1. The Very Large Array The VERY LARGE ARRAY (VLA), completed in 1980, is one of the most powerful radio telescopes ever built and is used by visiting astronomers from around the world. It is an astronomical observatory using radio telescopes to study celestial objects. The radio waves received from celestial objects are processed to produce images of these objects. All of the facilities needed for the operation and support of the instrument are at the site 2. Whisper Gallery Naturally occurring radio waves from space are very weak because of the enormous distances they must travel to reach the Earth. In order to detect them we must use large dish antennas to collect as many of the waves as possible. These two dishes demonstrate how the sound waves of a human voice can be collected and focused. Just as these dishes collect and focus sound waves, the VLA dishes collect and focus the celestial radio waves. 2a. Instructions Two people are needed, one at each dish. The speaker should face the dish and speak softly near the ball. The listener should place their ear near the other ball. 3. The VLA Site ... Why Here? This site was chosen for the VLA because it is high, flat and far from any city. The Plains of San Agustin offer a large flat area to permit proper placement and easy movement of the antennas. The mountains ringing the plains provide protection from man-made electrical interference. Additionally, the height above sea level minimizes the blurring effect of the atmosphere on radio images. 4. Aperture synthesis The VLA is used to produce radio images with as much detail as the largest ground-based optical telescopes. To accomplish this with a single dish antenna at radio frequencies would require that the antenna be 27 kilometers (17 miles) in diameter. 5. Optical Photo Songle Dish Radio MaP Multi-Dish Array Photo VLA Radio Map Instead, we can place many smaller antennas in a "Y" pattern with each arm of the "Y" 21 kilometers (13 miles) in length. By combining the data received from all of these antennas, we can synthesize a radio image with the same detail as if we had a single antenna 27 kilometers in diameter. 6. The VLA Antenna (Mechanical) These giant antennas with 25 meter (82 ft.) diameter dishes, were specially designed for the VLA. The aluminum panels of the dish are formed into a parabolic surface accurate to 0.5 millimeter (20 thousandths of an inch). Most of the time the antenna drive system is precisely tracking a radio source across the sky. Occasionally you will see the antenna rapidly slew from one radio-source to another. The sounds you hear are the drive motors moving the 100-ton dish, the cryogenic refrigeration compressors, and the air conditioning equipment. 7. The VLA Antenna (Electronics) Weak radio waves from celestial sources are collected by the highly directional antennas. The waves are focused into the receiver by the main dish, subreflector and feedhorns. The receiver is cooled to -427 degrees F(18 deg. K) to reduce the internally generated noise which otherwise would mask the very weak radio signals from space. These weak signals are amplified several million times, converted to an intermediate frequency and carried to the Control Building via a buried waveguide transmission system. 7a. Antenna specifications: Type: Offset cassegrain with asymmetric subreflector Mount: Altitude - Azimuth Focal Ratio: f/.35 Total Weight: 235 Tons Total Height: 29 Meters (94 ft.) when pointed straight up Dish Diameter: 25 Meters (82 ft.) Drive System: 4 Electric motors (2 per axis) Range of Motion: 540 degrees azimuth, 120 degrees elevation Tracking Accuracy: 15 arc seconds in winds up to 18 mph Foundations: Height above track 1.9 Meter (6ft. 4in.) Depth below track 9.7 Meters. (32ft.) 8. Transporters The antennas are moved along the array arms by rail. Two giant transporters carry the antennas on 63 kilometers (39 miles) of double standard gauge track. These transporters are specially designed to negotiate the 90 degree turns onto spurs at each antenna station. A fleet of special purpose rail vehicles is used for servicing and repairing antennas. 9. The Rail System The Central Electronics Room receives the astronomical data from each antenna, via the waveguide. In addition, the means to control, command and monitor the distant VLA antennas are provided here. A variety of electronic equipment performs all these tasks at the command and under the watchful eye of the Control Computer System in the Computer Room. 10. Control Room The control room is the VLA nerve center. Here the array operator implements the astronomer's observing program, monitors the performance of the array and serves as a central communications point for the observatory. Much of this activity involves using computer terminals to interact with the system. Observing generally goes on 24 hours a day, 7 days a week. Radio astronomy is not limited to nighttime or clear weather observing. 11. Computer Room Without computers, operation of the VLA would be impossible. Many thousands of instructions are issued to the array each second. In addition, thousands of items of monitor information and vast amounts of astronomical data are read every second. Data accumulated over a period of time, from a few minutes to many hours, must be porcessed and combined to form a radio image. to accomplise this requires millions of mathematical operatins which are feasible only with a large computer system. 12. SUMMARY The latter half of the 20th Century is often termed "The Golden Age of Astronomy". The VLA is a major contributor to tremendous surge of activity that is now going on in astronomical research. It is complemented by other new instruments such as the space telescope, orbiting x-ray observatories, and NRAO's own Very Long Baseline Array, a continent-wide cousin of the VLA. For astronomers, the VLA represents an instrument that will provide answers to fundamental questions about our universe. This promise will continue to be fulfilled by the astounding results obtained with this instrument. We are pleased that you took the time to visit us and share in our excitement. We invite you to continue the tour by driving to the Antenna Assembly Building display area in your car. Follow the signs as you exit the parking lot.