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Picture of a Stereo or Scissors Telescope, being used by a German Artillery Officer during 1914. The officer was sitting at the top of a ladder hidden behind a haystack.
The type of telescope with which you are probably most familiar is an optical telescope. These types of telescopes allow us to see things very far away, such as planets and other galaxies outside our own Milky Way galaxy. However, optical telescopes only allow us to see things that give off visible light. There’s a lot going on in our universe that falls outside the visible spectrum. That’s where radio telescopes come in.
As the name suggests, radio telescopes allow astronomers to observe radio waves and microwaves—which have much longer wavelengths than does visible light—coming from space. Because many astronomical objects emit radiation more strongly at longer wavelengths than at visible-light wavelengths, radio telescopes can show us things about the universe that optical telescopes cannot. Over the last half-century, radio astronomers have used radio telescopes to make important discoveries. By studying the sky with both radio and optical telescopes, astronomers gain a much more complete understanding of the universe.
A radio telescope typically consists of a parabolic (bowl-shaped) antenna similar to a modern satellite dish. This dish collects incoming radio or microwave radiation and focuses it onto a sensitive receiver located behind or below the antenna. Inside the receiver, the incoming waves are converted into electrical signals. Computers then process those signals to form images of the sky as it would look if our eyes were able to see at radio or microwave frequencies.
In the early 1930s, Karl Jansky, a young Bell Laboratories engineer, built the first highly directional radio frequency antenna ("highly directional" means it accepted radiation only from the direction in which it was pointed). Jansky was trying to determine the origins of static noise in transatlantic short-wave communications at wavelengths of about 10-20 meters (about 33-66 feet). In 1932 Jansky reported that some of this noise was coming from extraterrestrial sources, strongest toward the center of our Milky Way galaxy. Jansky planned to build a larger 30-meter (100-foot) diameter dish for further studies but was turned down by Bell Labs. A ham radio operator named Grote Reber picked up on the idea in 1937 and built a 10-meter (33-foot) diameter parabolic antenna in his back yard. With this home-built equipment, Reber performed the first systematic sky survey at radio frequencies. Advances made in radar technology during World War II became available to the growing radio astronomy community in the post-war years. As time went by, these techniques were extended to higher frequencies and shorter wavelengths, so that today observations of this type are also made at microwave, millimeter, and sub-millimeter wavelengths.
In 1965 microwave engineers Arno Penzias and Robert Wilson made one of the most important cosmological discoveries of the 20th century. Working at Bell Labs, they were also investigating noise in communications when they found microwave background radiation coming from all directions in space. The existence of this cosmic microwave background had been predicted as a consequence of the Big Bang theory by Robert Dicke. The Big Bang theory postulates that the universe was created in a primordial explosion followed by expansion. The data of Penzias and Wilson was interpreted as direct confirmation that the universe indeed began in a great explosion. This momentous discovery produced its own explosion in the building of many new radio telescopes around the world.
Modern computers now permit signals from multiple antennas to be combined to create effectively large apertures for better resolution, as in the Very Large Array (VLA) at Socorro, New Mexico. Dedicated in 1980, the VLA is composed of 27 individual antennas arranged in the form of a "Y". This creates an effective aperture up to 36 kilometers (22 miles) in diameter. Each individual antenna is 25 meters (82 feet) in diameter and weighs approximately 230 tons. A buried communications system carries control information to the antennas, and returns astronomical data from the receivers to a central computer in the control building. A similar system, completed in 1993, is the Very Long Baseline Array or VLBA. The VLBA uses ten antennas separated by thousands of miles for even better resolution.
The international community is currently building an even larger array of 64-millimeter-wavelength telescopes high in the Andes Mountains of Chile. The main purpose of this instrument will be to look farther back in time to the most distant galaxies, which cannot be seen at optical wavelengths, for clues to the evolution of the early universe.
THE ASTRONOMY DEPARTMENT IS BORN
In August 1921, the Board of Trustees authorized that the Division of Astronomy in the Department of Mathematics be organized as a separate Department of Astronomy. The following year, Charles Wylie earned the first University of Illinois PhD in Astronomy (the next Astronomy PhD would not be award until 1962, 40 years later!). 1922 also marked the departure of Joel Stebbins from Illinois, and Dr. Robert H. Baker took over as the new Director of the University of Illinois Observatory.
In 1925, the 30-inch telescope was rebuilt and moved to a new location on Florida Avenue (also since torn down). The "mirror blank" (i.e., slab of glass to which reflective metal would be applied) for the rebuilt telescope now resides in the Observatory.
In 1933, light from star Acturus falling on a photocell in the Observatory's annex activated an electric signal that turned on the lights at the Chicago World’s Fair.
The Illinois 400-Foot Radio Telescope
Aerial view of the 400-ft telescope. The full-size 1957 sedan parked at the far end of the reflector gives scale.
