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REALLY Big Telescopes

Week of April 12, 1999

Despite a popular saying, in astronomy, size really does matter.

Think of a telescope as a bucket that collects light. The bigger the bucket, the more light it collects. Many people think that the most important quality of a telescope is how much it magnifies an object (certainly cheap 'scope manufacturers advertise them that way!), but what is usually more important is how faint an object the 'scope can see. In that case, the more light you collect, the fainter you'll see. So over the years, telescope makers have focused on making telescopes bigger and bigger.

Still, magnification is important. A lot of important physics that goes on inside an object may be on a huge scale, but the objects can be so far away that the really interesting stuff is too small to see! So astronomers do want to be able to see small details, which means greatly magnifying an image. Astronomers say they need more resolution, which means we need to be able to distinguish (resolve) two objects that are very close together. As you make telescopes bigger, higher resolution comes about rather naturally; larger 'scopes tend to get better resolution. The problem is, you can only make a telescope so big before it gets too unwieldy. For optical telescopes (that is, telescopes that see the same kind of light our eyes do) that size limit is currently about 10 meters. That's still pretty big! The problem is, optical telescopes have to be very smooth or else they blur the light coming in. That's one reason why it's hard to make them so big.

For radio telescopes, however, the size limit is much bigger. Many astronomical objects emit radio waves, and as it happens telescopes built to detect radio don't have the same stringent smoothness criteria optical telescopes do. The largest radio telescope in the world is at Arecibo, in Puerto Rico (it was featured in the movie ``Contact''). That telescope is 300 meters across, so big it had to be built right in to the landscape! And right now, a huge 100 meter fully steerable radio telescope is being built in West Virginia.

But radio telescopes can get even bigger. It is possible to link up two separate telescopes so that they can act like one bigger one. This technique, called ``interferometry'', is massively complex and takes a huge amount of fine tuning. For one thing, you need to know very precisely how far apart the to telescopes are. However, it has proven itself so well that astronomers have been able to link up telescopes clear across the Earth! This is called "very long baseline interferometry (VLBI)", and it makes a telescope that is essentially the diameter of the Earth. When this is done, they can achieve resolution on the order of a milliarcsecond, which means they could easily tell how many fingers a person is holding up... if that person is 30,000 kilometers away (...and of course, that person is emitting enough radio waves for the telescope to detect)!

As a matter of fact, this technique is so precise that many years ago radio astronomers were using it in reverse. They would map out the sky using different telescopes, and use those observations to figure out just how far apart the telescopes were. To their astonishment, they discovered the telescopes were moving. The technique was so accurate they were seeing the effects of continental drift on their telescopes! This method is so good that astronomers and geodesists (people who measure the shape of the Earth) got together and share their data.

Sometimes I hear people argue that astronomy is a waste of money, because it doesn't impact our daily lives. Yet the pursuit of better astronomy had a literally down to Earth application which lets people study the interior of the Earth better. This can lead to better earthquake and volcanic eruption predictions, mountain formation, and may even lead to a better understanding of just how the Earth's magnetic field operates as well. You just never know where basic research will lead.

©2008 Phil Plait. All Rights Reserved.

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