Radio Astronomy

Ellen Blevins

CECS 5400

Fall 1999

Optical astronomy with the naked eye is at least 5000 years old and possibly much older. The optical telescope astronomy is about 400 years old. Radio astronomy is the youngest in the family at a mere 70. But it has only been since the 1970 the electromagnetic spectrum has been regularly explored for the astronomical information they may yield.

Until the twentieth century, astronomers had no way to "see" the nonvisible electromagnetic radiation that reached Earth from the universe. Radio astronomy is simply the study of the universe at radio wavelengths, its discovery was an accident that happened in the early 1930’s.

The first true radio astronomer was not trained as an astronomer at all, but as a physicist. Karl Jansky, the son of a Czech immigrant who settled in Oklahoma and later acquired a degree in physics at the University of Wisconsin. After graduating he went to work in 1928 as a telephone engineer with Bell Labs. The phone company was looking for a way to make telephone communication possible with short-wave radio. The transmissions were plagued with static. Jansky was to identify and eliminate sources of static at a wavelength of 14.6 m in an effort to track down the precise sources of radio interference.

On a farm in New Jersey, not far from the lab, Jansky set up an odd-looking device he called a merry-go-round. It was a large directional antenna that looked rather like a biplane wing, mounted on some discarded Model T Ford wheels. It could be rotated through 360 degrees by means of a motor. Jansky was soon able to identify all the known sources of radio interference except one. The unidentifiable sound, when amplified and sent to speaker, sounded like a faint hiss.

By the spring of 1932, Jansky traced the primary source of the radio noise to the direction of the constellation Sagittarius, essentially the center of the Milky Way Galaxy. He concluded that whatever the source of radio noise, it probably was not generated by the distant stars. The discovery was published in late 1932 and the detection of radio signals from space appeared in nation newspapers by the following year.

Strangely enough he did to pursue this discovery but another non-astronomer did. Grote Reber, who had grown up tinkering with radio transmitters, building one powerful enough to communicate with other ham radio operators all over the world during his youth. He became an electrical engineer, but never lost his interest in amateur astronomy. When he read about Jansky’s discovery that speculated the world was constantly being bombarded with cosmic radio waves, he tried to adapt his own short-wave receiver to pick up interstellar radio waves with wavelength of 10 cm.

He tried longer wavelengths, and in 1937 built a paraboloidal antenna 30 feet in diameter. With this, Reber not only confirmed Jansky’s discovery of radio waves from the direction of Sagittarius, but found other sources in the direction of the constellations Cygnus, Cassiopeia, and elsewhere. He confirmed that the radio signals did not coincide with the positions of visible stars. Directing his dish toward the bright stars of Sirius, Vega, or Rigel, he detected nothing. But looking toward a starless area in Cassiopeia, he picked up strong radio waves. He had unknowingly detected a supernova remnant know as Cassiopeia A.

The inventor of the first radio telescope, Reber is considered by many to be the father of radio astronomy. The basic unit of radio brightness is called a Jansky(Jy), and some consider the discovery his right to the title. Reber’s 30 foot dish was the only radio telescope in the world until after WWII, and by 1942 he had completed the world’s first preliminary radio maps of the Milky Way Galaxy. The spiral form of our own Galaxy was first mapped using the 21 cm radio spectral line. The very center of our own Milky Way Galaxy is hidden from optical probing, so that most of what we know of our galactic center has come from infrared and radio observation .

The end of the war started a period of intense research in radio astronomy all over the world. Radar engineers, particularly in England and Australia turned their talents to the pursuit of more distant sources using radio telescopes. Reber moved during this time with his telescope to Sterling, Virginia, and was given a government job in Washington, D.C. as chief of the Experimental Microwave Research Section. In 1951, he built a new radio telescope on an extinct Hawaiian volcano and mapped out low-frequency, or long-wave, celestial signals. In pursuit of a bigger radio window, he moved in 1954 to Tasmania, Australia, a place where the earth’s atmosphere is occasionally transparent to electromagnetic to work at the newly created National Radio Astronomy Observatory in Green Bank, West Virginia. Green Bank stills serves the NRAO as mainstay in their operations today.