In 1956 the University of Illinois invited the writer to join its faculty to design and build a large radio telescope. Professor George C. McVittie, Head of the Astronomy Department, believed with many other cosmologists that a very extensive and complete catalog of discrete, cosmic radio sources would help to distinguish among competing cosmological theories. Two major catalogues of sources had been published by radio astronomers in Cambridge, England, and Sydney, Australia, but they did not agree well in the region of the sky in which they overlapped and it was desirable to confirm them with different instruments. McVittie and Professor Edward C. Jordan, Head of the Electrical Engineering Department, agreed that such a program would be an appropriate area for cooperation between the departments and sought an engineer to undertake the job.
Copyright (c) 1986 Institute of Electrical and Electronics Engineers. Reprinted from the IEEE Antennas and Propagation Society Newsletter, vol. 28, no. 6, pp.13-16, December 1986. This material is posted here with permission of IEEE. Internal of personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by sending a blank e-mail message to [email protected]
Touring the World's Radio Telescopes
The first task was to study the existing instruments engaged in cosmic radio source cataloguing, to determine the errors to which they were most susceptible in order to design a new instrument which would best complement them. It was also desired to compile a deeper catalog than existed that is, a catalog complete to a lower flux level.
The University in 1957 sent me on a tour of the world's most prominent radio observatories, including those at Cambridge and Manchester (Jodrell Bank), England Sydney, Australia Paris, Nancáy, and Haute Provence, France and Dwingeloo, Netherlands. At that time the best-known catalogs had been compiled by Martin Ryle's group at Cambridge and Bernard Mills' group at Sydney. The Cambridge instrument was a compound interferometer operating at 159 MHz. One of its problems involved misidentification of the reception lobes of the interferometer, thus causing ambiguities in source position. The Sydney instrument, the Mills Cross, operated at 81.5 MHz and involved the multiplication of orthogonal fan beams produced by two linear arrays, in order to produce a narrow pencil beam. As the sidelobes of one array were multiplied by the main beam of the other, the resulting sidelobes of the instrument were inherently high. A strong source in a sidelobe could easily be taken for a weak source in the main beam.
It was apparent that a new instrument design should emphasize sidelobe reduction, should involve a different frequency range from existing instruments, and should be large enough and have sensitive-enough receivers to permit detection of sources an order of magnitude fainter than those in existing catalogs. The weaker the source to be catalogued the narrower must be the main beam and the lower the sidelobe levels, in order to avoid the "confusion" problem that had plagued earlier efforts. I thought a filled-aperture antenna had the best chance of achieving these goals however, it must be very large by the standards of the day. How to achieve the best compromise among size, surface precision, frequency, and cost was the problem.
Having learned the rudiments of the cosmic cataloguing business from Bernard Mills and Joseph Pawsey at Sydney, I mulled these questions over while touring the world's radio telescopes. At the Observatoire de Haute Provence, Dr. M. Laffineur had built an interferometer, each of whose elements was a 100-foot square (approximately) parabolic cylindrical reflector of poultry wire whose cylindrical axis was north-south. It was fed by a linear array of dipoles along the focal line, and the whole system was suspended above ground on wooden poles. Phase adjustment of the dipoles permitted steering of the main beam in declination, while the earth's rotation scanned the beam in right ascension. This "meridian transit" system is acceptable for a systematic survey and does not require any mechanical motions of the structure. It seemed a very economical scheme. I discussed it with Robert Hanbury Brown at Jodrell Bank and we agreed it might be a good approach to the problem.
There was a difficulty involving the precise phasing of the dipoles along the focal line. At the Carnegie Institution of Washington John Firor had built an array for solar studies in which he used helical antennas mechanically rotated about their helical axes to adjust their relative phases. Each element in Firor's array consisted of two helices of opposite sense, combined to be sensitive to linear polarization. If circular polarization were acceptable, each element might be a single helix however, the off-axis radiation is elliptical and this complicates the problem of appropriately illuminating a reflector antenna.
With these thoughts in mind, the conceptual design of the radio telescope was begun. Several design parameters were quickly obtained. The conventional wisdom of the day held that a resolution of several score beamwidths per source was necessary to avoid confusion. Adopting a lower flux level of 10-26 W/M2/Hz and extrapolating on the then-known source number/strength statistics led to a required beam-width of about 0.25 degree. A frequency of 600 MHz would be sufficiently higher than that of existing catalogs to yield useful spectral data and was still in the range where sensitive receivers were possible. Thus, the linear dimension of the antenna should be about 120 meters, and the configuration should be roughly square or circular. A reasonable focal-length-to-diameter ratio to permit good illumination of the reflector is 0.4. This would require very high structures if constructed on level ground, especially expensive if strong enough to withstand winter loading by snow and ice, and if precise enough to operate efficiently at l = 50 cm. A suspended structure was briefly considered but the construction costs and the effort needed to maintain dimensional precision promised to be excessive.
