Radio Astronomy in New Mexico:
The VLA and VLBA

David G. Finley
National Radio Astronomy Observatory*
P.O. Box O, Socorro, NM 87801
Published in the New Mexico Journal of Science
Vol. 35, November 1995
Pages 21-33
Esteban A. Herrera and Kurt S.J. Anderson, Editors


The Very Large Array (VLA) and the Very Long Baseline Array (VLBA) are two of the most important instruments available to the world's radio astronomers. The VLA, dedicated in 1980, has compiled an enviable record of scientific achievement and demonstrated tremendous versatility as a research tool. The VLBA, dedicated in 1993, already has begun to make landmark contributions to our understanding of the universe.

Both of these instruments of the National Radio Astronomy Observatory are facilities of the National Science Foundation, provided at no cost to the scientific community. Observing time is allocated on the basis of peer review. The instruments serve researchers in a wide variety of scientific specialties and from hundreds of institutions around the world. A significant percentage of observing time is provided to graduate students for thesis work, thus making these instruments important resources for training future generations of astronomers.

The administrative and scientific headquarters for both instruments is the Array Operations Center (AOC), a building located on the campus of the New Mexico Institute of Mining and Technology in Socorro. The AOC houses electronics laboratories where equipment for both the VLA and the VLBA is constructed and maintained. In addition, the AOC contains the control room from which the ten stations of the continent-wide VLBA are remotely operated.

The VLA is located 52 miles west of Socorro, on U.S. Highway 60. It is operated from a control room on the site. A number of maintenance facilities also reside on the site. There is a Visitor Center at the VLA, open every day from 8:30 a.m. to sunset, and more than 15,000 tourists take advantage of its free displays and self-guided walking tour every year.

Of the 600 to 800 scientists who are part of research teams using the VLA and VLBA each year, approximately 400 will come to the AOC. These visiting scientists come to New Mexico to prepare for and monitor their observing runs and to use the AOC's computing facilities for data reduction and image processing.

The Very Large Array

The VLA was conceived in the 1960s and built in the 1970s with the aim of producing a very versatile and sensitive radio telescope with angular resolution comparable to that of the best ground-based optical telescopes (1). The VLA was dedicated in 1980. The VLA (and the VLBA also) is an "aperture synthesis" interferometric instrument, designed to gain the resolving power of a very large antenna by utilizing a number of smaller antennas (2). In the case of the VLA, the information from all of its antennas is combined mathematically to produce resolving power equal to that of a single antenna as much as 36 km in diameter.

The VLA is arranged in a "Y" pattern, with nine antennas on each of the three arms, for a total of 27 antennas. The maximum antenna separation ranges from 1 km to 36 km. The antennas are fully-steerable parabolic dishes 25 meters (82 feet) in diameter, weighing approximately 230 tons. A communications system utilizing buried microwave waveguide carries control and monitor information to and from the antennas and the astronomical data from the antennas to the central control building (3).

Along the three arms of the VLA, there are 72 stations where the movable antennas can be mounted. A railroad system allows transporter vehicles to carry an antenna to any of the 72 stations. The entire array of 27 antennas is reconfigured approximately every four months. There are four standard configurations for the VLA, each offering a different range of resolving power, so completing the configuration cycle requires a total of 16 months. Changing from one configuration to another requires an average of two weeks.

The VLA currently is capable of observing at wavelengths from 90 cm to 7 mm, in seven separate segments, or bands. The resolution obtainable depends on the wavelength being observed and the configuration of the array. The greatest resolution ranges from 200 to 1.5 seconds of arc (arcseconds) in the smallest configuration to 6 to 0.05 arcseconds in the largest configuration. Typical sensitivity figures for the VLA range from 1.4 to 0.06 milliJanskys.** Observing projects at the VLA generally take from a half-hour to several days (4).

The astronomical data from all the VLA's antennas are assembled in real time in a process called correlation and the result is recorded on magnetic tape. The observer then takes the data in this form and performs post-processing and image processing on a workstation-class computer. Observers without sufficient computing resources at their home institutions can use NRAO computing facilities at the AOC.

Though a product of designs from the 1970s, the VLA has benefitted from technological advances throughout its service lifetime. The result has been a tremendous increase in capability and versatility over the original designs. Some of the major improvements have resulted from increased receiver sensitivity and the exponential growth in computing capability over the past 20 years. One example of the impact of these advances is that, while the original designers contemplated an instrument that would produce two or three images per day, the VLA now can produce images of strong, isolated radio sources in as little as a few minutes.

