- Green Bank Observatory - http://greenbankobservatory.org -

Reber Telelescope

Reber Telescope

Reber Telescope

Grote Reber

Grote Reber was born in Chicago on December 22, 1911 (d.12/20/2002). He was a ham radio operator, studied radio engineering, and worked for various radio manufacturers in Chicago from 1933 to 1947.

He learned about Karl Jansky’s discovery (1932) of radio waves from the Galaxy (i.e., the Milky Way), and wanted to follow up this discovery and learn more about cosmic radio waves.  Were the waves coming only from the Milky Way, or from other celestial objects?   What process produces the radio waves?

In the 1930s Reber applied for jobs with Karl Jansky at Bell Labs and with astronomical observatories to study cosmic radio waves, but none of them were hiring at the time, since it was in the middle of the great depression. Reber decided to study radio astronomy on his own.

The telescope was constructed by Grote Reber in 1937 in his back yard in Wheaton, Illinois (a suburb of Chicago). He built the telescope at his own expense while working full time for a radio company in Chicago. This shows the telescope as it was in Wheaton, Ill.

The mirror, made of sheet metal 31.4 feet in diameter, focuses radio waves to a point 20 feet above the dish. The cylinder contains the radio receiver which amplifies the faint cosmic signals by a factor of many million, making them strong enough to be recorded on a chart. The wooden tower at the left is used for access to the receiver.

Reber built a parabolic dish reflector because this shape focuses waves to the same focus for all wavelengths. This principle had been used for a long time by astronomers for design of optical telescopes, to avoid chromatic aberration. Reber knew that it would be important to observe a wide range of wavelengths of radiation from the sky in order to understand how the radiation was being produced. A parabolic reflector is usable over a wide wavelength range.

Observations

Reber spent long hours every night scanning the skies with his telescope. He had to do the work at night because there was too much interference from the sparks in automobile engines during the daytime.

The first receiver designed for 3300 MHz failed to detect signals from outer space. So did the second, at 900 MHz. Finally a third receiver at 160 MHz (1.9 meters wavelength) was successful in detecting radio emission from the Milky Way, in 1938, confirming Jansky’s discovery.

Chart recordings from Reber’s telescope made in 1943. The spikes or “fuzz” are due to interference from automobile engine sparks. The broader peaks are due to the Milky Way and the Sun. This chart recording is a copy of part of Figure 5 from “Cosmic Static”, by Grote Reber, in the Astrophysical Journal, Vol.100, page 279, 1944.

Discoveries

Reber surveyed the radio radiation from the sky and presented the data as contour maps showing that the brightest areas correspond to the Milky Way. The brightest part is toward the center of the Milky Way galaxy in the south. Other bright radio sources, such as the ones in Cygnus and Cassiopeia, were recognized for the first time.

The contour diagram at left is copied from “Galactic Radio Waves” by G.Reber, which was published in Sky and Telescope, Vol.8, No.6, April, 1949. The diagrams are plotted in galactic coordinates in which the galactic equator runs horizontally. Most of the radio radiation is in or near the galactic equator. The vertical axes are galactic latitude in degrees. The horizontal axes are galactic longitude, in which the direction toward the center of the galaxy has longitude=0.

In the years from 1938 to 1943, Reber made the first surveys of radio waves from the sky and published his results both in engineering and astronomy journals. His accomplishments insured that radio astronomy became a major field of research following World War II. Research groups in many countries began building bigger and better antennas and receivers to follow up on Reber’s discoveries.

Grote Reber donated his telescope to NRAO at Green Bank, WV, and supervised its assembly there in the early 1960s. It remains there as a historical monument. It was put on a turntable allowing it to point in any direction. This picture was taken in the late 1970’s after the telescope was painted red, white, and blue for the US bicentennial.
Reber visited NRAO in Green Bank on many occasions.  While he was supervising the assembly of his telescope, he also supervised the construction of a full scale reproduction of the Jansky antenna.

What produces the radio emission?

The process that produces the emission can be deduced from the spectrum, i.e., the graph of how power changes with frequency. Reber found that the radio power was weaker at higher frequencies, contrary to what was predicted by the theory of thermal radiation. This theory applies to the light from stars, or any hot object such as molten iron or stove burners, and predicts that the radio emission increases at higher frequencies. But Reber found just the opposite relation for the Milky Way. Some other, “non-thermal”, process had to be at work.

It was not until the 1950s that a Russian physicist, V.L.Ginzburg, worked out the theory of synchrotron radiation, which explains the observed radio spectrum. Synchrotron radiation results from electrons moving at speeds close to the speed of light in magnetic fields. Our galaxy is full of high speed charged particles, including electrons, known as “cosmic rays”. We now believe that these particles were blasted into interstellar space as a result of supernova explosions. This is the origin of most of the radio radiation from the Milky Way that Jansky and Reber measured.

Later work by Reber

In the 1950s, Reber sought a field that seemed neglected by most other researchers and turned his attention to cosmic radio waves at very low frequencies (1-2 MHz, or wavelength 150-300 meters). Waves of these frequencies cannot penetrate the Earth’s ionosphere except in certain parts of the Earth at times of low solar activity. One such place is Tasmania, where Reber lived for many years. He died in Tasmania on December 20, 2002.

Obituary

An obituary for Reber was written by Ken Kellermann and published in Nature for February 2003 (vol.421, page 596). [1]