22.09.2019
Radio telescopes and their characteristics, the operating principle of interferometers, space “radio astron”. The sharpest telescope
Characteristics of radio telescopes
Modern radio telescopes make it possible to study the Universe in detail that until recently was beyond the realm of possibility not only in the radio range, but also in traditional visible light astronomy. United in a single network of instruments located on different continents, we can look into the very core of radio galaxies, quasars, young star clusters, and emerging planetary systems. Radio interferometers with ultra-long bases are thousands of times more vigilant than the largest optical telescopes. With their help, you can not only track the movement of spacecraft in the vicinity of distant planets, but also study the movements of the crust of our own planet, including directly “feeling” continental drift. Next up are space radio interferometers, which will allow us to penetrate even deeper into the secrets of the Universe. |
The earth's atmosphere is not transparent to all types of electromagnetic radiation coming from space. It has only two wide “transparency windows”. The center of one of them falls on the optical region in which the maximum radiation of the Sun lies. It is to this that, as a result of evolution, the human eye has adapted in terms of sensitivity, which perceives light waves with a length of 350 to 700 nanometers. (In fact, this window of transparency is even slightly wider - from about 300 to 1,000 nm, that is, it covers near-ultraviolet and infrared ranges). However, the rainbow stripe of visible light is only a small fraction of the richness of the “colors” of the Universe. In the second half of the 20th century, astronomy became truly all-wave. Advances in technology have allowed astronomers to conduct observations in new spectral ranges. On the shorter wavelength side of visible light are the ultraviolet, x-ray and gamma ranges. On the other side are the infrared, submillimeter and radio ranges. For each of these ranges there are astronomical objects that manifest themselves most clearly in it, although in optical radiation they may not represent anything outstanding, so until recently astronomers simply did not notice them.
One of the most interesting and informative spectrum ranges for astronomy is radio waves. The radiation that is recorded by ground-based radio astronomy passes through a second and much wider window of transparency of the Earth's atmosphere - in the wavelength range from 1 mm to 30 m. The Earth's ionosphere - a layer of ionized gas at an altitude of about 70 km - reflects into space all radiation at longer wavelengths 30 m. At waves shorter than 1 mm, cosmic radiation is completely “eaten up” by atmospheric molecules (mainly oxygen and water vapor).
Modern radio telescopes make it possible to study the Universe in detail that until recently was beyond the realm of possibility not only in the radio range, but also in traditional visible light astronomy. Instruments united into a single network, located on different continents, allow us to look into the very core of radio galaxies, quasars, and young star clusters. |
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Arecibo in Puerto Rico has the world's largest fixed solid mirror - 305 m. A structure weighing 800 tons hangs above the spherical bowl on cables. The perimeter of the mirror is surrounded by a metal mesh that protects the telescope from radio radiation. | The world's largest fully rotating parabolic antenna at the Green Bank Observatory (West Virginia, USA). The 100x110 m mirror was built after a 90 m full-rotation antenna collapsed under its own weight in 1988. |
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The main characteristic of a radio telescope is its radiation pattern. It shows the sensitivity of the instrument to signals coming from different directions in space. For a “classical” parabolic antenna, the radiation pattern consists of a main lobe, which has the shape of a cone, oriented along the axis of the paraboloid, and several much (by orders of magnitude) weaker side lobes. The “visibility” of a radio telescope, that is, its angular resolution, is determined by the width of the main lobe of the radiation pattern. Two sources in the sky, which together fall into the solution of this lobe, merge into one for the radio telescope. Therefore, the width of the radiation pattern determines the size of the smallest details of the radio source that can still be distinguished individually.
A universal rule for telescope construction states that the resolution of the antenna is determined by the ratio of the wavelength to the diameter of the telescope mirror. Therefore, to increase vigilance, the telescope must be larger and the wavelength shorter. But as luck would have it, radio telescopes work with the longest waves of the electromagnetic spectrum. Because of this, even the huge size of the mirrors does not allow achieving high resolution. Not the largest modern optical telescope with a mirror diameter of 5 m can distinguish stars at a distance of only 0.02 arc seconds. Details of about one minute of arc are visible to the naked eye. A radio telescope with a diameter of 20 m at a wavelength of 2 cm gives a resolution even three times worse - about 3 arc minutes. A photo of an area of sky taken with an amateur camera contains more detail than a map of the radio emission of the same area obtained by a single radio telescope.