To take full advantage of newer technology, the NRAO has begun preparing for a large-scale upgrade of the VLA. This process began with a 1995 scientific workshop in which researchers from various specialties outlined their desires for improved capability of the instrument. Major parts of a proposed VLA upgrade program will include replacing the microwave waveguide system with an optical fiber system, building a new special- purpose correlation computer, and adding receivers to expand the wavelength coverage.

The Very Long Baseline Array

The desire for ever-increasing resolving power led radio astronomers to develop the technique of Very Long Baseline Interferometry (VLBI), in which the antennas of a synthesis array are so widely separated that a real-time communication system among them is no longer practicable. Instead, the signals from each antenna are recorded on magnetic tape and the tapes brought to a single location for the correlation process. This technique was first demonstrated in 1967 (5). VLBI systems currently offer the best resolving power available to astronomy.

Since the 1960s, VLBI observing has been conducted using existing radio observatories, which schedule coordinated observations at intervals of a few months. Such observations yielded valuable scientific results, but the VLBI "instruments" so assembled nonetheless had drawbacks. These included the difficulty of arranging and scheduling coordinated observations operated by multiple organizations, less than optimum siting of the antennas to produce a good synthesis image, and variations in sensitivity, pointing accuracy and frequency coverage of the individual antennas (6).

In 1982, the NRAO proposed to the National Science Foundation to build the VLBA, an array of ten identical radio telescopes dedicated to full-time VLBI observation. VLBA design work was authorized in 1984, and construction was authorized in 1985, with work on the first VLBA station, at Pie Town, NM, beginning in February of 1986. The final VLBA station, on Mauna Kea, Hawaii, was completed in the Spring of 1993, and the whole instrument was officially opened in ceremonies in Socorro on August 20, 1993.

The VLBA employs ten 25-meter diameter parabolic antennas at stations across the continental United States and on Mauna Kea, Hawaii, and St. Croix in the U.S. Virgin Islands. It offers resolution of less than one-thousandth of an arcsecond, high sensitivity, coverage of wavelengths from 90 cm to 7 mm in nine bands, full-time, user-friendly and reliable operation, and the ability to operate in conjunction with non-VLBA antennas (7).

Each of the ten VLBA stations is controlled by an on-site computer, which is in turn controlled by a computer at the Array Operations Center in Socorro. Commands and monitor information are exchanged between Socorro and the stations via the Internet. The array operator in Socorro controls the entire far-flung system and monitors the status of each station's equipment. Technicians at each antenna site conduct preventive and remedial maintenance and handle the magnetic tapes on which the observational information is recorded.

The VLBA tape recorders record data at extremely high density, 3.5 megabytes per square inch, and at very high rates, 128 megabits per second. An 18,000-foot reel of tape will record the data from 10.5 hours of observation.

To provide the precise timing references required to allow the VLBA stations to operate as a single instrument, a hydrogen maser system is used. This hydrogen maser, which also serves as a frequency reference for the receiving systems, has a stability of one part in 2 x 1015. The maser allows "time stamp" information to be included in the data recorded on the magnetic tape.

Tapes are shipped from the stations to the VLBA correlator in Socorro. The VLBA correlator, designed and built by NRAO, receives data from a bank of tape recorders. Tapes from all the VLBA stations, plus as many as ten additional observatories (including planned Japanese and Russian radio astronomy satellites), are played back simultaneously, and, through the precise "time tags" placed on the tapes by the hydrogen masers at each site, the observation is essentially re-created for the correlator. The correlator is a specialized, extremely high-performance computer operating routinely at a speed of 750 billion floating-point operations per second.

In the correlator, signals from different sites are manipulated mathematically to make it appear that all observing stations are on the same plane surface perpendicular to the direction toward the object observed, despite the great distances between the stations and curvature and rotation of the earth. Tiny differences in the arrival times of radio waves from different parts of the celestial body modify the combined signal detected by pairs of antennas. Data from all pairs of the array are combined to produce the information necessary to make an image of the observed object. The correlator's output can then be used by astronomers with workstations to perform post-processing and image processing.

Since its official opening in 1993, the VLBA has been used for scientific observing for a steadily-increasing percentage of the time, aiming toward a full, 24-hour, 7-day schedule. At the same time, system improvements and software development for the correlator have been ongoing. The wavelength coverage is being expanded by addition of receivers for a band at 3.5 mm.