The alternative was to build the reflector of earth, in large part below ground level. An extensive search disclosed a stream valley near Danville, Illinois, whose axis was nearly north-south and which had appropriate dimensions. The land was available for purchase. The geological conditions were acceptable from the structural viewpoint. The natural dimensions of the valley dictated only minor changes in the nominal parameters the final values are: width, 400 feet f/d, 0.39 length of reflector surface, 600 feet height of towers, 165 feet.
Television channel No. 37, 608-614 MHz, was at that time unassigned and unused in North America and the corresponding channel was unassigned elsewhere in the world. After a lengthy legal and political struggle, this frequency band was assigned to radio astronomy on a worldwide basis. This outcome was years in the making and represents a story all by itself. In the meantime, the telescope was completed, relying on hope and faith that the frequency use question would somehow be resolved in a satisfactory way.
The basic parameters and construction technique having thus been established, the multifarious details of the design remained to be determined. For help in this we turned to the outstanding Antenna Laboratory of the Department of Electrical Engineering, then led by Professor Victor H. Rumsey. One area of effort involved the phase adjustment of the focal line array and the illumination of the reflector. Professor Yuen Tse Lo undertook the detailed design of the array, taking into account the appropriate illumination tapering to minimize sidelobe levels. The number of elements needed in a uniformly-spaced array 122 meters long (actually 137 meters, to compensate for foreshortening at maximum zenith angles) was daunting and the dynamic range of element currents required to provide proper illumination tapering promised to be difficult to achieve. Lo and I decided to try to achieve this tapering in part by use of nonuniform element spacing.
Design of the array element was done by Professor John D. Dyson. The Antenna Lab team were then engaged in their pioneering studies of frequency-independent antennas, and the conical log-spiral proved to be appropriate, having circular polarization with a low axial ratio over a wide angle from the axis of the cone. Its primary radiation pattern gave the proper tapering for illuminating a parabola with the chosen f/d ratio. Dyson's design gave a precise adjustment of phase with mechanical rotation--no difficulty with phase adjustment was ever experienced during the operation of the telescope.
Conical log-spiral antenna: feed element of the University of Illinois 400-ft radio telescope. Design by J. D. Dyson.
Despite its excellent axial ratio, the conical log spiral still has a finite negative component of circular polarization. If the array is correctly phased to direct the positively polarized beam at a given zenith angle, there will also be a smaller negatively polarized beam at the negative of the given zenith angle. To eliminate this negative beam, Lo proposed an ingenious scheme. The feed lines to the several elements were of random lengths, and the phase for the positive polarization was corrected by mechanical rotation of the element. This merely doubled the random error for the negative polarization, and the negative beam never formed. These design efforts led Lo into his well-known studies of nonuniform and random arrays.
I also asked Lo to study the scattering of electromagnetic energy from simplified models of steel and of wooden towers to support the focal-line structure. His answer was that the two materials would have about the same net effect, so we decided to build the structure of wood, which we expected to be somewhat less expensive and to require less maintenance. After receiving the wooden structure design from the consulting engineers, we were dismayed to discover that each of the four towers contained four 155-foot-long, uniformly-spaced arrays of one-wavelength-long steel bolts. There wasn't time to calculate the cumulative effects of these hundreds of bolts, so we had them replaced by phenolic-impregnated wooden bolts. We also searched for suitable non-metallic guy ropes for the towers, but without success. We had to settle for steel guy cables and for steel elevator cables within one tower. In retrospect, these concerns were probably not important.
Feeding the array elements with the proper phases and amplitudes and with minimum attenuation proved to be a very difficult problem. Kwang-Shi Yang and I tried several feeding schemes. A full corporate structure feed system was considered too expensive by far and not easily realized because of the nonuniform spacing of the array elements. Instead, a traveling-wave transmission-line feed system was proposed, with the log-spiral elements loosely tapped into the line at the appropriate intervals, taking into consideration Lo's random-phase feeding system. This scheme proved to have insufficient bandwidth, so it was necessary to compromise, starting from the receiver port with a corporate structure, branching down to six ports feeding traveling-wave feed lines, each with several antenna elements. The individual elements were coupled to the transmission lines by adjustable capacitive probes. These latter were evolved through much experimentation by Yang and his assistants. Initially, inductive probes were used, but we were never able to make them work properly, so we abandoned them after making the first astronomical observations and redesigned the feeding hardware, much to the dismay of the astronomers who thought at the time that we were more interested in electronic experiments than in astronomy. Precise monitoring of the phase and amplitude at each element of the focal-line array was essential. The component of phase contributed by the individual log-spiral antenna element was precisely related to its mechanical rotation so it was only necessary to determine the phase and amplitude at the antenna port. Adapting a scheme he and his colleague Govind Swarup had developed for the Stanford Microwave Radioheliograph, Yang substituted an audio-modulated diode for the antenna at a given feed-system port and sent a signal into the line from the central receiver port. The phase delay of the audiomodulated reflection from the individual antenna port could then be compared with the phase of the signal generator. Adjustment for the proper co-phased condition for the broadside (vertical) beam direction could be made by rotating the index of the dial plate on the mechanical axis of the antenna element. Adjustment for an arbitrary zenith angle is made by further rotation with respect to this index, according to a table computed by Lo using the original Illiac computer. Design of the array transmission-line system probably took more engineering effort than any other task of the whole program. Every detail of the system had to be designed from scratch, as commercial components generally were unsuitable. In addition to the conical log-spiral antennas, these components included the coaxial transmission lines and all their hardware, including matching stubs, insulators, couplers, terminations, reference loads, refrigeration systems for reference loads, etc.
