emember the first magnifying glass you ever played with? It was a transparent lens a few inches across, which opened your eyes to the miniature landscapes and creatures all around you. If you had a second magnifying glass, or even a good strong pair of spectacles, you may even have independently invented the telescope. If so, you'll be interested to know there's a telescope in Wisconsin's Yerkes Observatory with a refractive lens 40" (about one meter) across--100 times the refracting area of your childhood toy. Though still in use today, the 1887 instrument belongs to an almost medieval era in astronomy; it is the largest refractive telescope ever built, little different in design from instruments first used by Galileo and Kepler nearly 400 years ago. It was also one of the last large refractors ever built, because that particular technology had reached the structural limits of the only available material: glass. Afterward, reflective telescopes, first invented by Isaac Newton in the 17th century, took over the lead in size and power, because large mirrors proved much easier to fabricate and control than large lenses.
Size is critical in telescope design, since the light-gathering ability of an instrument is proportional to the square of its radius. But as a nice bonus, reflecting telescopes also make it easier to focus light onto sensors or recording devices that are vastly more sensitive and specialized than the human eye. Of course, the 1990 debacle of the Hubble Space Telescope mirror reminds us that nothing is easy. But, in the seven years since its repair, the Hubble has fed down a steady stream of astonishing images that no other telescope could hope to reproduce. It's kind of a yawn, actually--Hubble was more newsworthy as a failure. Its successes, like other modern marvels, simply fade into the background noise of a technologically jaded society.
And while Hubble doesn't get nearly the press it deserves, it's nonetheless the media darling of the astronomy world. Meanwhile, less glamorous instruments have been scouring the heavens just as diligently, and pumping out volumes of data just as critical to our understanding of the universe. It's nothing short of a quiet revolution--one that overthrows cosmological theories almost as quickly as they can be invented.
Blue skies and twinkles
One key aspect of this revolution has been getting above the atmosphere. We're accustomed to thinking of air as transparent, but this is mostly because our eyes respond to the light frequencies that slip through most easily. In fact, our favorite cocktail of nitrogen, oxygen, water vapor and Other is mostly opaque, allowing electromagnetic radiation (a.k.a. "light") to pass through only in certain frequency windows. And even in the visible spectrum--wavelengths from 390 nanometers (violet) to 760 nanometers (red)--light is subject to scattering (the beloved "blue sky" effect), lensing (twinkle), obscuration (clouds and hazes) and simple overpowering from terrestrial sources. Only a handful of stars show up over a bright city skyline--a problem for astronomers as well as lovers.
In the 1970s, astronomers began placing crude X-ray and gamma-ray telescopes in low Earth orbit, to observe the sky in the higher frequencies that don't reach the ground. The '80s saw the addition of infrared and ultraviolet to the mix, but none of these instruments had enough resolution to accomplish more than a general sky survey and observations of a few highly visible objects. In recent years, however, these older telescopes have been replaced by the Space Infra-Red Telescope Facility (SIRTF); the Chandra X-ray Observatory; the Gamma Ray Observatory (GRO); and, of course, the Hubble, which observes in infrared as well as visible light. Each of these instruments has shown us what amounts to a whole new universe--a parallel universe with eerie resemblances to our own, but eerie differences as well. Today, we can see once-hypothetical features like dust clouds, galactic cores, brown dwarf stars, shock waves from exploding supernovae, and gas halos spiraling into black holes and neutron stars.
