers can now use telescopes that are on the other side of the planet, almost as easily as if they were actually in the observatory dome - and much more comfortably, when that dome is several miles up in the thin, freezing atmosphere.

I am indeed happy to recommend my old friend's book to all readers, even if they are not particularly interested in astronomy. And I wonder what Patrick will be talking about on the 50th anniversary of The Sky at Night. I hope he will be describing, with his usual enthusiasm, the successful operation of the first Moon-based telescope.

Arthur C. Clarke is an amateur astronomer based in Sri Lanka.