THE ILLINOIS 120-FOOT RADIO TELESCOPE
by Arno H. Schrieffer, Jr., Kwang-Shi Yang, and George Swenson, Jr.
Reprinted with permission from Sky and Telescope, vol. 41, no. 3, pp. 132-138, March 1971.
The 120-foot telescope at the Vermilion River Observatory in eastern Illinois is the result of a cooperative program by the astronomy and electrical engineering departments of the University of Illinois at Urbana-Champaign. The original intent was to build three such instruments on wheels, to be used together as an interferometer and as an aperture-synthesis telescope, capable of extremely high angular resolution and sensitivity.
Theoretically, two or more small telescopes movable with respect to one another on a one-mile base line can achieve the same sensitivity and the same angular discrimination as a single telescope a mile in diameter. The compromise is between initial cost (or structural feasibility) and observing time: the interferometer is relatively inexpensive but requires much time to complete its observations.
Partial funding for the first telescope was obtained in 1967 from the National Science Foundation and was matched by the university. But the budget was too slender to permit us to employ contractors, and it was decided that the observatory staff should build its own instrument, making maximum use of government surplus materials and equipment and of student labor. Professors, engineers, technicians, and students pitched in together.
First, they built a large shop and equipped it. Lathes, drills, saws, milling machines, welding and metal-burning gear, and materials-handling equipment were obtained from U. S. Department of Defense excess stocks with the assistance of the Office of Naval Research. Most of these items were overhauled or rebuilt by the staff.
A truck crane, bulldozer, trucks, hoists, and, eventually, a 200-ton stationary guy derrick were also borrowed from the government and overhauled by the staff and students. In the meantime, these personnel were learning the skills of mechanic, millwright, equipment operator, welder, and machinist. In general, every man has acquired some of the skills of all these trades.
The conceptual design was produced under contract by Neil Stafford of the Stanford Research Institute, Palo Alto, California. Detailed mechanical and structural design was by the first author of this article, Arno H. Schriefer, Jr., of the observatory staff, who also acted as project engineer. He and astronomy graduate students, advised by Prof. J. W. Melin, performed the structural analysis of the university's IBM 360/75 computer. The construction foreman was electronic technician Robert Fisher, whose crew consisted of maintenance worker Dan Hawker and several students.
The motion of the equatorially mounted telescope is controlled with an accuracy of one minute of arc by a digital computer built by the second author, senior research engineer Kwang-Shi Yang, assisted by technicians Lyle Hawkey and Jerome Oder. The same group, plus Gary Whittaker, has designed and built the sensitive radio receivers (radiometers) and all other electronic and electric-power components, including those for heating and lighting.
After this first telescope was well along in construction, it became apparent that money for the other two would not be forthcoming because of the general constriction in Federal funding of fundamental research. It was necessary to abandon the plan to do aperture-synthesis work and to concentrate on other research programs. At this time radio astronomers were discovering several complex molecules in the interstellar gas of the Milky Way, using radio telescopes as very sensitive microwave spectrometers (Sky and Telescope, November and December, 1970, pages 267 and 345).
This activity was highly appropriate for the new instrument, so a cooperative program was initiated with microwave spectroscopists led by Prof. Willis Flygare of the university's school of chemistry, and a complex spectrometer was designed by the observatory staff for construction in the chemistry shops.
Structural and Mechanical Design
In order to build such a large instrument on a low budget, it was necessary to compromise somewhat between cost and performance. To cover the entire sky from horizon to zenith, as would be most desirable, the huge paraboloid must be mounted well above the ground; its center being at least as high as the radius of the dish, 60 feet in this case. Regardless of the type of mounting chosen, this requires a high pedestal and, in the case of an equatorial mount, a long cantilever yoke. By accepting more limited coverage of the sky, however, we could reduce the tower's height considerably and eliminate the cantilever support entirely.