An additional task of comparable magnitude was the design and construction of the receivers and recording system. This also was a major task of K. S. Yang's, assisted by Kenneth Seib and several technicians. The Zenith Radio Corporation contributed electron-beam parametric amplifiers through the good offices of Dr. Robert Adler. These state-of-the-art amplifiers served well during the early years of observing, but they were difficult to maintain and adjust and they were eventually replaced by field-effect transistors when the latter became available. Maintaining proper balance between the effective temperatures of reference noise sources and the ambient-temperature-dependent losses in the hundreds of feet of transmission line in the feed system required many months of tinkering and adjustment.
Cataloguing the Sky: 1959-1969
The first installment of the radio source catalog was the PhD thesis of John M. MacLeod, published in 1964. Subsequently, most of the accessible parts of the sky (within 30 degrees of the zenith) were catalogued under the direction of Professor John R. Dickel, who also mapped a number of galactic extended sources, and by Professor John C. Webber. Professor Harold D. Webb also mapped considerable portions of the Milky Way galaxy, so that most of the accessible sky was covered at least once during the years 1959-1969. Among the many new cosmic radio sources discovered in this survey was the source VRO 42.22.01, which was later identified with the optical object BL Lacertae. This is the very small diameter core of an extended galaxy, and is the prototype of a large class of such objects whose extraordinary energetics are still unexplained. Two previously unknown supernova remnants were also discovered, expanding shells of gas expelled by exploding stars. A detailed study of the astrophysically interesting Cygnus X region of the Milky Way was begun, which has since been extended to other frequencies on other instruments. Many galactic H II (ionized hydrogen) regions were mapped.
One of the features of the original design concept was the provision of multiple reception beams, which would have permitted much more rapid surveying of the sky. The parabolic cylindrical antenna is inherently suited to multi-beam operation. Outputs of various sections of the focal-line array can be combined in several different beam-forming networks with appropriate phase shifts to produce a different beam angle for each network. The 400-foot telescope was ideally suited to this mode of operation: as there were six sections to the focal line array six closely-spaced beams were simultaneously possible, so that surveying could have proceeded at six times the rate of a single beam. It was never possible to secure funds to provide additional phase shifters and receivers (two per beam), so the catalog had to proceed at the most deliberate pace.
The reflector surface was of earth, precisely figured to 1/20th wavelength, covered with a heavy tarpaper liner topped with a galvanized steel one-inch (l/20) mesh. The earth base was subject to weathering. A period of each summer was reserved for precise surveying of the surface and for any necessary repairs which were effected with shovels, rakes, hoes and a portable tar kettle. This work and all other mechanical maintenance were supervised by Arno H. Schriefer. About $10,000 per year was required for mechanical maintenance, about two percent of the capital investment. Eventually, however, at about the time in 1969 that the sky coverage was being completed, there had accumulated substantial net erosion of earth from the upper slopes into the vertex of the parabola. This process resulted in progressive increase in the focal length, which eventually exceeded the available range of adjustment in the height of the focal line array. At that time the instrument was abandoned, its mission essentially complete.
In retrospect, several things probably should have been done differently. The variable-spacing tapering of the focal-line array saved many array elements, speeding both initial and daily phase adjustments and easing the amplitude tapering problem. However, it inevitably produced higher sidelobes in the meridian plane, which necessitated much re-observing of questionable sources. As the daily phase adjustment could never be precisely the same, the sidelobe levels differed from day to day, so re-observing usually answered any questions. Nonetheless, it probably would have saved time and money in the long run if a uniform array had been used. A more determined effort should have been made to institute a multi-beam system, though it's by no means certain funds for this could have been secured as the program was chronically underfunded. Less effort should have been expended on keeping metal components out of the near field of the reflector this was probably a needless concern. More effort should have been made to acquire a workable digital data recording system, despite the nascent state of that art.
