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BACKGROUND

The more recent history of radio astronomy begins in 1931 when an American engineer named Karl Jansky, while working for Bell Telephone Laboratories, conducted experiments on radio wavelength interference. Jansky detected three separate groups of static; local thunderstorms, distant thunderstorms and a steady hiss-type static of unknown origin. The unknown source that Jansky found is the center of the Milky Way as he was able to show by determining its position on the sky.

Jansky was the first to detect radio emission from the Galaxy. The image above shows Jansky standing with his antenna (Photo courtesy of Bell Laboratories). This rotatable antenna looks similar to a merry-go-round; the rotation allowed it to move along with the static. The work done by Jansky included receiving frequencies in the range of 15 to 30 MHz (approximately 15-m wavelengths). Jansky published three reports on his findings, which were largely ignored for many years to come. The field of radio astronomy would eventually recognize him with a unit named for him; the Jansky is equivalent to 10^(-26) watts per m^2 per Hz.

In 1937 Grote Reber, also a radio engineer, read about Jansky's work. Reber built a parabolic, 9.5-m diameter, reflector dish in his backyard. This was the first radio telescope used for astronomical research. Reber spent years studying cosmic radio waves at various wavelengths, while other astronomers still didn't get involved. He finally detected celestial radio emission at approximately 2-m. The image below shows Reber with his telescope, the prototype for modern radio telescopes (courtesy of NRAO). Reber continued his investigations of radio sources and confirmed that radio emission arose from the Galactic plane. Reber, in 1944, published the first radio frequency sky maps. Reber's telescope is displayed at the National Radio Astronomy Observatory (NRAO) in Green Bank West Virginia.

The first observation of radio emission from the sun was made in 1942, by J.S. Hey. Hey was working with the British Army Operational Research Group analyzing all occurrences of jamming of Army radar sets. A system for observing and recording jamming was organized. This eventually led Hey to conclude that the sun was radiating intense radio emission. Later that same year, G.C. Southworth made the first successful observations of thermal radio emission from the sun; he did this at centimeter wavelengths. The next important discovery regarding radio waves from beyond the solar system were discrete sources of emission. In 1946, J.S. Hey, S.J. Parsons, and J.W. Phillips observed fluctuations in the intensity of cosmic radio waves from the constellation Cygnus. In the next ten years thousands of discrete sources were identified, including galaxies and supernovae.

Most gases in galaxies are invisible to optical telescopes but can be seen by radio telescopes. Fast moving electrons, neutral atoms and molecules generously emit at radio wavelengths. In 1951, H. I. Ewen and E. M. Purcell, detected the spectral line emission from neutral Hydrogen that fell into the radio spectrum. For the first time, astronomers could determine the shape of our own home galaxy.

In 1963 Bell Laboratories assigned Arno Penzias and Robert Wilson the task of tracing the radio noise that was interfering with the development of communication satellites. Penzias and Wilson discovered that no matter where the antenna was pointed there was always non-zero noise strength, even where the sky was visibly empty. A simple solution would have been to reset their receivers to zero, but they persisted in tracing the source. This major discovery made by Penzias and Wilson was the cosmic background radiation and the strongest evidence for the big bang. Penzias and Wilson won the Nobel Prize in physics for their discovery in 1978. The horn shape was used because the field of view remains unobstructed allowing for a precise measurement of the effective collecting area of the antenna.

In the late 1960's, radio pulsars, predicted only by theories of stellar evolution, were discovered by Jocelyn Bell-Burnell and Anthony Hewish. Bell-Burnell and Hewish were working at what is now called the Nuffield Radio Astronomy Observatory at Cambridge, England. Pulsars are very strongly magnetized, spinning neutron stars. Neutron stars are so dense that one teaspoon of this star would weigh as much as all the cars and trucks in the U.S. put together. Anthony Hewish and Martin Ryle won the Nobel prize for this discovery in 1974.

What comes out of a radio telescope is not a picture, as we cannot picture radio waves. A change in the intensity of radio waves is marked by a change in the voltage at the output of the radio astronomy receiver. This was recorded on a strip chart in the early days, now the data, which is the time, the voltage output of the receiver and the point in the sky at which the radio telescope is pointing, is organized by a computer and recorded on a hard disk or on computer tape. Here is a sample from our radio telescope:

Sagittarius A, The Black Hole at the Center of Our Galaxy.

Observe the above graph. The numbers across the bottom are the number of seconds since the observation file started. The graph starts at 1800 seconds, which is 5 hours after the observation file started. The file name, 03290000.txt, appears in the upper right corner of the graph. The file name is based on the starting month, day, hour and minute in "mmddhhmm" format. This file started on March 29 at 00:00, which is midnight. Therefore, the graph starts at 5 am on March 29. At about 5:33 am, a radio source transited the south meridian ( moved across the beamwidth of the dish which was pointed south ). The increase in radio noise raised the total power in the 35 MHz wide spectrum the receiver "sees" at one time. That resulted in the output voltage of the receiver increasing, which is the vertical component of the graph. The voltage went up from 1.8 volts to 2.4 volts.

How did we know that it was the black hole first discovered by Jansky? We had access to the co-ordinates of Sagittarius A (right ascension and declination) from several sources. One source of that information is the astronomical database of radio, x-ray and gamma ray sources called the  Vizier database . If you check the time of the transit, you will discover that the dish was a pointing a few degrees away from due south.

If you looked at the graph with care, you noticed a second graph line just above the 0.5 volt grid line. That is the temperature, where 0 is 0 Deg. F and 1.00 is 100 Deg. F. The temperature affects the LNB, part of the receiver mounted right on the dish. The LNB has greater gain when it is cooler. However, that day the temperature stayed around 60 deg for the period of the graph. Since March 29, 1989, we have had a temperature sensor right on the LNB.

Actually, the temperature shown in the above graph is the temperature in a box on the post on which the dish is mounted. If you graph the entire file 03290000.txt on our graph a file on the web sub-page, you will see that we moved the temperature sensor that day, as the temperature goes to 0 when we pulled the power to the sensor. There are very strong spikes in the receiver output when we got in front of the dish on a stepladder to attach the sensor to the LNB! Apparently our bodies are warmer than cold sky!

You may have seen pictures showing part of the sky as it appears at radio frequencies. Those are a graphical combination of data from many observations at a particular frequency. Here is one:

The Plane of our Galaxy With Sag. A in the Center.

The above graphical composite is from NASA and is the result of many observations You can go to the Skyview Virtual Observatory  and see any area of the sky of your choosing at many radio frequencies and by X-ray and gamma ray emissions!

This information page is long enough, and we haven't begun to really explain radio astronomy. We really don't have to, as Haystack Observatory has a excellent tutorial on radio astronomy.  NASA also has a very good simple explaination of radio astronomy on the web in their Basics of Radio Astronomy . If that is not sufficient information, the real source of sources about radio astronomy is Radio Astronomy, 2nd Edition, by John D Kraus .