Four Hundred Years of the Telescope tells the dramatic story of how a simple instrument, put to use by brilliant astronomers, transformed our view of the universe and of ourselves. But, what exactly is a telescope, and how does it work?
Telescopes are devices to collect far more light than our eyes can bring together, and thus make things visible that are otherwise too faint for us to see. Telescopes can also magnify things (make them look bigger), something that is important if you are looking for the color of a bird in a tree or spying on your neighbors, but this is often less important for astronomers than sheer light-gathering power.
Think of telescopes as giant buckets for collecting light. If there has been a drought, and you run outside to catch the water from a sudden rainstorm, you will be disappointed if all you have with you is a small paper cup. Can't catch much rain in that! But if you could grab a big bucket, or even bring a picnic cooler outside, then you can collect more rain and perhaps water a few of your indoor plants.
Telescopes work the same way. The bigger the element in the telescope that collects and focuses the light, the more light you can gather from an object in the sky. Light gathering power depends on the area of the lens or mirror collecting light. Since the lenses in our eyes are so small (designed for looking at a sunlit scene), even Galileo's small telescope was able to gather about 100 times as much light as the human eye. This is how Galileo was able to see the moons of Jupiter, when no one had ever been able to see them before. The extra light collected made things in the night sky that were too faint for the unaided eye to see suddenly visible.
Scientists sometimes joke that the history of astronomy can be summed up as a search for bigger light buckets. Every time we have built a larger collector of light (or other waves), we have discovered things that were too faint to be detected before -- and opened up new worlds for exploration. The show gives a nice overview of how astronomers and engineers have worked together to build bigger and more efficient telescopes.
Types of Telescopes
The first telescopes used a lens to collect, bend, and focus light (just like our eyes do). But lenses have a number of disadvantages. As you know from the lenses in eyeglasses, the lens requires both sides to be clear. That's why the lenses in your uncle's glasses are held only around the edges. The larger the lens, the more it tends to sag because of its weight. And lenses tend to bend different colors of light differently, which made the various colors spread out in an image. We can compensate for these disadvantages up to a point, but large light buckets made with mirrors turn out to be significantly easier to construct.
It was the great physicist Isaac Newton who first showed how to make a practical telescope using mirrors. The big advantage of a mirror as your light bucket is that the light reflects off the mirror surface -- it doesn't go through the mirror like it does for a lens. Therefore only one side of the mirror must be kept clear and the bottom side can be used to hold and support it. Mirrors, when made the right shape, also don't separate the colors of light like lenses do. For these and other reasons, mirrors can be made much larger than lenses and all the modern telescopes astronomers use are "reflectors."
As discussed in the show, astronomers have found ingenious ways to make bigger and bigger mirrors in recent decades. Some giant mirrors are made from several segments which are kept aligned by computer-controlled devices. Other mirrors are made while the hot mirror material is spun at high speed and can be made much more light-weight through this process. Sometimes several large mirrors are used in the same telescope and the light they collect is brought together to make the equivalent of a much bigger mirror. As a result, we have light buckets today that Galileo and Newton did not even dare to dream about.
Another important characteristic of a telescope is its resolving power -- its ability to make out fine detail. In theory, the bigger the telescope, the better its resolving power -- the finer the detail it can make out -- whether we are looking at the weather on Jupiter, or the collision of two far-away galaxies of stars. Thus, another reason that astronomers build larger telescopes is to try to see the inner workings of distant objects.
Alas, many large telescopes cannot see fine detail anywhere near as well as theory predicts. That's because telescopes on Earth have to look through miles of air -- and our atmosphere dances, jiggles, and is full of water vapor, ash, and human pollution. The dancing and blurring of images by our atmosphere is the reason stars appear to twinkle when we look at them from the ground. The higher up we put our telescope, the less air it has to look through and the clearer our view. This is why all observatories with large telescopes are built on high mountaintops, where the air is clear and clean.
As you can imagine, the best solution to the blurring effects of air is to rise above the problem -- literally. Telescopes above our atmosphere, such as the Hubble Space Telescope, can enjoy the resolving power that laws of physics promise, but don't deliver on Earth.
A telescope is a passive collector of light (or other waves), just as your car antenna is a passive collector of waves from local radio stations. Before you can listen to the news and traffic, you need to attach that antenna to a detector, a device that actually receives the waves and can record or translate them. In the case of your radio, the detector is your radio set, which turns the radio waves sent by your favorite radio station into sound you can hear.
Telescopes also need detectors attached for us to be able to see the images they form. For centuries after Galileo, the only available detector was the human eye. Astronomers made sketches of what they saw -- we still treasure those early sketches for the pioneering information they contain. But in the second half of the 19th century, the invention of photography changed astronomy profoundly. A photograph allows us to make a permanent record of what is in the sky -- a record that is not fooled by the prejudices of the human brain, as early observers sometimes were.
Even more important, photographs allow us to add together the light that comes in over many minutes or even hours. This is called a long exposure, and it has the same effect as getting a bigger light bucket -- you can collect more light and see things in the sky that are fainter. It was photography that made the studies of galaxies discussed in our show possible and led to the discovery of the expanding universe.
Today, digital photography allows astronomers using electronic cameras to record even fainter objects than ever. Electronic detectors (whose technical name is CCD's) have revolutionized our search for faint objects -- such as asteroid, dwarf planets, or exploding stars in other galaxies. Together huge telescopes and super-efficient detectors are giving us unprecedented views of the universe. This is the revolution that 400 Years of the Telescope celebrates.
Contributed by Andrew Fraknoi (Foothill College)