Other technical aspects of the adjustment were very successful. The phase adjustment system worked well and there was never any problem with beam pointing. The instrument was mechanically stable and unaffected by weather. It observed through snow, ice and rain storms and through a substantial earthquake without any visible modulation of the data.
The History of Radio Astronomy
Radio astronomy is a relatively young branch of astronomical science. Today, some of the most important discoveries about our universe come from radio telescopes.
Karl Janksy circa 1930s
A Surprise Discovery Leads to Radio Astronomy
Radio astronomy was born early in the 20th century. In 1932, a young engineer for Bell Laboratories named Karl G. Jansky tackled a puzzling problem: noisy static was interfering with short-wave radio transatlantic voice communications. After months of tracking the source, he noticed that it shifted slowly across the sky. What could this be? Stumped, he consulted with an astronomer and came to a startling conclusion:
“I have taken more data which indicated definitely that the stuff, whatever it is, comes from something not only extraterrestrial, but from outside the solar system. It comes from a direction that is fixed in space and the surprising thing is that …[it] is in the direction towards which the solar system is moving in space. According to Skellett…there are clouds of “cosmic dust” in that direction…”
Jansky had discovered something at the heart of the Milky Way Galaxy. His work led to one of the most important papers in the history of astronomy in the 20th century, called “Radio Waves from Outside the Solar System”, published in 1933. His work laid the foundation for the science of radio astronomy!
Radio Astronomy and “Little Green Men”
One of the most famous radio astronomy discoveries occurred in 1967 when a young graduate student named Jocelyn Bell noticed a strange signal in a printout from a radio telescope she helped to build.
“My eureka moment was in the dead of night, the early hours of the morning, on a cold, cold night, and my feet were so cold they were aching. But when the result poured out of the charts, you just forget all that. You realize instantly how significant this is – what you’ve really landed on – and it’s great!”
What did she find? At first it wasn’t clear. The object produced strong radio pulses at a regular rate, about 30 times a second. Bell and her colleagues first called the object LGM-1, since they joked that the regular pulses could be from “Little Green Men,” though they understood that it was an as-yet-unexplained natural phenomenon.
The signals turned out to be flashes of radio emissions from a weird object called a pulsar. Pulsars are what remains after a massive star collapses and then explodes as a supernova Supernova The extremely violent explosion of a star many times more massive than our Sun after the nuclear furnace at its core can no longer balance out the force of gravity. During this explosion, these stars may become as bright as all the other stars in a galaxy combined, and in which a great deal of matter is thrown off into space at high velocity and high energy. The remnant of these massive stars collapse into either a neutron star or a black hole. . It sends clouds of debris into space, leaving behind a massive compressed object made entirely of neutrons. The pulsar Bell discovered spins on its axis 30 times a second, sending out a beacon with each spin. It is almost like the ticking of a clock.
Today, we know of more than 2,000 pulsars. For some, such as the pulsar at the center of the Crab Nebula, we can also see the glowing debris of the massive star. Radio astronomers focus on the ticking pulsar at the heart of the explosion.
Radio Astronomy and the Building Blocks of Planetary Systems
One of the most intriguing questions we can ask is about our own place in the universe, the solar system. How did it form? What conditions had to exist to allow life to form on our planet? The Atacama Large Millimeter/submillimeter Array (ALMA) was built to study the cold, dark portions of the universe, like the regions that give birth to infant solar systems. These regions are known as stellar nurseries. The Orion Nebula is a well-known stellar nursery that also appears to have infant planetary systems embedded in its clouds of gas and dust. ALMA scientists continue to study this nebula. Radio emissions as well as infrared light can travel right through those thick clouds of gas and dust, “lifting the veil” on the processes of star and planet formation.
This spectacular and unusual image shows part of the famous Orion Nebula, a star formation region lying about 1350 light-years from Earth. It combines a mosaic of millimeter wavelength images from the Atacama Large Millimeter/submillimeter Array (ALMA) and the IRAM 30-metre telescope, shown in red, with a more familiar infrared view from the HAWK-I instrument on ESO’s Very Large Telescope shown in blue. The group of bright blue-white stars at the left is the Trapezium Cluster, hot young stars that are only a few million years old.
What have astronomers found using radio astronomy to study this region? The cloud is threaded with filaments of cold gas that could well collapse someday to form stars and their planets.
Elsewhere in the galaxy, ALMA has detected traces of a molecule called called methyl isocyanate. This poisonous material turns out to be a precursor molecule that can combine with other molecules to form pre-biotic organic compounds called peptides which are one of the early building blocks of DNA.. In other words, ALMA is helping astronomers to detect precursors to life and giving clues to how life might have arisen in our own solar system billions of years ago.