Scientific Programs

Versatility is a hallmark of both the VLA and the VLBA. Both instruments, when brought on-line, offered new capabilities to the scientific community. Members of the community have proven highly resourceful in using the capabilities of these instruments for a wide variety of investigations. The VLA, for example, has been used to observe objects as near as the Moon and near-earth asteroids, as far as quasars at the edge of the observable universe, and nearly everything in between. Radio observations are often scheduled in conjunction with observation of the same objects at other wavelengths by ground-based and orbiting instruments.

Closer to home, the VLBA is an important tool for geophysics. The technique of VLBI can produce not only extremely high resolution for observing astronomical bodies, but also, using distant quasars as an inertial reference system, extremely precise three- dimensional positional information on the observing stations themselves. This information is used by geophysicists to learn about continental drift and the rotation and orientation of the earth in space. This can shed new light on such phenomena as: the earth's mantle convection, plate motion and earthquakes; atmospheric and ocean loading on the crust; and the gravitational effect of the Moon, Sun and planets.

Within the Solar System, the VLA routinely observes the Sun, planets, asteroids, and comets. The NRAO scientific staff includes both a full-time solar astronomer and a planetary scientist. Planetary observing at the VLA employs both passive observations and radar observations in conjunction with NASA's solar-system radar transmitter at Goldstone, California. Observations of Jupiter during the impacts of fragments of Comet Shoemaker-Levy 9 in 1994 revealed that the impacts significantly disrupted the pattern of microwave emission from the planet's radiation belts (8). The wide variety of other planetary studies ranges from monitoring atmospheric water vapor on Mars to detailed studies of Saturn's ring system.

Stellar astronomers use the VLA and VLBA to study the whole life cycle from protostars to supernova remnants. The recent addition of 7 mm receivers to the VLA, funded by the government of Mexico, has provided a powerful tool for probing young stellar objects and their environments. This includes studies of protoplanetary disks. Both the VLA and VLBA observe newly-discovered supernovae as targets of opportunity, and then monitor these objects to obtain radio light curves and, in the case of the VLBA, to measure the expansion of the shell of explosion debris in nearby galaxies.

The study of galaxies in all their variety and complexity is a mainstay of research with both instruments. This starts with studies of our own Milky Way galaxy, and both instruments are used to probe the region near its center, which contains a black-hole candidate. Studies of other galaxies probe the composition, dynamics and evolution of those galaxies. Galactic mergers are an expanding area of research.

The VLA and VLBA are frequently used to study the variety of active galactic nuclei, from different types of radio galaxies to extremely energetic quasars. Such studies seek to resolve the many outstanding questions about the environments and inner workings of these powerful "engines." Many observers hope to use detailed radio images to test theoretical models of these active galactic nuclei.

A large amount of VLA observing time currently is devoted to a project of long-term importance to the astronomical community -- the conduct of two large-scale surveys of the sky at radio wavelengths. These surveys are aimed at providing a reference resource to serve scientists for many years to come. The first of these, the NRAO VLA Sky Survey (NVSS), is designed to observe the 82 percent of the sky visible from New Mexico by the end of 1996. This survey is expected to produce a catalog of two million cosmic radio sources, most of which never have been seen before (9). The other survey, called Faint Images of the Radio Sky at Twenty-centimeters (FIRST), is producing more-detailed images of a region of the North Galactic Cap which also is being surveyed at optical wavelengths by the Sloan Digital Sky Survey (10).

Both these surveys are being made as a service to the astronomical community, and the data are being released as soon as they are reduced and verified. The products of these surveys are available freely through the Internet.

Finally, some recent results from both instruments indicate the significance of research done at NRAO's facilities in New Mexico.

The power of the VLBA was illustrated by that instrument's observations of the galaxy NGC 4258, some 21 million light-years distant, which produced the most elegant and conclusive evidence so far for the existence of an extragalactic black hole. An international team of astronomers used the VLBA to observe emissions from water molecules in a disk of material circling the core of this galaxy. From information on the velocity of the water molecules, the astronomers concluded that the disk is circling a central object of 40 million solar masses. This mass lies within a radius of less than half a light-year, making its minimum density nearly 100 million solar masses per cubic light-year, far greater than that of any known star cluster (11).