A wide radiation pattern limits not only the visual acuity of the telescope, but also the accuracy of determining the coordinates of the observed objects. Meanwhile, precise coordinates are needed to compare observations of an object in different ranges of e/magnetic radiation - this is an indispensable requirement of modern astrophysical research. Therefore, radio astronomers have always strived to create as many large antennas. And, surprisingly, radio astronomy ended up far surpassing optical astronomy in resolution.
Operating principle of radio telescopes
Fully rotating parabolic antennas - analogues of optical reflecting telescopes - turned out to be the most flexible in operation of the entire variety of radio astronomy antennas. They can be directed to any point in the sky, monitor the radio source - “accumulate the signal,” as radio astronomers say - and thereby increase the sensitivity of the telescope, its ability to isolate much weaker signals from cosmic sources against the background of all kinds of noise. The first large fully revolving paraboloid with a diameter of 76 m was built in 1957 at the British Jodrell Bank Observatory. And today the dish of the world's largest mobile antenna at the Green Bank Observatory (USA) measures 100 by 110 m. And this is practically the limit for single mobile radio telescopes. Increasing the diameter has three important consequences: two good and one bad. Firstly, the most important thing for us is that the angular resolution increases in proportion to the diameter. Secondly, sensitivity increases, much faster, in proportion to the area of the mirror, that is, the square of the diameter. And thirdly, the cost increases even faster, which in the case of a reflecting telescope (both optical and radio) is approximately proportional to the cube of the diameter of its main mirror.
The main difficulties are related to the deformation of the mirror under the influence of gravity. In order for the telescope mirror to clearly focus radio waves, the deviation of the surface from the ideal parabolic should not exceed one tenth of the wavelength. Such accuracy is easily achieved for waves of several meters or decimeters in length. But at short centimeter and millimeter waves, the required accuracy is already tenths of a millimeter. Due to structural deformations under its own weight and wind loads, it is almost impossible to create a fully rotating parabolic telescope with a diameter of more than 150 m. The largest fixed dish with a diameter of 305 m was built at the Arecibo Observatory, Puerto Rico. But in general, the era of gigantomania in the construction of radio telescopes has come to an end. In Mexico, on the Sierra Negra mountain, at an altitude of 4,600 meters, the construction of a 50-meter antenna for operation in the millimeter wave range is being completed. This may be the last large single antenna being built in the world.
In order to discern the details of the structure of radio sources, we need other approaches, which we have to understand. Radio waves emitted by the observed object propagate in space, generating periodic changes in electrical and magnetic field. A parabolic antenna collects radio waves incident on it at one point - the focus. When several electromagnetic waves pass through one point, they interfere, that is, their fields add up. If the waves arrive in phase, they strengthen each other; if they come in antiphase, they weaken each other, down to complete zero. The peculiarity of a parabolic mirror is precisely that all waves from one source come to focus in the same phase and reinforce each other in the maximum possible way! The functioning of all reflecting telescopes is based on this idea.
A bright spot appears at the focus, and a receiver is usually placed here, which measures the total intensity of the radiation captured within the telescope’s radiation pattern. Unlike optical astronomy, a radio telescope cannot take a photograph of an area of the sky. At each moment, it detects radiation coming from only one direction. Roughly speaking, a radio telescope works like a single-pixel camera. To build an image, you have to scan the radio source point by point. (However, the millimeter radio telescope being built in Mexico has a matrix of radiometers at the focus and is no longer “single-pixel.”)
"Team game of radio telescopes"
However, you can do it differently. Instead of bringing all the rays together at one point, we can measure and record the electric field fluctuations each ray produces at the surface of the mirror (or at another point through which the same ray passes), and then “add” these records in a computer device processing, taking into account the phase shift corresponding to the distance that each of the waves had to travel to the imaginary focus of the antenna. A device operating on this principle is called an interferometer, in our case a radio interferometer.