Radio Astronomy Helps Understand the Cause of Gravitational Waves
Gravitational waves grabbed the headlines in recent years when they were first detected. But what causes them? The Karl G. Jansky Very Large Array played a role in studying the physics of these cosmic catastrophes. Three months of observations from several observatories led researchers to a titanic collision of neutron stars in a galaxy 130 million light-years from Earth. Neutron stars Neutron Star A small compressed core of a star that has gone through supernova (star explosion). These stars are almost completely made up of only neutrons and have a strong gravitational field. are superdense objects left over from the deaths of massive stars. They are magnetically active, and have very strong gravitational fields. When these monsters collide, they not only send gravitational waves out through space, but they also give off strong radio emissions. That’s what the VLA, plus the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India combined to study. The merger set off a huge outburst of energy and material and gave scientists new insight into the phenomenon behind the creation of gravitational waves.
M. Cassegrain - Man without a Name
On April 15, 1672 , the Journal de Scavans published an excerpt of a letter from a M. de Berce describing a telescope design proposed by a M. Cassegrain. The diagram that was used to illustrate the device is shown above. It had a large (primary) concave mirror which reflected light onto a smaller (secondary) convex mirror which then reflected the light back through a hole in the primary mirror to an eyepiece. Variations of this type of design would dominate the construction of research telescopes from the start of the twentieth century onward. In spite of the importance of the design, the true identity of this M. Cassegrain would not be known for more than three centuries.
You see therefore, that the advantages of this design are none, but the disadvantages so great and unavoidable, that I fear it will never be put in practise with good effect [_8_] .
For his work, 'M. Cassegrain' received one of the great smackdowns in the history of science. The quote above is from Isaac Newton. Christian Huygens was more caustic. Both of these scientists were wrong. Large research optical telescopes have been predominantly Cassegrain designs for over a century. But M. Cassegrain was never to be heard from again. The identity of M. Cassegrain was finally discovered in 2000. He was Laurent Cassegrain, a French Catholic priest from the region around Chartres, France. He was a teacher at a lycee (high school).
The Illinois 120-Foot Radio Telescope
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.
Students in the shop assemble the track for the chain drive in hour angle.
Big dish, big science
For most of its 57 years, the 305-meter-wide dish of the Arecibo Observatory was the largest in the world. Researchers used it to study Earth’s upper atmosphere, the rocks and planets of the Solar System, and more distant astrophysical objects. Here are some of its milestones.
1974 Finds first binary pulsar, a pair of neutron stars that emits regular radio bursts.
1978 Tracks inspiraling of pulsar pair— the first indirect evidence for gravitational waves.
1980 Radar maps of 25% of Venus’s cloud-shrouded surface reveal signs of volcanic repaving.
1989 Radar reveals an asteroid’s peanut shape—a “contact binary” held together by weak gravity.
1991 Reflections in shadowed craters near Mercury’s north pole suggest ice deposits.
1994 Pulsar fluctuations point to the tug of rocky worlds: the first exoplanets.
2007 NANOGrav begins to use Arecibo to monitor pulsars for signs of passing gravitational waves.
2016 Finds a repeating “fast radio burst,” showing some sources survive the outbursts.
1963 Opens under Cornell University management. Built with Department of Defense funding.
1969 National Science Foundation (NSF) takes over as owner.
2006 NSF advisory panel recommends closure.
1974 Message sent to a star cluster with pictures of the Solar System, DNA, and a human figure.
1997 Gregorian dome added to platform, along with six auxiliary cables.
2011 SRI International takes over management.
2017 Hurricane Maria batters Puerto Rico. “Line feed” antenna falls into dish.
2018 University of Central Florida takes over management.
2020 Suspension cables break and platform collapses.
The mission to save the telescope was now urgent. The engineers had to reduce the load on the three main cables still attached to Tower 4, now shouldering more than 75% of their breaking load, but they couldn’t risk putting people on the towers or platform. They looked at using helicopters to install extra cables or sever platform components to reduce its weight. They even considered sacrificing the entire 110 tons of the Gregorian dome, but the violent recoil of the platform after the dome was cut loose would have been “a bad thing,” Lugo says. There was no good option.
One firm—Wiss, Janney, Elstner Associates—favored stabilizing the telescope by relaxing the backstays that stretch from the towers to the ground, installing extra support cables, and removing mass from the platform before starting restoration work. But Thornton Tomasetti and the third firm, WSP, concluded that, after two cables had broken well below their design strength, none of them could be trusted. “Although it saddens us to make this recommendation, we believe the structure should be demolished in a controlled way as soon as pragmatically possible,” principal engineer John Abruzzo of Thornton Tomasetti said in his report. So, at a 19 November press briefing, NSF called time on the telescope. “We understand how much Arecibo means to [the scientific] community and to Puerto Rico,” said Sean Jones, head of the Directorate for Mathematical and Physical Sciences. “There is no path forward that allows us to do so safely.”
