Ever since 1609, when Galileo pointed his rickety "spy glass" at the heavens and made discoveries that turned the scientific world upside down, people have been star-gazing with the help of ever bigger and better instruments.
To see more, the most obvious solution is to increase the size of the telescope. By doing so, we collect more light (each doubling of the diameter produces a fourfold increase in the amount of light collected), and stars that were previously below the threshold of visibility can now be seen. These may be inherently faint objects quite close to us, such as asteroids or the so-called "brown dwarf" stars, or they may be very bright objects out on the edge of the observable universe the quasars.
As the size of telescopes is increased, so is their resolution — their ability to distinguish fine detail or to see as separate two objects close together. By eye, we can just separate two objects one arc minute (1/60°) apart, whereas the giant Hale telescope in Mt Palomar, with its 5m (200-inch) diameter mirror, reduces this to almost one arc second — a sixtyfold improvement.
Recently completed or planned are the "new technology telescopes", their mirrors made up of between 10 and 20 computer-controlled segments which can be independently oriented to keep the mirror true to shape, whatever its angle of tilt. At the European Southern Observatory in Chile an array of four 8m telescopes is planned. These will work in unison, with the light-collecting ability of a single 16m diameter instrument, or be able to function as four independent telescopes when needed.
But size is not the only consideration. Getting a better view also means getting a clearer view, and that means reducing the depth of atmosphere we have to look through. The components of the atmosphere, particularly water vapour, cause the atmosphere to act as a filter, and wavelengths like the ultra‑ violet and infra-red are almost completely blocked. Worse is the fact that the atmosphere is not steady or uniform in composition, so that light from the stars is being refracted first one way and then another. This is why stars appear to twinkle, their images dancing back and forth as if seen through the surface of a swimming pool.
The best viewing sites are as high as possible, with clean, steady air and wind streams. The greatest collection of major telescopes on earth is at 4150m on the summit of Mauna Kea, Hawaii, well above sea fogs and low-level cloud, and in relatively non-turbulent oceanic air.
Clearly, the ultimate viewing conditions are to he found in space, and the solution is to lift the telescope into orbit. This was accomplished with the successful launching of the Hubble Space Telescope from the space shuttle Discovery in April, and now we have a medium-size telescope in the ideal optical environment of space.
Unfortunately, a design flaw in the Hubble's primary mirror (discovered after the launch) means that the telescope's observations in the visible part of the spectrum will be severely affected. If working properly, Hubble would have been able to see objects 10 times smaller than the largest telescope on earth could. For example, while giant telescopes like the Hale 200-inch can resolve to roughly the stellar equivalent of reading the headlines of a newspaper at 1km, the Hubble is theoretically capable of reading the fine print as well.
Now such improvements in resolution will have to wait until the mirror system can be repaired and that may mean bringing Hubble back to earth.
No telescope can "see" anything without a light detector of some sort. Originally this was the astronomer peering through the eyepiece, but in the Hubble he is replaced by the Charged Coupled Device, the "retina" of the video camera. By converting light into electrical signals which are then transmitted to the astronomers on earth, the CCD has made observing from space possible. Like photographic emulsion, the CCD can be given long exposures to faint objects, thus building up a detectable image. So, in time, the Hubble will let us see more and fainter detail than ever before. We will also be able to see at all wavelengths of the ultra-violet, which are filtered out by water vapour at ground level.
From nineteenth century astronomers' pencil drawings, we have moved through monochromatic and three colour photographic reproductions into the garish world of colour-coded CCD images with their arbitrary fluorescent blues, lurid greens and searing magentas. One result of these developments is that the art of peering at the night sky through a telescope appears to have fallen into disrepute — an activity more suited to Boy Scouts in pursuit of badges.
But no one who has spent time looking through even a modest telescope in a light-polluted urban sky would agree with this attitude. The mid-winter view of the Sagittarius star cloud is addictive. Once seen, its reappearance is impatiently awaited each autumn. This is also true of the Orion nebula in summer.
Eighty years ago astronomy, like sketching, was one of the polite accomplishments, and modest refractors were in popular demand. During the 1920s, amateur telescope-making became the core of many astronomical societies, and the manufacturing of parabolic mirrors one of their major functions. By the late 1950s, the availability of the commercially produced Schmidt-Cassegrain telescopes seduced many amateurs away from the work bench. However, a handful of dedicated amateur telescope-makers have kept the craft alive and abreast of developments.
Such people belong to one of two schools: there are the craftsmen who produce instruments of a quality, ingenuity and performance that would put them in the five-figure bracket if priced commercially; on the other hand, the photon-grabbers place a premium on seeing the stars and cheerfully accept that their telescopes may look like something left over in a builder's yard. But these assemblages of second-hand plywood, plumbers' fittings and scroungings from car wreckers work, and with care will continue to do so for years at a price affordable by anyone.