Interferometers eliminate the need to build huge, solid antennas. Instead, you can place dozens, hundreds, or even thousands of antennas next to each other and combine the signals they receive. Such telescopes are called in-phase arrays. However, they still do not solve the problem of “vigilance” - for this you need to take one more step. As you remember, as the size of a radio telescope increases, its sensitivity increases much faster than its resolution. Therefore, we quickly find ourselves in a situation where the power of the recorded signal is more than enough, but the angular resolution is sorely lacking. And then the question arises: “Why do we need a continuous array of antennas? Is it possible to thin it out?” It turned out that it is possible! This idea is called "aperture synthesis" because from several separate independent antennas placed on large area, a mirror of a much larger diameter is “synthesized.” The resolution of such a “synthetic” instrument is determined not by the diameter of the individual antennas, but by the distance between them - the base of the radio interferometer. Of course, there must be at least three antennas, and they should not be located along the same straight line. Otherwise, the resolution of the radio interferometer will be extremely inhomogeneous. It will be high only in the direction along which the antennas are spaced. In the transverse direction, the resolution will still be determined by the size of the individual antennas.
Radio astronomy began to develop along this path back in the 1970s. During this time, a number of large multi-antenna interferometers were created. Some of them have fixed antennas, others can move along the surface of the earth to carry out observations in different “configurations”. Such interferometers build “synthesized” maps of radio sources with much more high resolution than single radio telescopes: at centimeter waves it reaches 1 arc second, and this is comparable to the resolution of optical telescopes when observing through the Earth’s atmosphere.
The most famous system of this type is the "Very Large Lattice" ( Very Large Array, VLA) - built in 1980 at the US National Radio Astronomy Observatory. Its 27 parabolic antennas, each with a diameter of 25 m and weighing 209 tons, move along three radial rail tracks and can be removed from the center of the interferometer at a distance of up to 21 km. Today other systems are in operation: Westerbork in Holland (14 antennas with a diameter of 25 m), ATCA in Australia (6 antennas of 22 m each), MERLIN in the UK. The latter system, along with 6 other instruments scattered throughout the country, includes the famous 76-meter telescope. In Russia (in Buryatia), the Siberian Solar Radio Interferometer was created - a special antenna system for the operational study of the Sun in the radio range.
In 1965, Soviet scientists L.I. Matveenko, N.S. Kardashev, G.B. Sholomitsky proposed to independently record data on each interferometer antenna, and then process them together, as if simulating the phenomenon of interference on a computer. This allows the antennas to be spread over arbitrarily large distances. Therefore, the method was called very long baseline radio interferometry (VLBI) and has been successfully used since the early 1970s. The record base length achieved in experiments is 12.2 thousand km, and the resolution at a wavelength of about 3 mm reaches 0.00008'' - three orders of magnitude higher than that of large optical telescopes. It is unlikely that this result will be significantly improved on Earth, since the size of the base is limited by the diameter of our planet.
Currently, systematic observations are carried out by several networks of intercontinental radio interferometers. In the USA, a system has been created that includes 10 radio telescopes with an average diameter of 25 m, located in the continental part of the country, in Hawaii and Virgin Islands. In Europe, the 100-meter Bonn telescope and the 32-meter one in Medicina (Italy), MERLIN interferometers, Westerbork and other instruments are regularly combined for VLBI experiments. This system is called EVN. There is also the global International Network of Radio Telescopes for Astrometry and Geodesy IVS. And recently, Russia began operating its own interferometric network “Quasar” consisting of three 32-meter antennas located in Leningrad region, in the North Caucasus and Buryatia. It is important to note that telescopes are not rigidly assigned to VLBI networks. They can be used standalone or switch between networks.