On 1 December, less than 2 weeks later, Lugo, who had temporarily relocated to Puerto Rico, stopped to buy breakfast before driving up to the observatory. Just after 8 a.m., he got a call telling him the platform had collapsed. “I felt like throwing up,” he says. One hour later he was on-site talking to staff who had heard and felt the crash. “There were a lot of glazed over expressions, they were all crying,” he says. Cameras on a drone had caught the remaining Tower 4 cables snapping within seconds of each other while a fixed camera watched the platform fall. Arecibo’s giant telescope was no more.
So why did cables that had held up the platform for decades suddenly fail so spectacularly? Decades earlier, staff noted cable wires snapping and suspected that corrosion from water was to blame. In 1976, managers tackled the problem by painting the cables to seal them off from the elements and installing fans to blow dry air through the length of the cables. Phoenix says that reduced the rate of wire breaks, but it’s unclear how long those practices were maintained. Kerr says the fans weren’t in use when he took over in 2007, nor was he aware of when the cables were last painted. “Someone may have dropped the ball,” he says.
Lugo insists procedures were continued since UCF took over in 2018. “We were doing what was being done prior,” he says. “It was not poorly maintained,” Rankin agrees. “The Puerto Rico staff are incredible: They did every possible thing.”
Natural disasters hastened the end, Lugo says. Hurricane Maria battered Puerto Rico in 2017. Phoenix says it was “an opportunity for trouble,” because the storm’s winds could have picked up seawater, whose salt makes it especially corrosive, and dumped some on the telescope. The observatory was also shaken by a series of earthquakes in December 2019 and January 2020.
Others say the NSF astronomy division’s efforts to hand off the telescope didn’t help. In 2006, the division convened an independent panel of astronomers for one of its “senior reviews” of existing facilities. To pay for planned new telescopes, such as the Atacama Large Millimeter/submillimeter Array in Chile and the Daniel K. Inouye Solar Telescope in Hawaii, economies were needed. Among other measures, the panel recommended closing Arecibo by 2011 unless partners were found to share operating costs. The astronomy division began to ramp down its roughly $10 million annual spending on Arecibo. NSF’s atmospheric and geospace division increased its funding from $2 million to $4 million and NASA chipped in a few million dollars for tracking near-Earth asteroids. But Arecibo wasn’t out of the woods.
Following an open competition, management of the observatory was transferred in 2011 from Cornell to a collaboration led by SRI International, a nonprofit research institute. NSF’s astronomy division still wanted more savings, however. In 2018, UCF stepped up to take over management, with support from Puerto Rico’s Metropolitan University and the company Yang Enterprises, on the understanding that the astronomy division would gradually reduce its contribution to $2 million annually.
Two management changes in 7 years and the slow dwindling of funds took a toll, supporters say. “People would leave or retire when there are no raises. The best people would go elsewhere,” says planetary scientist Michael Nolan of the University of Arizona, who was Arecibo director from 2008 to 2011. And when old hands move on, something goes with them, Phoenix says. “Knowledge gets lost without that continuity.” In response to questions from Science, an NSF spokesperson says, “Funding from NSF covered scheduled maintenance for the facility and should not have negatively affected the observatory’s ability to maintain the 305-meter telescope.”
Although Kerr is convinced neglect was a factor, he believes the collapse had no single cause. “We drove that telescope hard. It’s an old piece of steel in the tropics, too heavy, it failed.” But he does think the 1997 upgrade, although scientifically valuable, was a mistake. “If it had not been upgraded, it would still be standing.”
After the shock of last month’s collapse wore off, observatory managers gave a group of staff and outside researchers 3 weeks to come up with a plan to replace the telescope. “We need something concrete to put in front of people,” Lugo says. “We want to develop a system that will be relevant for another 50 years.” The planners are aiming for a replacement that would surpass the capabilities of the original, be more flexible, and satisfy the needs of planetary and atmospheric scientists as well as astronomers. And they are trying to do that for less than $400 million—roughly the cost of making a Hollywood blockbuster.
First we mourned, then we had a wake, then we got down to work.Joanna Rankin, University of Vermont
The researchers first considered a new fixed dish, along with an array of independently steerable smaller ones. But in the white paper delivered to NSF last month, they went with something more ambitious: a flat, 300-meter-wide, rigid platform, bridging the sinkhole, and studded with more than 1000 closely packed 9-meter dishes. The dishes would not steer but the disk would, with hydraulics tilting it more than 45° from the horizontal. At such an extreme tilt, one edge of the disk would be higher than Arecibo’s existing support towers. Steering “will be a great mechanical challenge,” says Anish Roshi, head of astrophysics at the observatory.
In this design, modern receivers built into each dish could cover a broader frequency range than its predecessor and, fired synchronously, the collective radar of 1000 dishes could send out a more powerful beam than a single transmitter. Dubbed the Next Generation Arecibo Telescope, it would be nearly twice as sensitive and have four times the radar power of the original. The steerable platform would enable it to see more than twice as much of the sky as its predecessor, while the field of view of its 1000 dishes would cover a swath 500 times larger.
