Astronomy and Radio Waves
Astronomy may be the oldest of the sciences. In ancient times, the
term included astrology (the idea that the positions of the stars and
planets influence our lives), navigation, medicine, metaphysics, and
religion--any subject believed to be related to or influenced by the
stars.
Nowadays "astronomy" refers to the methodical scientific study of
the physical nature of the planets, stars, and galaxies of our
universe. It no longer includes astrology. Modern astronomers hold
doctoral degrees in physics, chemistry, or mathematics and conduct
research to understand the universe and its processes. Topics involve
sizes, masses, forces, and energies impossible to duplicate in
laboratories on Earth. This research brings not only a new
understanding of our universe but also contributes new developments to
our technology--many of which have commercial and educational
applications.
Cosmic Radio Waves
Astronomers make use of all possible information coming from
astronomical objects, even information we cannot see. For example, all
objects in the universe emit radio waves naturally. We call these
cosmic radio waves. Observations of cosmic radio waves has
revealed an "invisible" universe whose existence was undreamed of a
century ago.
Radio waves are a part of the family of electromagnetic
waves, which include gamma rays, x-rays, ultraviolet rays, light, and
infra-red rays. Those waves to which our eyes respond are called
"light". Our bodies sense infra-red waves as radiant heat. At long
wavelengths, cosmic radio waves tend to come from violent processes in
the universe such as exploding stars or galaxies. At short
wavelengths, they tend to come from cold objects like the dense gas
clouds from which stars form. All electromagnetic waves travel to us at the
"speed of light".
The photograph above shows contours of very high-frequency radio
emission overlaid on an optical photograph of the Great Nebula in the
constellation Orion. The radio emission comes from dust behind the
nebula in which stars are forming. This dust cannot be seen by optical
telescopes. The new stars heat the dust and gas that surrounds them to about
60 to 100K (-350 to -280F), and this warm dust emits principally in the
far infrared and sub-millimeter radio ranges. Astronomers refer to
such regions as "stellar nurseries" and to the dust as "cocoons" for
the new stars forming inside them. When the larger stars are fully formed,
their surfaces are extremely hot, and their radiation ionizes much of the
gas surrounding them to form a glowing nebula like the Great Nebula.
When they reach Earth, radio waves from astronomical bodies are
extremely weak compared with those produced artificially by radars,
broadcast stations, microwave ovens, garage door openers, and other
devices. Finding observatory locations far away from man-made radio
transmitters is a problem for radio astronomers.
Although all radio waves vibrate much more slowly than light waves,
some radio waves vibrate more quickly than others. Radio waves can
also be characterized by their wavelengths, long wavelength
radio waves vibrate more slowly than short wavelength radio waves. The
12-m telescope is used to observe radio waves whose wavelengths are
only a few millimeters. These millimeter waves are best
observed in dry climates, where there is little atmospheric water vapor
to absorb them. These conditions exist on southwestern mountaintops
like Kitt Peak during the non-summer months and at high altitudes.
The 12-m Telescope
The metal "dish" shown in the photograph is an astronomical
telescope. "Telescope" comes from a Greek word meaning "far-seeing".
The telescope collects radio waves from the small region of sky toward
which it is pointed. These waves fall upon its parabolic surface,
which redirects them up to a smaller hyperbolic mirror at the
apex of the telescope, which redirects them downward through the
central hole in the primary mirror to the Cassegrain focus
behind the mirror and into specially designed radio receivers. The
receivers transfer the information carried on the incoming waves, such
as intensity and polarization, that can only travel through pipes known
as waveguides to much lower frequencies that travel easily on
wires. This transfer process is known as a heterodyne
conversion. Computers process the information and display it on
television-like monitors in a form that astronomers can interpret.
The 12-m telescope consists of five parts: the pedestal at the
bottom, the fork or yoke which holds the main reflector, the
main reflector or mirror, the small secondary mirror or
subreflector at the top, and the 95-ft astrodome
enclosing the telescope. Although the telescope structure is mostly
steel, its reflecting surfaces are aluminum.
Critical to the telescope's performance is the smoothness or
surface accuracy of the primary mirror. The mirror's diameter
is 12 meters (40 feet). To reflect waves efficiently, the surface
accuracy of the reflecting surfaces must be many times smaller than the
length of the waves. The secondary mirror and each of the 72 aluminum
panels have been machined to an accuracy of 25 micrometers (microns),
or 1 thousandth of an inch. Engineers have adjusted the positions of
these panels to form a parabolic surface accurate to 75 micrometers or
3 thousandths of an inch--approximately the thickness of a sheet of
letter paper. The steel tubular framework or back structure
supporting these panels is stiff enough to maintain this accuracy
regardless of where the telescope is pointed. This accuracy allows
observations at wavelengths as short as 0.8 millimeters, or frequencies
up to 345,000 megahertz (cycles per second)--about 4,000 times higher
than frequencies used by FM broadcast stations and about 400,000 times
higher than those used by AM broadcast stations.