Both the VLA and the VLBA have been involved in the discovery and study of a new class of objects within our own Milky Way galaxy. In March and April of 1994, VLA observers made a series of images of an energetic object that had been discovered by an orbiting X-ray observatory. They discovered that the object was emitting "jets" of subatomic particles and were able to track condensations in those jets that were moving at apparent speeds greater than that of light. This illusion, called "superluminal motion," has been observed frequently in the much more massive and distant active galactic nuclei and quasars, but never before had been seen in our own Galaxy. The researchers believe the new object is a binary-star system some 40,000 light-years distant, with one of the pair a neutron star or black hole, which is accelerating particles to an actual speed of 92 percent of light speed (12). Another X-ray emitting object, discovered in July of 1994, also was observed with both the VLA and VLBA. The high-resolution VLBA images of this object, at a distance of 10,000 light-years, have revealed similar particle speeds and complex motions. (13) Optical studies of this binary system show that it contains a black hole. (14). These unexpected, nearby Galactic objects will be the target of intense study to decipher the physics of such "jet" phenomena, which are seen at greater distances in large numbers.

For additional information about the National Radio Astronomy Observatory, its instruments, or the scientific programs, see the NRAO World Wide Web Home Page, at URL:


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2. Ryle, M., and Hewish, A., The Synthesis of Large Radio Telescopes, Monthly Notices of the Royal Astronomical Society, vol. 120, pp. 220-230, 1960.

3. Napier, Peter J., Thompson, A. Richard, and Ekers, Ronald D., The Very Large Array: Design and Performance of a Modern Synthesis Radio Telescope, Proceedings of the IEEE, Vol. 71, No. 11, November 1983, pp. 1295-1322.

4. VLA Observational Status Summary, NRAO, 1995.

5. Thompson, A.R., Moran, J.M. and Swenson, G.W., Interferometry and Synthesis in Radio Astronomy, New York: Wiley, 1986.

6. Kellermann, Kenneth I. and Thompson, A. Richard, The Very-Long-Baseline Array, Scientific American, January 1988, pp. 54-63.

7. Napier, Peter J., et al., The Very Long Baseline Array, Proceedings of the IEEE, vol 82, No. 5, May 1994, pp. 658-672.

8. de Pater, Imke, et. al., Outburst of Jupiter's Synchrotron Radiation After the Impact of Comet Shoemaker-Levy 9, Science, Vol. 268, 30 June 1995, pp. 1879-1883.

9. Condon, J.J., et. al., The NRAO VLA Sky Survey, NRAO, 1994.

10. Becker, Robert H., White, Richard L., and Helfland, David J., The VLA's FIRST Sur- vey, in Astronomical Data Analysis Software and Systems III, ASP Conference Series, v. 61, eds. D.R. Crabtree, R.J. Hanisch, and J. Parnes., 1994, p. 165.

11. Miyoshi, Makato, et. al., Evidence for a black hole from high rotation velocities in a sub-parsec region of NGC 4258, Nature, Vol. 373, No. 6510, 12 January 1995, pp. 127- 129.

12. Mirabel, I.F. and Rodriguez, L.F., A superluminal source in the Galaxy, Nature, Vol. 371, No. 6492, 1 September 1994, pp. 46.48.

13. Hjellming, R.M. and Rupen, M.P, Episodic ejection of relativistic jets by the X-ray transient GRO J1655-40, Nature, Vol. 375, No. 6531, 8 June 1995, pp. 464-468.

14. Bailyn, C.D., et. al., IAU Circular No. 6173 (1995).

Antennas of the VLA: The Very Large Array, on the Plains of San Augustin, west of Socorro, uses 27 dish antennas, each weighing 230 tons, to make detailed images of the sky. Photo Courtesy NRAO/AUI.

The radio galaxy Cygnus A, imaged with the Very Large Array in all four of its antenna configurations. The galaxy's nucleus is the small point in the center of the image. The nucleus is emitting jets of material in opposite directions. These jets impact material surrounding the galaxy, giving rise to the giant "lobes" of radio emission seen in this image. The energy required to produce these jets is believed to come from a black hole millions of times more massive than the Sun. Photo courtesy R.A. Perley, J.J. Cowan, J.W. Dreher, and NRAO/AUI.

New Mexico is host to two of the world's premier radio astronomy facilities, the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), operated by the National Radio Astronomy Observatory for the National Science Foundation. These instruments, open to the scientific community on a peer-review basis, are extremely versatile resources, capable of supporting a wide range of research programs within astronomy, planetary science and geophysics. The technical capabilities and research applications of these instruments are reviewed. Future plans include use of the VLBA in conjunction with orbiting radio observatories and a major upgrade for the older VLA.

* The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.

** A Jansky is the standard unit of flux used in radio astronomy. It represents 10-26 Watts per square meter per Hertz. A milliJansky is one-thousandth of a Jansky.