The basic anatomy of a radio telescope hasn’t changed much from Reber’s dish. The instruments have become much larger and the electronics more sophisticated. A radio telescope works just like an optical telescope. It collects radio frequency waves and focuses them on a detector. The large, curved, metal dish is supported on a moveable mount. A detector, called a receiver horn, is mounted on legs above the dish (prime focus) or below the surface of the dish (Cassegrain focus). The telescope is pointed toward the radio source, the dish collects the radio waves and focuses or reflects them on to a focus point above the center of the dish to the receiver. At this point, an aerial intercepts the radio waves and turns them into a weak electrical signal which is sent to a computer. Radio telescopes detect very weak waves, and can also communicate with spacecraft. By detecting radio waves coming from galaxies and other objects in space, radio telescopes have discovered the existence of may previously unknown bodies. It is possible to make visible images of radio sources by scanning the telescope or a group of signals from different parts of the source, which the computer can process to form an image. Differences in frequency of the signals give information about the composition and motion of the radio source. Television programs broadcast from a satellite are received by a satellite dish, which is like a small radio telescope. The curved surface reflects the incoming radio waves to meet at a central antenna. The picture signal then goes from the antenna to the television set.

Astronomers used to categorize themselves by the wavelength of the observation that they made. Increasingly, though, they define their work more by what they study, as the field expands and specialization becomes more prevalent. And although professional radio astronomy typically involves elaborate equipment and massive facilities, amateur radio astronomers can use equipment as modest as an ordinary FM radio.

Since the radio spectrum is so broad, astronomers have to decide which portion of the radio spectrum they will observe. Different receivers are used for observation at different frequencies. Receivers are either swapped in and out, or more typically the radio signal is directed to the correct receiver by adjusting a secondary reflecting surface.

The resolution of a telescope depends not only on its diameter, but the wavelength of the detected radiation. Radio waves are big and telescopes that detect them are correspondingly huge. Very good optical telescopes located on the earth’s surface can resolve celestial objects to 1". The best angular resolution that a very large single dish radio telescope can achieve is ten times this, about 10", and this is possible only with the largest single dish radio telescopes in the world. The NRAO is currently building at Green Bank, West Virginia the world’s largest fully steerable radio telescope.

The world’s largest non-steerable single dish radio telescope was built in 1963 at Arecibo, Puerto Rico, and has a dish, 300 meters in diameter sunk into a natural valley. Because it is totally immobile it is limited to observing objects that happen to pass roughly overhead as the earth rotates.

Low angular resolution due to the size of radio waves may be overcome by linking together a lot of smaller telescopes to obtain greater detail. Steerable telescopes should not be built bigger than approximately 330 feet in diameter. A radio interferometer is a combination of two or more radio telescopes lined together electronically to form a kind of virtual dish, but it does not have the sensitivity to faint sources of a truly gigantic telescope. Since radio interferometers are detecting an interference pattern, radio data has to be processed in ways different from optical data. But the end result is either a radio image, showing the brightness of the source on the sky, or a radio spectrum, showing a spectral line or lines.

The National Radio Astronomy Observatory maintains and operates, the Very Large Array interferometer near Socorro, New Mexico, consisting of 27 larges dishes arrayed on railroad tracks in a Y-shaped pattern. Each arm is 12.4 miles long, and the largest distance between 2 of the antennas is 21.7 miles. The effective result, the VLA has the resolving power of a radio telescope 21.7 miles across.

The Very Long Baseline Array can link radio telescopes in different parts of the world to achieve incredible angular resolutions. From Socorro, New Mexico, the NRAO also operates the VBLA which consists of 10 radio dishes scattered over the United States, from Mauna Kea, Hawaii, to St, Croix, U.S. Virgin Islands. In 1969, Japanese astronomers launched into Earth’s orbit a radio telescope to be used in conjunction with the ground based telescopes in order to achieve the resolution of a telescope larger than the earth itself.