Interferometry with ultra-long baselines requires very high measurement accuracy: it is necessary to record the spatial distribution of the maxima and minima of electromagnetic fields with an accuracy of a fraction of a wavelength, that is, for short waves up to a fraction of a centimeter. And with the highest accuracy, mark the time points at which measurements were taken on each antenna. Atomic frequency standards are used as ultra-precise clocks in VLBI experiments. But don’t think that radio interferometers have no drawbacks. Unlike a solid parabolic antenna, the radiation pattern of the interferometer, instead of one main lobe, has hundreds and thousands of narrow lobes of comparable size. Mapping a source with such a radiation pattern is like feeling the computer keyboard with splayed fingers. Image restoration is a complex and, moreover, “incorrect” (that is, unstable to small changes in measurement results) task, which, however, radio astronomers have learned to solve.
Achievements of radio interferometry
Radio interferometers with angular resolution of thousandths of an arc second “looked” into the very interior areas The most powerful “radio beacons” of the Universe are radio galaxies and quasars, which emit tens of millions of times more intense radio waves than ordinary galaxies. It was possible to “see” how clouds of plasma are ejected from the nuclei of galaxies and quasars, and to measure the speed of their movement, which turned out to be close to the speed of light. Many interesting things have been discovered in our Galaxy. In the vicinity of young stars, sources of maser radio emission (a maser is an analogue of an optical laser, but in the radio range) were found in the spectral lines of water, hydroxyl (OH) and methanol (CH 3 OH) molecules. On a cosmic scale, the sources are very small - less solar system. Individual bright spots on radio maps obtained by interferometers may be the embryos of planets.
Such masers have been found in other galaxies. The change in the positions of maser spots over several years, observed in the neighboring galaxy M33 in the constellation Triangulum, made it possible for the first time to directly estimate the speed of its rotation and movement across the sky. The measured displacements are negligible, their speed is many thousands of times less than the speed of a snail crawling along the surface of Mars, visible to an earthly observer. Such an experiment is still far beyond the capabilities of optical astronomy: it is simply not capable of detecting the proper movements of individual objects at intergalactic distances. Finally, interferometric observations have provided new confirmation of the existence of supermassive black holes. Around the core of the active galaxy NGC 4258, clumps of matter were discovered that move in orbits with a radius of no more than three light years, while their speeds reach thousands of kilometers per second. This means that the mass of the central body is at least a billion times the mass of the Sun, and it cannot be anything other than a black hole.
A number of interesting results were obtained by the VLBI method during observations in the Solar System. Start with at least the most accurate quantitative test to date general theory relativity. The interferometer measured the deflection of radio waves in the gravitational field of the Sun with an accuracy of one hundredth of a percent. This is two orders of magnitude more accurate than optical observations allow. Global radio interferometers are also used to track the movements of spacecraft studying other planets. The first time such an experiment was carried out was in 1985, when the Soviet spacecraft Vega-1 and -2 dropped balloons into the atmosphere of Venus. Observations confirmed the rapid circulation of the planet's atmosphere at a speed of about 70 m/s, that is, one revolution around the planet in 6 days. This amazing fact, which is still awaiting its explanation.
In 2004, similar observations involving a network of 18 radio telescopes on different continents accompanied the landing of the Huygens spacecraft on Saturn's moon Titan. From a distance of 1.2 billion km, the vehicle’s movement in Titan’s atmosphere was monitored with an accuracy of ten kilometers! It is not widely known that almost half of the scientific information was lost during the landing of Huygens. The probe relayed data through the Cassini station, which delivered it to Saturn. For reliability, two redundant data transmission channels were provided. However, shortly before landing, it was decided to transmit different information through them. But at the most crucial moment, due to an as yet unexplained glitch, one of the receivers on Cassini did not turn on, and half of the images were lost. And along with them, data on wind speed in Titan’s atmosphere, which was transmitted precisely through the disconnected channel, also disappeared. Fortunately, NASA managed to hedge their bets - the descent of Huygens was observed from Earth by a global radio interferometer. This would likely help salvage missing data on Titan's atmospheric dynamics. The results of this experiment are still being processed at the European Joint Radio Interferometric Institute, and, by the way, our compatriots Leonid Gurvits and Sergei Pogrebenko are doing this.