In radio astronomy, the principal need for large hour-angle coverage arises from the need for long integration times in detecting faint sources. With the advent of very-long-base-line interferometry, it is also desirable to have large hour-angle coverage so that telescopes separated widely in longitude may have significant periods of overlap on mutually visible sources. We compromised here, too, deciding that it would be sufficient for our dish to follow a celestial object from 2 1/2 hours before meridian transit until 2 1/2 hours after; a total of five hours.
Likewise, declination coverage from the celestial equator to the north celestial pole was deemed acceptable, in view of the very substantial savings in cost by not going to southern declinations. To look more than a few degrees below the equator would require that the declination bearings be mounted well behind the support structure of the dish and that substantial counterweights be added to balance the moving structure. Thus, the total moving load would increase rapidly if the declination range were extended. In our instrument, the declination axis is mounted well within the structural hub of the dish, minimizing the need for counterweights.
With our limited sky coverage, neither of the two most popular mounting systems--altitude-azimuth or equatorial--has a significant advantage over the other with respect to cost or ease of construction. But as is well known, an equatorial mounting needs no two-coordinate conversion to follow a source in sidereal motion. It simplifies the design and maintenance of the electronic system that controls the telescope and generally simplifies the drive system.
Our drive machinery has some novel features. Most equatorial radio telescopes use large spur gears for the hour-angle drive. These are expensive. The Illinois group decided to use roller chain, a precision analog of a large bicycle chain, from the Link-Belt Co. of Indianapolis, Indiana. This was tested in the university laboratories and found to have the necessary precision, strength, and elastic properties. When wound around a circular track, roller chain performs as well as a much more expensive spur-gear system.
The declination drive is a jackscrew operating between the rocking platform and the structural hub of the dish. It is 23 feet long and six inches in diameter and has a nut containing recirculating ball bearings. These carry the dish up and down the rotating screw. This is similar to the landing-gear extension system on some large airplanes, and it is simpler and less expensive than an equivalent spur-gear system. One aesthetically negative feature is that the screw extends through the surface of the dish at low declinations. However, care was taken to have the screw fall under the "shadow" of one of the feed-support legs so that it has little or no effect on the reception pattern of the telescope.
The dish has a central structural-steel hub of inner diameter 30 feet, outer diameter 50 feet, and average depth 9 feet. Extending outward and inward from this hub are 60 cantilevered aluminum trusses which support 40 concentric aluminum hoops to carry the reflecting surface. This consists of lightweight expanded aluminum mesh, with the mesh openings 0.6 centimeter (about 1/4 inch). All steel members are welded together, while all aluminum members are riveted or bolted.
While the giant paraboloid was still resting on the ground in the summer of 1970, its mesh was being installed. The template at the right was pivoted on the center post to provide an accurate surface gauge. The reflector has a very large sagita, the depth to its center being 21 feet from the midpoint of the 120-ft chord from rim to rim.
The pedestal is made of 10-inch steel pipe welded into a pyramid with a square base, 28 1/2 feet on a side. The choice of design was dictated by the availability of surplus pipe and an elegant, computer-controlled saddle-cutting machine that greatly simplified the fitting of the pipe joints.
We designed the telescope to survive 100 mile-per-hour winds when it is coated with an inch of ice and stowed in the zenith-pointing position. At this wind velocity the critical corner of the pedestal experiences an upward force of 140,000 pounds. To counteract this, a foundation block consisting of 1,000 cubic feet of reinforced concrete is buried beneath each corner of the pedestal, which is bolted to these foundation blocks except when it is being moved.
The telescope can travel on wheels over a monorail system. The diagonals of the square pedestal base are oriented north-south and east-west, and a double-flanged crane wheel is mounted under each of the two diagonal corners that are on the axis of motion. However, the wheels can be placed under the other two corners to provide for motion at right angles to the first. Rubber-tired wheels under the outer corners, running on gravel paths, balance the telescope against wind forces while it is moving. But all observing is to be done from prepared foundations.
