If you have never paid much attention to the state of the
art in telescopes, then you might believe they consist of either
the $99 Tasco jobs in Walmart's toy department or the 200" Hale
atop Mount Palomar. You might also think that objects which
emit or reflect light in the visible spectrum are the only things
studied. You might not know that telescopes have been built
to detect frequencies ranging from HF all the way up into cosmic
rays. Newer telescopes search for gravity waves and even neutrino
and 'dark matter' pseudo particles. You might not know that
there is a broad selection of optical telescopes for amateur
and research grade telescopes and that the line between amateur
and research grade telescopes grows less distinct every year.
Even radio telescope equipment is beginning to hit the market.
This article gives a nice intro to the subject of radio telescopy;
the basics haven't changed.
How We Listen to Stars and Satellites
Radio and radar help astronomers search outer space
It wasn't very long ago that astronomy, like the mythical
Cyclops, had only one eye - the optical telescope. Astronomers
expected that bigger and better giant "eyes" would unlock the
remaining closed doors of the universe. Everything, it seemed,
was progressing quietly in its well-ordered way.
Long-range radar is used to track the Sputniks
in their orbits. A typical transmission-response pattern appears
above the symbol for the International Geophysical Year.
Then a second eye was developed, that of radio astronomy.
A whole new universe opened - an incredible dimension they didn't
know existed. Today radio astronomy is flourishing like a lusty
youngster; it may some day equal or even exceed in importance
the 4000-year-old science of visual astronomy.
Another branch of the science is long-distance radar, which
is now coming into its own in tracking the Russian Sputniks.
As a matter of fact, some of the giant radio telescopes have
had radar antennas installed in them for this job.
Static Identified. Radio astronomy is the science
dealing with radio-frequency emissions from the stars, a phenomenon
never suspected until about 1930, when Karl Jansky, a Bell Laboratories
engineer, began to study and measure different kinds of static
at frequencies around 20 mc. Before long he realized that the
hiss-type static which he encountered was being emitted from
definite points in space. Working in his own backyard observatory,
Grote Reber, another radio engineer and a radio amateur, confirmed
Jansky's discovery. Using a small parabolic antenna, he plotted
the first radio star sky map.
Astronomers call this probable radio source
"NGC 5128" - it is thought to be two galaxies in collision.
This photo was made with the 200-inch Palomar optical telescope.
Giant strides have been made from these small beginnings.
Dozens of radio telescopes are scattered across the earth, their
sensitive antennas constantly probing the heavens, recording
the strange radio impulses. We now know that three types of
"stars" emit radio waves: huge hydrogen gas clouds, made up
of such a thin diffusion of atoms that they would be called
vacuums on earth; novae, which are stars that have exploded
with awesome violence; and collisions of huge star clusters
or universes called galaxies.
Heretofore, one of the stumbling blocks for astronomers had
been the huge masses of dust scattered about the galaxies which
prevented light from more distant stars from being seen on earth.
Scientists could only guess what lay beyond them. Now, however,
radio astronomy is unlocking even this secret. By focusing on
gas cloud signals coming from behind the dust pockets (at a
frequency of 1420 mc.) , astronomers have been able to "count"
the number of stars beyond. Mind you, they are not only counting
the number of gas clouds whose radio emissions pass easily through
the dust but - mathematically - the actual number of "visual"
stars. They have learned that the greater the radio emission
from an area, the greater the number of stars that are located
Sources of Emission. Radio emission from gas clouds
was first predicted in 1944. It wasn't until 1955, however,
that the signal was picked up on radioscopes. But since then
astronomers have made up for lost time. They have used the 1420-mc.
signal, and shifts in this frequency caused by the Doppler effect,
to determine how fast and in which direction the gas clouds
are moving. Such data have allowed them to plot the movement
of our own galaxy, the Milky Way, as well as to gather more
information on our expanding universe.
The second most intense radio source, in the constellation
of Cygnus, has been found to be from two whole galaxies in collision
about 200 light years away. Another is listed only as NGC 5128
by astronomers. Others are being charted.
Our own sun has been proving a fruitful source of radio propagation.
Although the study of the radio spectrum of the sun was begun
only within the past few years, it has been determined that
a huge amount of radio energy comes from the areas around large
and active sunspots. This is in the 5-meter band. It is thought
that the flares, being highly ionized gas, may produce strong
electric fields when given rotational motion, which in turn
may be the cause of the radio emissions.