The future of radio interferometry
At least in the next half century, the general line of development of radio astronomy will be the creation of more and more large systems aperture synthesis - all large instruments being designed are interferometers. Thus, on the Chajnantor plateau in Chile, through the joint efforts of a number of European and American countries, the construction of the ALMA (Atacama Large Millimeter Array) millimeter-wave antenna system began. In total there will be 64 antennas with a diameter of 12 meters with an operating wavelength range from 0.35 to 10 mm. The maximum distance between ALMA antennas will be 14 km. Due to the very dry climate and high altitude above sea level (5100 m), the system will be able to conduct observations at waves shorter than a millimeter. In other places and at lower altitudes this is not possible due to the absorption of such radiation by water vapor in the air. Construction of ALMA will be completed by 2011.
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Radio telescopes of the present and near future on Earth and in Space |
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Project "Radioastron", launched in 2007 |
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The European LOFAR aperture synthesis system will operate at much longer wavelengths - from 1.2 to 10 m. It will go into operation within the next three years. This is very interesting project: to reduce cost, it uses the simplest fixed antennas - pyramids of metal rods about 1.5 m high with a signal amplifier. But there will be 25 thousand such antennas in the system. They will be united into groups that will be placed throughout Holland along the rays of a “curved five-pointed star” with a diameter of about 350 km. Each antenna will receive signals from the entire visible sky, but their joint computer processing will make it possible to highlight those that came from scientific directions of interest. In this case, a directive pattern of the interferometer is formed purely computationally, the width of which at the shortest wavelength will be 1 arc second. The operation of the system will require a huge amount of calculations, but for today's computers this is a completely feasible task. To solve this problem, the most powerful supercomputer in Europe, IBM Blue Gene/L, with 12,288 processors, was installed in Holland last year. Moreover, with appropriate signal processing (requiring even greater computer power), LOFAR will be able to simultaneously observe several or even many objects!
But the most ambitious project of the near future is SKA (Square Kilometer Array - Square Kilometer System). The total area of its antennas will be about 1 km 2, and the cost of the instrument is estimated at a billion dollars. The SKA project is still at an early stage of development. The main design option under discussion is thousands of antennas with a diameter of several meters, operating in the range from 3 mm to 5 m. Moreover, half of them are planned to be installed in an area with a diameter of 5 km, and the rest are to be spread over considerable distances. Chinese scientists proposed an alternative scheme - 8 fixed mirrors with a diameter of 500 m each, similar to the Arecibo telescope. Suitable dry lakes have even been proposed for their placement. However, in September, China dropped out of the list of countries competing to host the giant telescope. Now the main fight will take place between Australia and South Africa.
The possibilities for increasing the base of ground-based interferometers are almost exhausted. The future is the launch of interferometer antennas into space, where there are no restrictions related to the size of our planet. Such an experiment has already been carried out. In February 1997, the Japanese satellite HALCA was launched, which operated until November 2003 and completed the first stage in the development of the international project VSOP (VLBI Space Observatory Program - VLBI Space Observatory Program). The satellite carried an 8 m diameter umbrella antenna and operated in an elliptical Earth orbit that provided a base equal to three Earth diameters. Many extragalactic radio sources have been imaged with resolutions of thousandths of an arcsecond. The next stage of the space interferometry experiment, VSOP-2, is scheduled to begin in 2011-2012. Another instrument of this type is being created within the framework of the Radioastron project by the Astrospace Center of the Physical Institute. P.N. Lebedev RAS together with scientists from other countries. The Radioastron satellite will have a parabolic mirror with a diameter of 10 m. During launch it will be folded, and after entering orbit it will unfold. Radioastron will be equipped with receivers for several wavelengths - from 1.2 to 92 cm. Radio telescopes in Pushchino (Russia), Canberra (Australia) and Green Bank (USA) will be used as ground-based antennas of the space interferometer. The satellite's orbit will be very elongated, with an apogee of 350 thousand km. With such an interferometer base, at the shortest wavelength it will be possible to obtain images of radio sources and measure their coordinates with an accuracy of 8 millionths of an arcsecond. This will make it possible to look into the immediate vicinity of the nuclei of radio galaxies and black holes, and into the depths of the regions of formation of young stars in the Galaxy.