The Electrical Drives
To follow a source in sidereal motion it is only necessary to drive the roller-chain sprocket at a rate synchronous with the earth's rotation. This would most obviously be done with an a.c. synchronous motor fed by a precision oscillator through adequate power amplifiers. In the present case, this would require several kilowatts of power and would introduce complications and high cost. An alternative would be a servomechanism in which a d.c. motor would drive the telescope under the control of a feedback loop. Again, this is complicated and expensive. Either of these systems may generate radio-frequency noise, which is undesirable in the neighborhood of a radio telescope.
Instead, we decided upon a polyphase induction motor, the most inexpensive, simple, and rugged electrical prime mover. When loaded well below its rated power, it runs at very nearly synchronous speed. The 120-foot telescope requires less than one horsepower to drive in hour angle in a 25 mile-per-hour wind, and after an hour's tracking our 3-hp. induction motor is out of step by a maximum of one minute of arc. This error includes both the induction-motor slip and the inaccuracy of the commercial power frequency. With a proper hour-angle readout system, such a small error is easily corrected by the operator periodically, much as if he were guiding an optical telescope.
However, to permit scanning the source in hour angle it is necessary to have some flexibility in the drive rate. This is provided by a Graham variable-speed transmission, which is essentially a variable-ratio clutch between the motor and the main hour-angle speed-reduction gears. The ratio is controlled electrically from the operator's console.
An hour-angle slewing motor is also provided, capable of moving the telescope 10 degrees per minute of time. Declination slewing is at approximately the same rate. Slow motion in declination is at a degree per minute or 1/4 degree per minute, at the operator's choice. These rates can be reset by adjusting the field current of a d.c. slow-motion motor which is coaxial with the slewing induction motor.
Electronic Control System
A good sidereal clock is a requirement for any telescope. Ours is provided by a 4-megahertz quartz-crystal oscillator with appropriate integrated-circuit frequency dividers. The local sidereal time and the local civil time are indicated by Nixie lamps on the operator's console.
The most easily read, precise readout system to show telescope position would utilize digital indicators. Unfortunately, digital shaft-angle encoders of high precision have been extremely expensive and therefore were initially thought to be unsuitable for the project. However, Baldwin Electronics, Inc., Little Rock, Arkansas, manufactures a simplified encoder that gives an output impulse for each minute of arc, rather than a complete and unambiguous coded readout of each angle. It requires an electronic counting system to accumulate the pulses and to indicate the total angle.
This, in turn, suggested using a small wired-program computer, not only to indicate the angles but also to provide logical interlocking of the operator's controls to prevent blunders or accidents. The computer was designed and built by observatory personnel. It performs the counting and display functions for hour angle and declination, does the necessary arithmetic to compute and display the right ascension, and checks the accumulated angles periodically with unambiguous angle codes generated every 15 degrees by the encoders. Safety limits on the shaft angles can be set into the computer from the consoles; if a limit is reached, the telescope will either stop or reverse, as the operator wishes.
Should the power supply fail momentarily, our computer memory would be erased. To provide control in this event, a set of "synchro" analogue shaft-angle readouts is provided as a backup system. These readouts are less accurate than the digital ones and are intended for emergency use only. The telescope can thus be controlled mechanically even if the computer should fail, provided power is available for the motors.
All of the clock and computer circuitry is built into four drawers of the operator's console. It involves some 550 integrated solid-state circuits, the equivalent of 8,250 transistors. At the time the telescope system was designed, it appeared more economical to build than to buy the computer. With the recent substantial reductions in cost of small computers, this is probably no longer true, though funding problems might still have made it necessary for us to build our own. Also, those who built the computer will be operating the telescope, which should expedite our troubleshooting when that is required.
