The original telescope as designed by Galileo and others helped people see farther and deeper into space. That was a great advantage, and its original discovery was one of the great accomplishments of our civilization. But the first telescopes only showed what is visible. As time went on, scientists and astronomers started to realize that the human eye limited what could be discovered through the telescope. First of all, one person looking through a telescope had to depend on his or her memory and skill in recording what was observed. But once photography was invented and astronomers thought of attaching photographic plates to telescopes, taking pictures instead of just looking through telescopes proved far more practical. Not only could several astronomers instead of just one look at the same patch of sky captured at the same instant of time, but with careful preservation, astronomers for years to come can view the same picture. A permanent record of how the sun, moon, and planets look over long periods of time can be assembled. A photographic survey of all the sky was now possible, too, which would be much more accurate than any drawings or other records which astronomers of past centuries made. [Charged couple device attached to telescope] Also, by leaving a film shutter open longer, fainter stars appear in the film, even ones invisible to the ordinary human eye. Observations with today's largest telescopes are now almost exclusively done by camera-like devices, since photography has been replaced by machines that electrically detect light to produce an even better, sharper image than a photograph. This recording of the telescope's images is a great advantage and allows teams of astronomers to see the same area of sky without having to schedule hard-to-obtain viewing time on a large telescope. In fact, astronomers can live far from any observatory and yet conduct their research, using images taken at the 200-inch Hale Telescope or even the orbiting Hubble Space Telescope. The invention and development of photography and electronic imaging is one of the most important advances in astronomy since the invention of the telescope itself. The other big limitation discovered about the human eye is that visible energy is only a small part of what is radiated outwards from stars, our sun, and actually all matter in general. There's a wide range of information to be gathered by investigating what lies outside the visible spectrum. The trick is inventing the right "receiver" to detect that invisible world. Most of the energy in the world cannot be seen, so regular telescopes cannot detect it. For example, the heat which radiates off a hot stove is energy, but it is not visible. You can feel it, but you can't see it because the eye isn't made to detect it. Sound is energy, but it also cannot be seen. Our eardrums are built to detect sound. But all these different kinds of energy, visible light, heat, and sound, have one thing in common - they move outward in waves from their source. One way they are different is in how short or long their wavelengths are. Sound has longer wavelengths than heat, and heat has longer wavelengths than light. Another way this is often described is by measuring an energy source's "frequency." When something has a high frequency, it means that the crests or tops of the waves of energy are arriving quickly, or frequently. Sound waves from a radio transmitter have very long wavelengths, so a complete wave arrives at our ears a lot less frequently than the shorter or higher-frequency waves of energy coming to us from the sun. [Wavelength diagram] Interestingly enough, the color-fringe problem of refracting telescopes was what led astronomers to find that energy has a much wider spectrum than just what we can see. As astronomers and other scientists focused on how to remove color blurs from refracting lenses, they experimented with the way white light breaks up into the colors of the rainbow. On one side of the rainbow spectrum is red, and William Herschel in 1800 identified the energy that has slightly longer wavelengths than red, called infrared. At the other side of the rainbow of visible light is violet, and William Huggins in 1875 was the first to detect the energy that has slightly shorter wavelengths than violet, called ultraviolet. Both infrared and ultraviolet are invisible to the human eye, but scientists found they could build special instruments which could detect those wavelengths. There are other shorter and longer wavelength energies with familiar names: the high-frequency ones above ultraviolet are X rays, gamma rays, and cosmic waves; and low-frequency energy sources, below red and infrared, are microwaves, TV broadcast waves, short-wave radio, long-wave radio, and the energy sent through powerlines as a source of household electricity. Instruments to detect these other wavelengths were then attached to telescopes to see what could be discovered. The earliest spectrum experiments took place when Joseph Fraunhofer (1787-1826), a glass and lens maker, constructed what would later be called a spectroscope as he searched for a solution to the color problem of refractors. His spectroscope was a combination of a lens, a prism, and a small telescope, all positioned in front of a slit in a window shade. Light came through the slit, traveled through the lens and into the prism, and the separated colors were then picked up through the small telescope. To his surprise, he not only got bands of color, but also dark lines running at different intervals among the colors. [Fraunhofer spectrum with dark lines] Fraunhofer had no idea that the spectrum he produced was really a coded map of the chemical composition of our sun. The information in those lines, eventually called Fraunhofer's lines, would later tell scientists what elements our sun is made of. Fraunhofer did notice that the moon and planets, when shining their light through his device, also produced the same arrangement of colors and dark lines. This is because they reflect the same light as the sun. But once he viewed other stars with his spectroscope, the dark lines in the colors changed position. Fraunhofer made drawings of each star's resulting spectrum colors and lines, but he didn't know why each star had a unique look to its spectrum. It wasn't until 33 years later that Gustav Robert Kirchhoff (1824-1887), while studying luminous gases, was looking at some vaporized