Authors of the material: Mikhail Prokhorov, Doctor of Physical and Mathematical Sciences and Georgy Rudnitsky, Candidate of Physical and Mathematical Sciences Magazine "Around the World": The most vigilant telescope
Russian scientists are also developing a more advanced space radio telescope for operation in the millimeter and submillimeter ranges - “Millimetron”. The instrument's mirror will be cooled with liquid helium to a temperature of 4 Kelvin (-269°C) to reduce thermal noise and increase sensitivity. Several options for the operation of this interferometer are being considered according to the “Space-Earth” and “Space-Space” schemes (between two telescopes on satellites). The device can be launched into the same elongated orbit as in the Radioastron project, or to the Lagrange point of the Sun-Earth system, at a distance of 1.5 million km in the antisolar direction from the Earth (this is 4 times further than the Moon). In the latest version, at a wavelength of 0.35 mm, the Space-Earth interferometer will provide an angular resolution of up to 45 billion fractions of an arcsecond! |
Using VLBI for Earth The radio interferometry method also has purely practical applications - it is not for nothing that, for example, in St. Petersburg, the Institute of Applied Astronomy of the Russian Academy of Sciences is working on this topic. Observations using VLBI technology make it possible not only to determine the coordinates of radio sources with an accuracy of up to a ten-thousandth of an arcsecond, but also to measure the positions of the radio telescopes themselves on Earth with an accuracy of better than one millimeter. This, in turn, makes it possible to track variations in the Earth's rotation and movements with the highest accuracy. earth's crust. For example, it was with the use of VLBI that the movement of continents was experimentally confirmed. Today, registration of such movements has already become routine. Interferometric observations of distant radio galaxies have firmly entered the arsenal of geophysics along with seismic sounding of the Earth. Thanks to them, periodic displacements of stations relative to each other caused by deformations of the earth’s crust are reliably recorded. Moreover, not only solid-body tides that have been measured for a long time (first recorded by the VLBI method) are noted, but also deflections that arise under the influence of changes atmospheric pressure, the weight of ocean water and the weight of groundwater. Leonid Petrov, Space Flight Center. Goddard, NASA |
table 2
Telescope characteristics
Perigee - 350,000 km.
Apogee-600km. /2/
The reflective parabolic antenna of the radio telescope has a diameter of 10 meters, consists of 27 petals and a 3-meter solid mirror.
The total mass of the scientific payload is approximately 2600 kg. It includes the antenna mass (1500 kg), electronic complex, containing receivers, low-noise amplifiers, frequency synthesizers, control units, signal converters, frequency standards, a highly informative scientific data transmission system - about 900 kg.
Currently, the largest antenna complexes in Russia, P-2500 (diameter 70 m) in the coastal city of Ussuriysk and TNA-1500 (diameter 64 m) in the village of Medvezhye Ozera near Moscow, are used for two-way communication sessions.
Communication with the Spektr-R device is possible in two modes. The first mode is two-way communication, including the transmission of commands to the board and the reception of telemetric information from it.
The second communication mode is the release of radio interferometric data through a highly directional antenna of a highly informative radio complex (VIRK).
Conclusion
I believe that this work sufficiently describes the available methods for obtaining cosmic radio emission. Using this work, you can follow trends in the development of radio telescopes. It can be noted that scientists have focused their efforts in improving telescopes more on increasing the angular expansion characteristics than on increasing the sensitivity of radio telescopes. This is most likely due to the fact that increasing sensitivity requires increasing the area, and therefore the diameter, of the antennas (2.5), which is very difficult to do after a certain threshold (150m). Since the observations carried out with the help of 'RadioAstron' turned out to be very productive, I think that radio astronomy will continue to develop in this direction (increasing resolution by increasing the aperture) by placing new orbital observatories that will be similar to 'RadioAstron'. My idea is confirmed by the presence of such a project as SNAP (SuperNova Acceleration Probe), which is planned to be launched in 2020. /5/
List of sources used
1. Kraus D. D. 1.2. Short story the first years of radio astronomy // Radio astronomy / Ed. V. V. Zheleznyakova. - M.: Soviet radio, 1973. - P. 14-21. - 456 s.
