TELESCOPES - 2


Built next to the 200-inch Hale the year before is the Big Schmidt, now called the Oschin Telescope, a 48-inch telescope-camera that is both a reflector and a refractor in one. This combination was invented by Bernhard Schmidt (1879-1935). Its advantage over the much larger Hale Telescope is its wider field of vision. Using the Schmidt, astronomers made a complete photographic map of the sky, something it would have taken the 200-inch Hale reflector millions of individual photographs to do.

After the observatories on Palomar were built, 30 years passed before the construction of larger ground-based telescopes, although a number of telescopes between the 100- to 200-inch range were constructed. In 1996, the two 10-meter (400-inch) Keck telescopes were completed atop Mauna Kea in Hawaii. The twin Keck reflectors, when used together, have the resolving power of a single telescope 90 meters or 295 feet in diameter. With its very thin atmosphere, the area around the extinct volcano of Mauna Kea is almost perfect for sky watching; a total of nine huge telescopes, with two more under construction, are taking advantage of these ideal conditions.

Efforts to create even larger Earth-based telescopes continue. The invention of the rotating furnace allowed Roger Angel to cast an 8-meter or 330-inch mirror for the Large Binocular Telescope to be built in Arizona. The final telescope will have the equivalent light-gathering power of a single 11.8-meter or 464-inch instrument, with the image sharpness of a 22.8-meter or 897-inch mirror, making it the largest telescope anywhere.

But despite the great achievements of large ground-based telescopes, there are others who focused on improving our view of outer space by getting rid of the atmosphere's influence altogether. As early as the 1940's, a telescope was proposed that would be completely above the atmosphere. The plan was not to use an even higher mountain, but to place the telescope in outer space itself, positioned in orbit around the Earth and operated by remote control. Today the Hubble Space Telescope is the realization of that dream, although it took many other smaller efforts and projects to make that dream a reality.

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.

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.

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 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 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.

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. 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.

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.

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.

Telescopes above the Atmosphere

Once the technical problems with telescopes were solved and instruments were invented to help detect energy beyond what we could physically see, only one big hindrance remained for astronomers - the Earth's atmosphere. Even on the tops of mountains, the atmosphere prevents the hugest telescopes from getting a really sharp picture of distant objects.

But how do we get telescopes into space, and how would they operate? Rockets were one way found to get such devices above the atmosphere, but rockets could only stay up for a few minutes before falling back down. They also couldn't point a telescope in a specific direction, so only simple detectors could be used that would just scan the general area. Airplanes and big weather balloons were tried, but neither one escaped the atmosphere altogether, and they, too, couldn't stay up for a very long time.

Balloons also couldn't point the equipment in a specific, steady direction. What was needed was advanced equipment with complex controls that could make a flying telescope point steadily at a chosen target. This would require an expensive satellite or spacecraft, but such projects only get financial support if scientists can show that a lot can be learned with such a telescope. To get satellites and spacecraft observatories funded, smaller projects using those rockets, planes, and balloons had to come first, to justify spending the money on putting remote-control telescopes in space.

The first rocket carrying an instrument to investigate ultraviolet radiation was launched in 1949. Ultraviolet was a good choice to begin such studies, since it was already known to have a big influence on human health. A small dose of ultraviolet builds up vitamin D, but large amounts cause sunburn and skin cancer. Earth's atmosphere keeps most of the harmful ultraviolet radiation from affecting us, and that's why a detector in space could be useful, to measure the full amount that the sun and stars radiate.

The ultraviolet detector was carried in the nose cone of a German V-2 rocket being fired to test its propulsion system. It was really just along for the ride, installed in the place where bombs had once been. When scientists were given this opportunity to put an experiment in the rocket, an ultraviolet photometer was chosen specifically because the photometer could read the sun's spectrum without being carefully pointed. Once launched, the rocket was above the cloud layers only for a few minutes, but the photometer was able to detect not only the sun's ultraviolet radiation, but solar X rays as well. As experiments continued, some X rays were detected from sources other than the sun. Scientists became eager to find out more about these mysterious cosmic X rays.

To determine if there were enough X rays coming from outside our solar system to study in the first place, a detector which would specifically target the X-ray wavelength was built into a rocket. The first two rockets launched with this instrument ran into technical difficulties. The first rocket engine failed, and while the second rocket launched successfully, the door to the instrument area got stuck, and so the only thing detected was the inside of the rocket chamber. The third try on June 18, 1962, was finally a success. Once above 80 kilometers, the rocket doors opened and the instruments found very strong X rays coming from the southern sky. The five-minute flight had discovered a cosmic X-ray source hundreds of times brighter than anyone thought existed.

After a few more rocket surveys, an X-ray detector was sent up on one of the Orbiting Solar Observatories. The more experiments done with X rays, the more promising were the discoveries. Invisible X-ray stars were found. Some were also strong radio sources. Scientists now pushed to get an X-ray telescope in space.

The X-ray Explorer, nicknamed "Uhuru," was launched from Kenya in 1970. Its success led to the launch of the Einstein X-ray Observatory in 1978, which operated for two and a half years. Observations were made of 5,000 objects ranging from comets in our solar system to quasars billions of light-years away. One key achievement was the discovery of a uniform glow of X rays throughout the sky, probably coming from far outside our galaxy. If you compare it to how our eyes see the night sky as black with bright points of light scattered here and there, when viewed in the X-ray wavelength, there is no black sky at all. This bright X-ray background could mean that very hot gas exists between galaxies, or perhaps it is produced by millions of distant X-ray sources, like quasars, which are star-like radio sources. More study and observation will not only clear up such mysteries, but will likely reveal more amazing things about X-ray energy sources.

