Michelle Thaller
The Keck Interferometer: Double-Teaming the Universe
Everyone knows that two heads are better than one, and on March 14th, 2001, astronomers proved it.
High atop the volcanic peak of Mauna Kea, Hawaii, a team of scientists and engineers from the Jet Propulsion Laboratory succeeded in linking the world's two most powerful optical telescopes into one gigantic instrument. This new behemoth, called the Keck Interferometer, has the equivalent optical power of a telescope 85 meters across.
That's a telescope so big, you could almost fit a football field inside of it. But when looking at an impressive new project like this, a lot of questions get raised. Why do astronomers always want to build bigger and bigger telescopes? Why is size so important? Linking two telescopes together sounds like a neat idea, but is a colossal pair of binoculars worth all that work and money? The answers to these questions lie in the very basic nature of light, and the tricks that scientists use to wring every last clue about the universe out of it.
The first telescopes invented by humankind were actually nautical spyglasses, used by Dutch traders during the Renaissance. People on shore could spot the masts of ships sailing in to port, and even do a little speculative commodities trading on the spot (especially if they recognized the ship and knew what cargo it was about to bring in).
The famous and tragic Galileo Galilei was one of the first people to train a spyglass up into the starry night sky, and all kinds of wondrous details emerged. Dark spots whizzing across the sun, craters on the moon, mysterious stars orbiting around Jupiter (now called the Galilean Moons), and thousands upon thousands of tiny stars, invisible to the naked eye. Galileo must have barely completed his first curious scan of the heavens before wondering what else he could see if he had a better, bigger, telescope. And then, despite the painful amounts of political and religious trouble Galileo's observations engendered, the race was on to build the next telescope.
In essence, telescopes are nothing but light-collecting buckets. The bigger your bucket, the more light you take in. Collecting more light means that images of the faint night sky become brighter and clearer; that much seems obvious. But something else happens as your telescope gets bigger: you get better resolution.
Resolution is something we commonly talk about in the context of TV or computer screens. If you have an old computer monitor, any image on the screen seems block-y, like it's made out of lots of little squares. These squares are the "pixels" on the screen, and they're the smallest components the image is made out of. The better your monitor, the smaller the pixels are, and the smoother your image looks. High-definition televisions have great resolution; the pixels are so small, the human eye can barely see them any more.
In the case of a telescope, the wider a telescope is, the better the resolution it can see on the sky. So, as telescopes got bigger and bigger, images of space got clearer and clearer the pixels got smaller. And that's exactly what you want if you're an astronomer looking for tiny river channels on Mars, or studying two stars locked in close orbit around each other, or searching for dim, distant planets around other stars.
For the last 500 years or so, astronomers have been in the business of building bigger and bigger telescopes. About a hundred years ago, this diameter-craze led to the use of reflecting telescopes (which use mirrors to focus light), because it became impossible to make glass lenses big enough that wouldn't distort under their own weight. In the end, astronomers got pretty good at building giant mirrors, culminating in today's current champions, the twin Keck telescopes, which are each ten meters (about 30 feet) wide.
But then astronomers ran into a predictable set of problems. Building gargantuan mirrors was getting prohibitively expensive, not to mention difficult. Keeping a 30-foot mirror smooth and evenly supported is quite an engineering challenge, keeping in mind that the entire thing has to be able to steer and point to any spot on the sky. Building a telescope twice as large seemed virtually impossible (how do you deal with a 60-foot mirror?), so how would the next generation of astronomers improve its measurements?
Like with many things in technology, when the going got tough, people got clever. Instead of building bigger and bigger mirrors, astronomers are now turning to a new technology called interferometry. The basic concept is simple: take the light coming into two or more separate telescopes, and combine it all together. What you create, in essence, is a telescope with the power of a single mirror that's as wide across as the distance between the telescopes. In the case of the Keck Interferometer, the two ten-meter telescopes are located about 85 meters away from each other. When they're combined, the Keck can now act like a single telescope 85 meters across!
The way this all works is, admittedly, not very intuitive, but it also gets back to the way light works. The word interferometry takes its name from the way light is combined from the two telescopes: it's interfered. What does that mean? You may have heard that light can act like a wave, and the wave analogy helps us to understand what goes on inside an interferometer.
Think of two stones thrown into a smooth pond, each creating rings of ripples that move toward each other. Ripples are made of a series of crests (where the water is higher than the surface of the pond) and troughs (where the water is lower), arranged in rings. As two sets of ripples over-lap each other, scientists call what happens interference. If a crest from one ripple meets another crest, the two combine together into a bigger, higher crest. The same thing happens if a trough meets another trough (it combines into one, lower, trough). But if a crest from one ripple meets a trough from the other, then the two just cancel each other out, and the water becomes smooth again.
In the case of light, when waves of light combine together, they make patterns of bright lines (where two "crests" of light come together) and dark lines (where a light "crest" and "trough" cancel each other out). These patterns of bright and dark lines are called "interference fringes." Astronomers study and measure these fringes, and with a good amount of computer-time and modeling, are able to re-create images from the sky.
In order to get this rather delicate technique to work, the position of the telescopes has to be accurate to about a micron, or a millionth of a meter. Think about swinging around a couple of ten-meter telescopes and getting everything lined up to a millionth of a meter. Now you know why this technology wasn't possible until just a few years ago. Any small vibration, or even a good sneeze at the wrong time, can throw an interferometer off. In the case of the Keck, the entire light-combining facility is enclosed within an air-tight "room within a room" beneath the ground. The isolation keeps temperature changes to a minimum, making sure the instrument stays stable.
Despite the delicacy, the boon that interferometry offers astronomers is enough to send ripples (or perhaps fringes) of excitement through the community. Instead of building bigger single mirrors, now the race is on to link as many telescopes as possible into interferometers. European astronomers are now completing the VLT (Very Large Telescope - how cute), which will link four eight-meter telescopes into an interferometer. Both the Keck Interferometer and the VLT have plans to add smaller telescopes to the mix as well, spreading out their coverage of the sky.
Space is the next frontier. If we can build an interferometer on the ground, why not build one in space where it can view the stars without the churning, hazy impedance of our atmosphere? NASA now has a fleet of missions planned to address this new challenge, and create the technology to make it all possible.
Starting with the Space Interferometry Mission (SIM), plans are in place to launch interferometers into space. SIM will consist of a series of telescopes connected together on a single spacecraft, but the next planned mission, the Terrestrial Planet Finder (TPF) will take the concept even further. If a larger distance between the telescopes give you better resolution, why not fly the telescopes on completely separate spacecraft? Then they can get as far away from each other as you like, as long as all the light can be bounced through space to a common combining instrument. Instead of a single, rigid interferometer, TPF will be a small armada of independent telescopes, all flying in formation through space.
With this advance, our view of the sky will be almost unlimited. One of the first questions we're looking to interferometers to answer is whether there are planets like Earth elsewhere in the universe. Astronomers have found about fifty planets orbiting other stars (we don't have direct pictures of any of them yet), but all the planets we've found are nothing like the Earth.
We've found massive, Jupiter-like gas giants, some of them trapped up against their stars and heated to tens of thousands of degrees. Fascinating, but an unlikely environment in which to search for beings like ourselves. We're still looking for that gentle, blue dot in the darkness, but with our current telescopes, anything the size of the Earth would be too tiny and faint to find.
With the formation-flying interferometers after TPF, we may, in the next few decades, be able to take a picture of a distant world, and see unfamiliar oceans and continents on its surface. We'll need interferometry to get there, and the Keck Interferometer just made the next step.