2. Related definitions[ Electronic resource] // Electronic Encyclopedia: website. - URL: http://ru.wikipedia.org/wiki/(access date: 05/12/2014)
3. Around the world.-M.: Popular Science. 2006-2007
4. Project Radioastron and space radio astronomy [Electronic resource] //Federal Space Agency: website. - URL: http://www.federalspace.ru/185/ (access date: 05/12/2014)
5. Information about the SNAP project [Electronic resource] // Supernova Acceleration Probe:
website. - URL: http://snap.lbl.gov/index.php (access date: 05/12/2014)
Application
Photos of the VLA radio interferomater and photos of the images obtained from them
Rice. 1VeryLargeArray(earthviews)
Rice. 2VeryLargeArray(satellite view) 
Rice. 3Image of the black hole 3C75 in the radio range
) and studies of their characteristics, such as: coordinates, spatial structure, radiation intensity, spectrum and polarization.
Radio telescope occupies the initial position, in terms of frequency range, among astronomical instruments for studying electromagnetic radiation - higher frequencies are thermal, visible, ultraviolet, x-ray and gamma radiation.
Radio telescopes preferably located far from the main settlements to minimize electromagnetic interference from broadcast radio stations, television, radars and other emitting devices. Placing a radio observatory in a valley or lowland protects it even better from the influence of man-made electromagnetic noise.
Device
Radio telescope consists of two main elements: an antenna device and a very sensitive receiving device - a radiometer. The radiometer amplifies the radio emission received by the antenna and converts it into a form convenient for recording and processing.
Radio telescope antenna designs are very diverse, due to the very wide range of wavelengths used in radio astronomy (from 0.1 mm to 1000 m). The antennas of radio telescopes that receive mm, cm, dm and meter waves are most often parabolic reflectors, similar to the mirrors of conventional optical reflectors. An irradiator is installed at the focus of the paraboloid - a device that collects radio emission, which is directed at it by a mirror. The irradiator transmits the received energy to the input of the radiometer, and, after amplification and detection, the signal is recorded on the tape of a recording electrical measuring instrument. On modern radio telescopes, the analog signal from the radiometer output is converted to digital and recorded on the hard drive in the form of one or more files.
To calibrate the obtained measurements (bringing them to absolute values of radiation flux density), a noise generator of known power is connected to the radiometer input instead of an antenna.
Depending on the antenna design and observation technique, the radio telescope can either be aimed in advance at a given point on the celestial sphere (through which the observed object will pass due to daily rotation), or operate in object tracking mode.
To direct antennas to the studied area of the sky, they are usually installed on Azimuth mounts, which provide rotation in azimuth and height (full rotation antennas). There are also antennas that allow only limited rotation, and even completely stationary. The reception direction in the latter type of antennas (usually very big size) is achieved by moving the feeds, which perceive the radio emission reflected from the antenna.
Principle of operation
The operating principle of a radio telescope is more similar to the operating principle of a photometer than an optical telescope. A radio telescope cannot construct an image directly; it only measures the energy of radiation coming from the direction in which the telescope is “looking.” Thus, to obtain an image of an extended source, a radio telescope must measure its brightness at each point.
Due to the diffraction of radio waves at the telescope aperture, measuring the direction to a point source occurs with some error, which is determined by the antenna radiation pattern and imposes a fundamental limitation on the resolution of the instrument:
where is the wavelength and is the aperture diameter. High resolution allows you to observe finer spatial details of the objects under study. To improve resolution, you need to either reduce the wavelength or increase the aperture. However, the use of short wavelengths increases the requirements for the quality of the mirror surface. Therefore, they usually take the path of increasing the aperture. Increasing the aperture also improves another important characteristic - sensitivity. A radio telescope must have high sensitivity to ensure reliable detection of sources as faint as possible. Sensitivity is determined by the level of flux density fluctuations:
where is the inherent noise power of the radio telescope, is the effective antenna area, is the frequency band, and is the signal accumulation time. To increase the sensitivity of radio telescopes, their collecting surface is increased and low-noise receivers and amplifiers based on masers, parametric amplifiers, etc. are used.