Scientist measures and records radio observations
of satellites at Lincoln Lab's new long-range radar station,
Of the planets, Jupiter was the first to be picked up on
radioscopes; the signals are apparently due to large-scale atmospheric
disturbances. Venus was next to be detected. Radio-frequency
measurements showed this planet to have a temperature higher
than that of boiling water. Optical measurements had shown only
half that temperature, but since Venus is covered by a layer
of clouds, the optical measurements only took the cloud surface
into account. Mercury, Mars and Saturn are expected to be heard
from soon via their radio signals. And some cosmic static comes
from "dark" areas, where stars have never been seen.
Types of Scopes. The radiotelescope is usually one of two types,
the parabolic reflector (dish), and the interferometer. There
are other types - helical, horn, and combinations of two or
more types. The first two, however, are most generally used.
The largest "dish" is the one just completed and now being
tested at Jodrell Bank in England. This has a diameter of 250
feet, and is steerable, which will allow it to cover all of
the visible sky. It will complement the fixed 220-footer in
operation there for many years. Work has also begun on a 140-foot
steerable dish at Green Bank, West Virginia, which will be the
largest of its type in the United States.
The largest parabolic radio. telescope is
at Jodrell Bank, England; a movable dish, it is a 250-foot monster
which has also been used in tracking the Russian Sputniks.
Another new scope is under construction at the University
of Michigan - this 85-foot dish will be completed in time to
aid the International Geophysical Year effort. Recently completed
was a long-range radar station at Lexington, Mass. Built by
the Lincoln Laboratory, this radar has been used successfully
to track the Sputniks. It is also being used to check the radio
effects of meteors and the aurora.
One of the largest interferometers is near Sydney, Australia.
This consists of an array of dipoles 1500 feet long.
Essentially, the job of the radio telescope is to focus the
radio waves it receives and feed them into a sensitive receiver.
This is analogous to the action of the optical telescope. The
parabolic dish, either solid or made of wire screen or mesh,
reflects incoming radio waves to a focal point, where they are
picked up by a rod or dipole and fed to a receiver. The signal
is amplified, then sent to a mechanical recorder, usually a
pen tracing the signal on graph paper.
Photo of Jupiter, showing the huge "Red Spot"
in the upper left-hand segment; this largest of planets is a
radio source as well.
The interferometer, on the other hand, works in a different
manner. The typical telescope of this sort consists of a flat
array of dipoles. When they face directly toward the emission
source, the wavefront reaches all the dipoles at the same time.
This is shown as a signal of maximum strength. When the signal
comes in at a slight angle, it reaches one dipole earlier than
the next, and the interference of the out-of-phase waves cuts
the signal strength. To improve resolution, interferometers
are built with a second array of dipoles arranged at right angles
to the first array. Where the two signals of maximum strength
intersect, they produce a "pencil" beam which has the resolution
of a huge parabolic antenna.
The 61-foot movable parabolic antenna at
Stanford Research Institute in Menlo Park, California.
Diffuse Definition. Radio waves from space are relatively
unaffected by daylight, cloud or fog, which is a tremendous
advantage over light waves for observation, but their long wavelength
compared to light makes it difficult to gain good resolution.
The beam width depends on the ratio of the wavelength to the
diameter of the telescope. Therefore, in order to gain the resolution
of even a small optical telescope, the antenna of a radioscope
would have to be thousands of miles long. For that reason, radioscopes
are able to define a radio source only diffusely, causing the
observers to concentrate on the shorter wavelength. But as the
larger scopes are built, they will be able to push their studies
up into the longer wavelengths. There is no doubt that even
more and possibly greater surprises are awaiting them in this
With the earth trembling on the threshold of space, it would
not be out of place to predict that man may one day soon construct
even larger radio telescopes between orbiting space stations,
or even on the moon itself. It is impossible to guess what tremendous
discoveries there will be, but one thing is certain - radio
astronomy will some day rank with optical astronomy as one of
the most important of sciences. Indeed, it may even outgrow
its older brother in unlocking the secrets of the universe.
It won't be long now!
How a Radio Telescope Works
Radio telescopes are tuned to receive certain radio frequencies
and indicate the direction from which they come. The two most
common, the parabolic reflector (dish) and the interferometer,
use two different methods to gain the same end. Note that in
the dish (Fig. 1) there is a single response pattern, a fairly
wide one. The dish is used just like an optical telescope: it
"focuses" on a point as closely as possible - by using the maximum
response point "A" on the radio source. On the other hand, the
interferometer (Fig. 2) responds with a series of lobes as the
angle of observation of the radio source changes, alternately
reinforcing and canceling. Using the angles between these peaks,
such as between "X" and "Y," the position of the radio source
can be calculated with a somewhat better degree of accuracy.
February 1958 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Posted October 31, 2013