The extreme tilt was designed to bring an important target within view: the supermassive black hole that sits in the galactic center. The 2020 Nobel Prize in Physics was awarded in part to astronomers who peered through a haze of dust and gas at the heart of the Galaxy to painstakingly track a star following a tortuous orbit in the grip of the black hole. If radio astronomers could discover a pulsar in a similar orbit, its steady clock would allow them to study the behemoth’s gravitational field in fine detail. “It would be a better probe than anything that exists now,” Roshi says.
But some think the plan is a pipe dream. When choosing major projects, NSF and funders in Congress traditionally follow the recommendations of the decadal survey in astrophysics, a priority-setting exercise that at the turn of each decade asks the field what it wants to do next. The current one is already complete and will report in the coming months. “If you skip to the front of the line, those other projects would be furious,” Behnke says.
In theory, Congress could choose to set aside extra funds for a pet project, as happened after the 90-meter telescope at Green Bank Observatory collapsed in 1988. West Virginia’s influential senator pushed through funding for a replacement, resulting in the Robert C. Byrd Green Bank Telescope, inaugurated in 2000 and the world’s largest steerable dish. But Puerto Rico, with only a nonvoting representative in Congress, has little clout, even though it could use a leg up after being battered by earthquakes and hurricanes. “In terms of economy, [Puerto Rico] needs it,” Méndez says.
Lugo says advocates for a new telescope are talking to private foundations. And late last month Puerto Rico Governor Wanda Vázquez Garced allocated $8 million to clean up the site and design a replacement. Lugo says the money will go to a feasibility study of the new design. “We have to be optimistic that we will make this happen.”
But for researchers who relied on data gathered by Arecibo’s big eye, it won’t happen soon enough, leaving them to cast around for other, less capable instruments to continue their work. “I had so many projects in mind,” Vaddi says. “Along with the cable, this broke all my projects.”
With over 600 years in our presence, telescopes managed to become significant part of our science, enabling everyone between ordinary people, workers, amateur astronomers and scientists to easily take advantage of their powerful optic capabilities to make distant objects look closer.
Telescopes was shaped not only by the state of our industry and science, but also by famous inventors who managed to improve telescopes in very significant ways and pave the way for future inventors who all gave us the telescope technology that we have today.
Stereo Telescope - History
Isaac Newton reveals a new type of telescope, which uses polished mirrors instead of glass lenses.
December 25, 1642
March 20, 1727
• Invented the reflecting telescope
• Calculated the color-distortion effects of refractive lenses
While Newton’s reflecting telescope overcame the rainbow-like haloes around astronomical objects, it left the view somewhat distorted because of the mirror’s spherical curvature. Eventually, astronomers would adopt designs using mirrors with parabolic mirrors instead of spherical ones. (If you extend the curve of a parabolic surface into space it remains open, with the ends never joining.) James Gregory proposed this design in 1663, but there was no way to create parabolic mirrors at the time, so the design was not implemented for decades.
Grinding glass to the right shape for refracting telescope lenses was a daunting task, and Galileo’s telescopes produced a slightly blurry view of the sky, with colored “haloes” around astronomical objects. And the glass contained chemical impurities that colored the lenses green.
Isaac Newton, who is best known for devising his laws of motion and gravity, realized that part of the problem was with the glass itself. Any glass lens acts like a prism, splitting a beam of light into its individual wavelengths or colors, so there was no way to eliminate the colored haloes with lens-based telescopes.
A replica of Isaac Newton's reflecting telescope. Newton's mirror was made not of glass, but of a metal alloy consisting of three parts copper and one part tin, mixed with a small amount of arsenic, which would make the metal easier to polish. [© Andrew Dunn]
So Newton devised a new type of telescope, which he presented to his colleagues in England’s Royal Society in January 1672. Instead of glass lenses, Newton’s telescope used two polished metal mirrors.
The primary mirror, at the bottom of the telescope tube, curved inward slightly, in a spherical shape. (In other words, if you extend the curve of the mirror into space, it will form a sphere.) Light from an astronomical object struck this mirror and reflected back up the telescope tube, where it hit a flat secondary mirror. This mirror, which was tilted at a 45-degree angle, in turn reflected the light to an eyepiece at the side of the tube, where the observer saw an image of the star, planet, or other astronomical object.
Although it took a while to work out some problems and gain acceptance by most astronomers, Newton’s creation of the reflecting telescope ushered in a new era of astronomical study. By the early 18th century, most astronomers were using reflectors, and although refractors made a brief comeback a century later, all large modern-day research telescopes are reflectors, and Newtonian-style reflectors are popular among amateur astronomers.