MilliMeter Astronomy


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Millimeter Astronomy


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.

Written by Mark A. Gordon


 Copyright Arizona Radio Observatory.
For problems or questions regarding this web contact [tfolkers{at}email{dot}arizona{dot}edu].
Last updated: 11/08/11.