Gamma rays, which are produced by the decay of radioactive material, were first found in space by sending detectors up in balloons 20 miles above the Earth's surface. Enough interesting waves in that spectrum were discovered to argue for a small gamma-ray telescope to be included on board the second Small Astronomy Satellite (SAS-2) in 1972. SAS-2 made a gamma-ray map of the entire sky. [SAS-2 image] Gamma rays are associated with neutron stars, which are stars once bigger than our sun that have exploded and collapsed into very dense material, so dense that a piece of a neutron star the size of a grape would weigh about a billion tons.

Then in 1979 the first spacecraft flown to detect gamma rays from outer space was the third High Energy Astronomical Observatory. HEAO-3 had lots of difficulties at first, because many false gamma ray readings hit the detector. Eventually scientists sorted out the false readings and learned that gamma rays and their radioactive sources are probably coming from novae, which are partial explosions of stars, a process which happens 1,000 times more frequent than supernovae, the complete explosion of a star.

Infrared is another wavelength in which scientists wanted to map the sky. Infrared energy is like heat, and every living thing, and even nonliving things which retain heat, emit an infrared glow, though humans can't see it. Certain snakes have an infrared detector so they can catch mice and small animals at night by sensing the prey's body heat. At one point, some scientists were anxious to view Mars in the infrared, thinking that we could sooner determine if life existed there if infrared pictures showed large concentrations of infrared energy.

The first airborne infrared survey was done in a plane. The success of this and other experiments with infrared detectors eventually led to an internationally sponsored Infrared Astronomical Satellite carrying an infrared telescope, launched in 1983. This device ran mainly by computer and made almost four complete surveys of infrared energy in outer space. Since there is a lot of gas, dust, and general space debris like burned-out rocket stages orbiting Earth, many of the readings had to be thrown out as just interference. But these thrown-out readings were also kept, since a mistaken infrared source could be an unknown distant 10th planet or a dark star companion to our sun. Scientists will be able to refer to this infrared map in the future if some interesting object is discovered at a later time.

While these specialized projects uncovered many interesting things about the universe, other astronomers insisted that we also had to have a large all-purpose telescope in orbit, one with a range of sensitivity from the infrared through the visual spectrum and into the ultraviolet. Many years of planning, development, and battles over funds finally produced the Hubble Space Telescope, named after Edwin Hubble (1889-1953), the astronomer who discovered the redshift in numerous galaxies, proving the universe was expanding. Launched in 1990, the Hubble was flown to outer space in the Space Shuttle, which limited the size of its mirror and overall structure, since it had to fit into the shuttle's cargo bay. But a 2.4-meter or 96-inch mirror in outer space still promised to get sharp images of distant star systems and clouds of gases 10 times better than possible from Earth.

The Hubble telescope is arranged so that all instruments are installed behind the main mirror, and a hole in that mirror faces a smaller mirror which reflects images back into the instrument area for recording and analysis. A wide-field camera and a faint-object camera are on board, as well as devices for analyzing the color spectrum of very distant objects. The Hubble is not studying the sun or moon, because the light from these bodies is too bright and would damage the telescope. In fact, the Hubble is not usually pointed at any object which lies within 50 degrees of the sun or 15 degrees of the sunlit moon or Earth. The telescope completes an orbit every 95 minutes and holds steady by locking onto guide stars. Flying about 600 kilometers above the Earth, it is expected to operate for 15 years, with the Space Shuttle visiting it every three years to service it and install any updated equipment it might need.

Soon after the Hubble Space Telescope was launched, a problem was discovered with its main mirror. The mirror had a very slight flattening at its edge, so slight that it was hardly detectable. But this tiny flaw produced images which just weren't the sharp quality which was expected of a space telescope. Since replacing the mirror in space would have been extremely difficult and expensive, Hubble engineers decided to trick the mirror into working properly. They built duplicates of some of the equipment that worked with the mirror and made those devices with an opposing flaw, to "correct" the defect of the slightly warped mirror. In this way, the images the Hubble produced would come out right. It was like the color problem of early telescopes all over again. Just as flint and crown glass lenses made images bend in complementary ways to produce one perfect image, the Hubble's mirror and altered equipment together create a correct image. Sometimes two wrongs can make a right!

Plans are already underway for a bigger and better orbiting telescope, presently called the Next Generation Space Telescope. Early ideas for this next space telescope included possibly installing it on the moon, which would allow it a stable foundation instead of needing complex control systems to point it steadily in space. But although the moon has no atmosphere to interfere with such a telescope, there are limitations to any telescope which stands anywhere on firm ground. The telescope would be restricted to pointing only to the half of the sky it is facing, and the sun and sunlit Earth would have to be constantly avoided. The current plans are instead to put an 8-meter or 314-inch reflecting infrared telescope in deep space around the year 2007. A far-Earth orbit is planned to help keep the equipment at a colder temperature and to eliminate the problems of having to avoid a close sunlit Earth and moon so much of the time. [NGST planning drawing]

While the Hubble can detect the near infrared, which is closest to the visual wavelengths, the Next Generation Space Telescope will cover longer wavelengths as well so it can study the first stars and galaxies that formed after the universe cooled. This is possible to see when looking with an infrared telescope, since the process of star formation is thought to be very violent, releasing energies hundreds of billions of times more than our sun. Even though these events happened so long ago, they still exist visibly for us, since the light we see from these distant stars was radiated billions of years ago.

Understandably, once telescopes got very powerful and could see to the visible edges of the universe, the planets in our own solar system sometimes got neglected in favor of the farthest stars, nebulae, and mysterious quasars. But during the last 30 years, we no longer had to peer at planets like Jupiter and Saturn using just a mere 200-inch mirror. With robotic spacecraft, we can now travel to the planets and take our pictures close up!

- The History of the Telescope Website







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