Astronomers aim the telescope at astronomical objects by electric
motors controlling the azimuth (East-West angle) of the yoke and the
elevation (Up-Down angle) of the primary mirror. Specially designed
electrical instruments, called angle encoders, read the azimuth
and elevation angles to a fraction of a second of arc. Comparing these
readouts with calculated positions, a computer points the telescope to
an accuracy of a few seconds of arc--an angle corresponding to a dime
seen edgewise from a distance of 10 miles. Because the large telescope
surface acts like a sail, the surrounding astrodome ensures accurate
pointing by protecting the telescope from winds.
Observations are made through the opening, or slit, of the
astrodome. If the winds are high, or if it rains or snows, a door is
rolled across the slit to protect the telescope. Of course, it is
usually impossible to observe in such weather.
Unaffected by light, this radio telescope observes astronomical
objects 24 hours each day. It closes for 6 hours one day a week for
scheduled maintenance during the dry non-summer months. The wet summer
months are used for repairs and improvements.
Constructed in 1967, this telescope helped open a new area in
astronomical research through its ability to observe millimeter waves
and because of the high quality of its electronics. Because of its
success, many countries have built similar telescopes. The 12-m
telescope was resurfaced in 1983 to enable it to operate at higher
frequencies.
The Radio Receivers
Because astronomical radio waves are extremely weak, the radio
receivers have to be as sensitive as possible. Usually, the receivers
are simple mixers that use exotic semiconductors called
Superconductor-Insulator-Superconductors, or SIS junctions, to
detect and convert the incoming waves to much lower frequencies. These
mixers are cooled to a temperature of only a few degrees above absolute
zero (about 4K or -455F) to reduce radio noise generated internally.
The cooling device is like that in a home refrigerator or
air-conditioner where liquid Freon expands into a gas through a tiny
hole. This device is called an evaporator. For radio
receivers, we use helium instead of Freon to enable the evaporator to
reach temperatures as low as 1K (-458F). When the isotope 3He is used,
one can reach temperatures as low 0.3K (-459F). Large thermos bottles
or dewars enclose these mixers to insulate them from the warm
atmosphere.
The signal emerging from these receivers goes to a
spectrometer, which measures the variation of the strength of
the cosmic radio waves as a function of frequency. A computer plots the
resulting radio spectrum on a television-like monitor for the
astronomer to examine.
Astronomical Use of Millimeter Waves
The millimeter-wave range is especially useful for studying the
characteristics of enormous, cold gas clouds in which stars form.
Because they are as cold as 20K (-440F), these clouds emit no light,
rendering them invisible to optical astronomers except when the lie in
front of bright stars. Even at these low temperatures, chemical
reactions occur within these clouds, producing molecules like carbon
monoxide, formaldehyde, ethyl alcohol, methyl alcohol, silicon monoxide,
formic acid, and countless others. These molecules emit radio waves in
the millimeter range. Studying these molecules tells astronomers about
the physical conditions within these clouds associated with formation
of stars, with the evolution of galaxies, and about chemical reactions
peculiar to astronomical environments that cannot be duplicated in
laboratories on earth.
The figure above is a spectrum taken with the 12-m telescope of the
dust region behind the Great Nebula in Orion, also shown above. This
graph is what the astronomer sees at the telescope. Most of the
baseline in the spectrum is random radio noise that decreases as
additional spectra are added together. The strongest spike is emitted
by deuterated water (HDO) molecules in the dark cloud. Other spikes
are identified as deuterated formic acid (DCOOH), cyanamid (NH2CN),
dimethyl ether ((CH3)20), and ethyl cyanide (EtCN).
A publicly-funded, national resource, the 12-m telescope is
available to astronomers and graduate students world-wide on a
competitive basis judged by the content of their observing proposals.
Typically, an observing program is scheduled for 4 days, depending upon
the complexity of the project. Even operating 24 hours each day, the
12-m telescope accommodates less than half of requests.
The data gathered during an observing period is stored and
transported on magnetic tapes. The analysis process takes many weeks
and, normally, occurs at the astronomer's home institution. A
successful observing program results in a publication in a scientific
journal, through which other astronomers worldwide learn what was
observed and what was discovered. Subsequent observing proposals (and
theories) build upon the results of previous observations.
The National Radio Astronomy Observatory
The NRAO equips, maintains,
schedules, and operates the 12-m telescope. It is a service
organization that makes cutting-edge telescopes available to all
qualified astronomers and graduate students at no charge through a peer-review
selection process. Founded in 1956, the NRAO operates radio telescopes in
Arizona, New Mexico,
and West Virginia. It also
operates the
Very Large Baseline Array -- a continental-size radio telescope
consisting of ten 25-m (82-ft) antennas situated across the country from St.
Croix, US Virgin Islands, to Mauna Kea in Hawaii. The NRAO
headquarters are in Charlottesville,
Virginia. It has approximately 400 employees, of whom about 50 are
astronomers who must also go through the same peer-review process to use
NRAO telescopes.
The NRAO is a facility of the National
Science Foundation, operated by
Associated Universities, Inc.,
under a cooperative agreement. AUI is a non-profit organization
incorporated in 1948 consisting of Brown University, Columbia
University, Cornell University, Harvard University, the Massachusetts
Institute of Technology, Princeton University, the University of
Pennsylvania, the University of Rochester, and Yale University.
Written by Mark A. Gordon
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