Newton's quiet revolution
Still, these space telescopes are expensive and occasionally cranky, and they can observe, at most, one thing at a time. However valuable they are, they remain a supplement to ground-based telescopes, which are a whole lot cheaper and easier to maintain. But the quiet revolution has struck here, too. Passive glass mirrors reached their structural limit in 1949, with the 200-inch Hale Telescope on Arizona's Mount Palomar. Any larger and the mirror would begin to distort or even shatter under its own weight. But there is another kind of reflector: the Liquid Mirror Telescope (LMT), first proposed (once again) by that gallant knight of the hard sciences, Sir Isaac Newton. Observing that a pool of spinning liquid will automatically assume the perfect parabolic shape, Newton suggested that mercury--the shiniest element known--could serve as a telescope mirror. Alas, since mercury is both heavy and highly toxic, keeping its vapors contained during spinning was beyond the technology of the day; the idea wasn't successfully put to use until the 19th century. And since an LMT can look only straight up, it was never really a strong competitor against glass mirrors, anyway. Today, though, LMTs can be made that are not only larger than their glass cousins, but cost less than one percent as much. Too, by synchronizing the telescope's sensors with the rotation rate of the Earth, astronomers can take daily snapshots of the same pieces of sky, to look for movement or other changes that might signify, for example, undiscovered asteroids or orbiting space junk. The widest LMT today is Canada's super-cheap Large Zenith Telescope, with a $0.5 million price tag and a whopping six-meter mirror.
The 1990s also saw huge advances for steerable optics, with the completion of the Keck Telescope atop Hawaii's Mauna Kea in 1996. With flexible mirrors shaped by computer-controlled pistons, the Keck boasted a pair of 10-meter telescopes that could not only counteract the distorting effects of gravity, but--using careful measurements made with laser beams--could actually compensate dynamically for the "twinkle" of atmospheric density changes. And as if that weren't enough, there is a computerized technique called "long baseline interferometry" that uses multiple sensors to construct a single synthetic image whose resolution is proportional to the distance between the sensors (although its light-gathering ability remains a function of their actual size). This trick is very difficult to accomplish at visible-light frequencies, because it requires (among other things) clocks accurate to within one wavelength period, or about one billionth of a microsecond. But this stunning technology is now in daily use at Mauna Kea, where the two Keck telescopes form a single instrument equivalent to a 90-meter reflector. And in 2002, the Keck will be surpassed by Chile's creatively named Very Large Telescope (VLT), which will use four telescopes to simulate a 200-meter reflector.
Of course, long baseline interferometry has been possible for 60 years in the radio spectrum, where frequencies and computing demands are millions of times lower. The Very Large Array near Socorro, New Mexico (featured in the movies 2010: The Year We Make Contact and Contact--those 28 big radio dishes mounted on railroad tracks), was completed in 1982 to exploit this very principle. But VLA, with a mere 36-kilometer aperture, is a child's plaything compared to the new, U.S.-owned Very Long Baseline Array, which uses ten 82-foot dishes scattered from St. Croix to Honolulu, from New Hampshire to northern Washington, to create a radio telescope with an aperture over 8,000 kilometers wide! Peek out the window for a moment and consider the 80-centimeter satellite TV receiver on your neighbor's roof, which is capable of decoding up to 500 channels of full-motion video from a 300-watt transmitter located 23,000 miles away. Now imagine something one hundred thousand billion times more sensitive. Yeah.
The size of their toys
And believe it or not, the VLBA itself will become a toy as astronomers add signals from new orbiting radio telescopes and--eventually--fixed dishes on the moon and asteroids. By the end of the 21st century, our telescopes may well be the size of the solar system itself, capable--literally!--of reading the license plates of a car parked 700 light-years away.
Still, as telescopes get bigger and their imaging power gets bigger-squared, the areas of sky they observe grow proportionally smaller. A paradox: the more we see, the less we can look at. This is a real problem for astronomers who study sudden or fleeting events such as gamma-ray bursts and supernova explosions, and also for sky-survey projects such as the Search for Extraterrestrial Intelligence, and the search for small, fast-moving asteroids and comets that may sooner or later endanger the Earth.
This is why there'll always be a place in astronomy for small telescopes. Preferably lots of small telescopes, looking every direction at once. Fortunately, in our computerized society, building or purchasing a fully automated ground-based observatory costs less than a cheap car or a trip overseas. Thousands of people have already built home observatories, and hundreds more join them every year. Because astronomy is one of the very few sciences where major discoveries and observations are still routinely made by amateurs, these expenditures are far from idle whimsy. In fact, in strict bang-for-buck terms, the most powerful observatory in the world may always be the same one favored by Galileo and Newton: our own backyards.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major publications, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and upcoming The Collapsium.