A good deal of thought and effort went into the interlock system. The observatory cannot usually supply operator assistants for the astronomers who use the telescope and it will frequently be necessary to turn the instrument over to relatively unskilled persons. A foolproof system of safeguards was therefore deemed essential. The hour-angle rate is read out independently of the hour angle, by means of an optical angle-rate encoder built by observatory personnel.
Radio Receiving Equipment
The receiving antenna is placed at the prime focus of the 120-foot paraboloid, which has a focal length four-tenths its diameter (f/0.4). At the focus are an electronic-equipment shelter and machinery for focusing and polarization control. A complex, welded-steel space frame connects the legs of the feed-support tripod and surrounds the electronic housing. The latter is a box 22 inches square and five feet long salvaged from the National Radio Astronomy Observatory in West Virginia. It is equipped with solid-state Pelletier-effect heating-refrigerating units. This temperature-control system is similar to the one long used at NRAO, but it was redesigned to meet the needs of the 120-foot telescope. The temperature within the box is held constant at 25 degrees centigrade (plus or minus 0.5 degrees) at all seasons.
The entire box can be rotated about the axis of the paraboloid by means of a ring gear from the gun turret of an airplane. Two recirculating-ball jackscrews provide for focusing along the axis. Motion perpendicular to the axis requires adjustment of threaded studs at the lower ends of the tripod legs. The feed antennas are mounted on the lower end of the electronics box. Thus, focusing and polarization rotation can be accomplished from the operator's console.
The first receiving systems have been built for 18-cm and 49-cm wavelengths, respectively. The former is the band occupied by four spectral lines of the hydroxyl (OH) molecule, and the latter is the wavelength that was used for the Vermilion River Observatory's 400-foot-telescope survey of faint continuum sources. (This instrument was described in Sky and Telescope, December 1962, page 322.) Both wavelengths have some measure of legal protection for radio astronomy on a national and international basis.
As the two wavelengths are to be observed simultaneously, the feed antennas are concentric. The 18-cm. (1670 MHz) antenna is a square horn with periodic internal flanges to equalize the reflector illumination in the two principal planes. Two modes of reception are provided for, their polarizations being mutually at right angles. These can be combined electronically to provide plane polarization at any position angle or either sense of circular polarization. The 49-cm (610 MHz) antenna consists of four full-wave dipoles arranged in a square around the 18-cm horn. The same polarization versatility is provided in this case. The whole antenna assembly is enclosed in a hemispherical weatherproof radome.
Other antennas can be substituted as the need arises. An electric winch-driven cable can lower the entire front-end-box assembly to a platform at the vertex of the reflector for servicing. On-the-spot servicing must be accomplished by climbing a tripod leg and perching on the space frame like a seaman on a square-rigged sailing ship. We hope that acrophobia can be overcome by practice and determination, since for our budget an elegant service tower, like the one for the 140-foot telescope at Green Bank, was out of the question.
The continuum receivers for 18 and 49 centimeters are conventional superheterodynes, using front-end switching against resistive loads. The only unusual feature is provision for remotely monitoring a great many electrical quantities in the front-end box, such as voltages, currents, oscillator power and frequencies, noise figures, and passband shapes. All this is to minimize the necessity for climbing that tripod. It should be possible to diagnose most malfunctions from the equipment racks in the control room. The cosmic signals come down from the focus at 30 MHz. intermediate frequency and are further processed in the control room.
A 50-channel, frequency-filter spectrometer is under construction, and a frequency-agile receiver has been designed to permit the telescope to be used for spectroscopy in the wavelength range 10 to 300 cm. Incidentally, while the surface accuracy of the dish is unknown, we believe it to be good enough for operation at wavelengths as short as 10 cm.
Vermilion River Observatory's new radio telescope is located at latitude 40 degrees, 3 minutes, 54 seconds north, longitude 5 hours, 50 minutes, 13.75 seconds west, 213 meters above sea level. This is about 45 miles by road east of the campus at Urbana and five miles southeast of Danville.