In recent year, astronomers have launched instruments into orbit that can detect all segments of the electromagnetic spectrum, from infrared, through visible, and on to ultraviolet, X-ray, and gamma rays. The highest frequency radiation(X-rays and gamma rays) comes from some of the most energetic and exotic objects in the universe. Ultraviolet radiation, which begins in the spectrum at frequencies higher hen those of visible light, is also being studied with new telescopes. Since our atmosphere blacks all but a small amount of ultraviolet radiation. Ultraviolet studies must be made by higher-altitude balloons, rockers, or orbital satellites.

Telescopes equipped to detect infrared radiation, the portion of the spectrum just below the red end of visible light, have been used at high altitudes on the earth as well as in high-altitude balloons and orbital satellites to image celestial objects otherwise invisible, as well as the infrared-emitting aspects of visible objects. With many applications the Infrared Astronomy Satellite was launched in 1983 and sent images back to Earth form many years. Like all infrared detectors, the one on IRAS must be cooled to low temperatures so that their own heat does not overwhelm the weak signals that they are trying to detect. Although the satellite is still in orbit, it has run out of coolant, and can no longer make images. The infrared capability of Hubble Space Telescope has already yielded spectacular results.

The Hubble space telescope is part optical telescope and part satellite. It orbits the Earth and, under radio control from the ground, takes pictures and measurements of planets, stars, galaxies sand other bodies in space. Operating in the total clarity of space, above the obscuring effects of the atmosphere, the space telescope produces sharper pictures than those of Earth-bound telescopes. It has detected faint and distant objects and produced more detailed pictures of known objects, greatly expanding our knowledge of the Universe. The space telescope was launched by the space shuttle in 1990 and improved in 1993 and 1997. It should operate until at least 2005. The Hubble Space Telescope has the capability to detect ultraviolet photons as those with frequencies in the visible and infrared. Ultraviolet astronomy is one way to study the so-called interstellar medium, the matter, gas and dust, between stars, and stars lithe surface temperatures higher than the sun’s.

Electromagnetic radiation at the highest end of the spectrum in now being studied as well. Since x-rays and gamma rays cannot penetrate our atmosphere, all of the work must be done by satellite. An X-ray telescope was launched in 1978, called the High energy Astronomy Observatory , and yet another similar telescope was launched in 1991. X-rays are detected from very high energy sources, such as the remnants of galaxies. AXAF premiere instrument, doing on this region of the spectrum what the Hubble Space Telescope has done in the optical range. The Gamma Ra Observatory was launched yet the space shuttle.

These advance instruments typically have a portion of their observing time set aside for the astronomer who spent years developing the instrumentation. Once more satellites are launched, any astronomer can make a proposal at to obtain observing time. New instruments have opened the electromagnetic spectrum to an unprecedented degree. Astronomers now have the ability to ask questions that can be answered with observations at many different wavelengths. Today’s astronomers have the unique ability to turn to almost any region of the electromagnetic spectrum for answers to their questions.

Electronic information that your eyes take in on clear night comes from a very thin slice of the entire spectrum. The earth’s atmosphere screens out much of the electromagnetic radiation that comes from space. It allows only visible light and a bit of infrared and ultraviolet radiation to pass through a so-called optical window and broad portion of the radio spectrum to pass through a radio window.

References

Axelrod, A. and De Pree, C.(1999). Astronomy: the complete idiot’s guide. Macmillan

Company.

Krupp, E.C.(1984).The Early years of Radio Astronomy: reflections fifty years

after Jansky’s discovery. Cambridge University Press.

Macaulay, D.(1998). The New Way Things Work. Houghton Mifflin Company.

Peterson, C. C. (1995).Hubble Vision: Astronomy with the Hubble Space Telescope.

Cambridge University Press.

Rohls, K.(1996). Tools of Radio Astronomy. Springer.

Verschuur, G.L.(1987).The Invisible Universe Revealed: the story of radio astronomy.