sodium through a spectroscope and noticed two bright lines in the same position as Fraunhofer's dark lines. By further experimenting with sunlight, different elements, and Fraunhofer's spectrum drawings, Kirchhoff decoded the sun's spectrum. The dark lines and their position indicated that the sun contained such familiar elements as sodium, magnesium, iron, calcium, copper, and zinc. Astronomers now had the key to what elements made up the farthest stars - they could find out just by using a spectroscope attached to the familiar telescope. Another important event associated with a different use of the telescope was the discovery that we can "hear" the universe. Karl Jansky (1905-1950), an engineer working for Bell Telephone Laboratories, discovered the first extra-terrestrial radio signals coming from the center of our galaxy. He had been charged with tracking down an annoying static which was interfering with new transatlantic phone lines, and he decided to build a large movable antenna in a field in New Jersey to find the source of the noise. What he discovered was a radio source which sounded like hissing, moving across the sky in the same direction as the sun. At first he thought it was coming from the sun, but in time Jansky realized it moved ahead of the sun, coming about four minutes earlier each day. Eventually the sound was louder at midnight when the sun was nowhere around. [Jansky's movable radio telescope] Jansky knew enough about astronomy to know that there was a four minute difference between a solar day and a sidereal day. A solar day is 24 hours long, the time it takes for the Earth to fully rotate from, for example, one noon when the sun is directly overhead, to the next noon, when the sun appears to return there. But a sidereal day is only 23 hours and 56 minutes long, because that's the time it takes Earth to rotate so a particular star returns to the same place in the sky. The sun appears to move a little each day, and it's that small movement that adds four minutes to a full day when measured relative to the sun. Soon Jansky realized the strange hissing was coming from a fixed point in the sky, somewhere among the stars beyond our solar system. After a few more years of investigating this phenomena, he published what he found. At first people thought Jansky had discovered some kind of intelligent signal from an extra-terrestrial civilization, but it was soon understood that many bodies in outer space give off strong radio signals. What Jansky had found were radio waves coming from the center of our Milky Way galaxy. Since then, many other stars, some not even visible, have been found giving off radio signals. The planet Jupiter, for instance, is a strong source of radio noise. Because of Jansky's work, an entire new field of exploration opened up. Radio telescopes were built along the same lines as regular telescopes, but with a much larger dish to catch the much longer, weaker radio waves. Like the mirrors of reflecting telescopes, early pioneers in radio astronomy like Grote Reber (1911- ) made their collecting device concave, or bowl shaped. Carefully polished mirrors weren't necessary, since radio reception doesn't need the fine resolution that visual images need. So Reber made his antenna dish out of 45 wedge-shaped pieces of sheet iron. Radio astronomers early on realized the need for radio telescopes to be as large as possible, to catch what would be much weaker sound wavelengths than the shorter, stronger visual wavebands. [Large radio antenna] Just as those who developed regular telescopes pushed for larger and larger lenses and mirrors, the radio astronomers thought up ways that they could get better reception of the radio waves coming from outer space by making stronger and larger radio dishes. One of the largest single radio receivers in the world is at Arecibo, Puerto Rico, where a 1,000-foot radio telescope is installed into a natural depression in the ground. The large dish cannot be moved, but positioned above the dish are instruments which help funnel a particular source in the sky into the dish for collection of its radio energy. [Arecibo dish. image 32, p. 363 Hoskin] By placing the huge dish into the ground, the Arecibo telescope avoids the problems which eventually plague the movable radio antennas. There are limits to how big a movable radio dish can get before its own weight will make it impossible to maneuver or maintain. A key advance in making more sensitive radio telescopes was the discovery that two or more radio antennas can be linked together in an array. The first large set of connected antennas, called the Very Large Array, was completed in 1981 on a plain in New Mexico. The VLA is made up of 28 linked antennas (27 working ones plus a spare), each with an 82-foot antenna dish. Each dish is mounted on a pedestal which can point the antenna upward in all directions, and each pedestal is on a kind of railroad track so that all the antenna dishes can be moved into different configurations, all spread out for some observations and all bunched up for others. [Very Large Array. image 33, p. 357 Hoskin] The largest array of radio telescopes today is an array of ten 82-foot radio antennas spanning the entire U.S., from the Virgin Islands, across the mainland states, and out to Hawaii. Called the Very Long Baseline Array, these radio telescopes are connected by the Internet and synchronized by super-accurate atomic clocks. Each antenna weighs 240 tons and is nearly as tall as a ten-story building when pointed straight up. [Very Long Baseline Array. image 33A, p. 357 Hoskin] Shorter wavelengths like the ultraviolet, X-ray, gamma ray, and cosmic ray energy sources are prevented from fully reaching Earth by our protective atmosphere. To detect these high-frequency energy sources, plans had to be made to put specially constructed telescopes where they could work without the atmosphere's interference. Only above Earth's cloud layers would an X-ray telescope or ultraviolet spectrometer work at its best. The radio telescopes can detect radio waves during the day or in cloudy weather, but these other instruments needed space-age technology to get above the limitations of Earth's atmosphere. With rocket telescopes, satellites, and orbiting telescopes, we could finally begin to explore the universe's many other, invisible faces, as well as see farther out into the visible world than we ever could from Earth.