Radio interferometers
In addition to increasing the aperture diameter, there is another way to increase resolution (or narrow the radiation pattern). If you take two antennas located at a distance d(base) from each other, then the signal from the source to one of them will arrive a little earlier than to the other. If the signals from the two antennas are then interfered with, then from the resulting signal, using a special mathematical reduction procedure, it will be possible to recover information about the source with effective resolution. This reduction procedure is called aperture synthesis. Interference can be carried out both in hardware, by supplying a signal through cables and waveguides to a common mixer, and on a computer with signals previously digitized by exact time stamps and stored on a storage medium. Modern technical means have made it possible to create a VLBI system, which includes telescopes located on different continents and separated by several thousand kilometers.
The first radio telescopes
Home - Karl Jansky
Replica of the Jansky radio telescope
Story radio telescopes originates in 1931, with the experiments of Karl Jansky at the Bell Telephone Labs test site. To study the direction of arrival of lightning interference, he built a vertically polarized unidirectional antenna of the Bruce canvas type. The dimensions of the structure were 30.5 m in length and 3.7 m in height. The work was carried out at a wavelength of 14.6 m (20.5 MHz). The antenna was connected to a sensitive receiver, at the output of which there was a recorder with a long time constant.
Record of radiation received by Jansky on February 24, 1932. Maxima (arrows) repeat after 20 minutes. - period of full rotation of the antenna.
In December 1932, Yansky already reported the first results obtained with his installation. The article reported the discovery "... constant hissing of unknown origin", which “... difficult to distinguish from hissing caused by the noise of the equipment itself. The direction of arrival of the hissing interference changes gradually throughout the day, making a complete revolution in 24 hours.". In his next two papers, in October 1933 and October 1935, Karl Jansky gradually came to the conclusion that the source of his new interference was the central region of our galaxy. Moreover, the greatest response is obtained when the antenna is directed towards the center of the Milky Way.
Jansky recognized that progress in radio astronomy would require antennas large sizes with sharper diagrams that should be easily oriented in different directions. He himself proposed the design of a parabolic antenna with a mirror 30.5 m in diameter for operation at meter waves. However, his proposal did not receive support in the United States.
Rebirth - Grout Reber
In 1937, Grout Reber, a radio engineer from Weton (USA, Illinois) became interested in Jansky's work and designed an antenna with a parabolic reflector with a diameter of 9.5 m in the backyard of his parents' house. This antenna had a meridian mount, that is, it was controlled only by elevation , and the change in the position of the diagram lobe in right ascension was achieved due to the rotation of the Earth. Reber's antenna was smaller than Jansky's, but operated at shorter wavelengths, and its radiation pattern was much sharper. The Reber antenna had a conical beam with a width of 12° at half power, while the beam of the Jansky antenna had a fan shape with a width of 30° at half power at its narrowest section.
In the spring of 1939, Reber discovered radiation at a wavelength of 1.87 m (160 MHz) with a noticeable concentration in the plane of the Galaxy and published some results.
Radio map of the sky obtained by Grout Reber in 1944.
Improving his equipment, Reber undertook a systematic survey of the sky and in 1944 published the first radio maps of the sky at a wavelength of 1.87 m. The maps clearly show the central regions of the Milky Way and bright radio sources in the constellations Sagittarius, Cygnus A, Cassiopeia A, and Puppis. Reber's cards are quite good even compared to modern maps, meter wavelengths.
After World War II, significant technological improvements were made in the field of radio astronomy by scientists in Europe, Australia and the United States. Thus began the flowering of radio astronomy, which led to the development of millimeter and submillimeter wavelengths, allowing one to achieve significantly higher resolutions.
Classification of radio telescopes
A wide range of wavelengths, a variety of research objects in radio astronomy, the rapid pace of development of radio physics and radio telescope construction, and a large number of independent teams of radio astronomers have led to a wide variety of types of radio telescopes. It is most natural to classify radio telescopes according to the nature of filling their aperture and according to the methods of phasing the microwave field (reflectors, refractors, independent recording of fields):
Filled Aperture Antennas
Antennas of this type are similar to the mirrors of optical telescopes and are the simplest and most common to use. Filled aperture antennas simply collect the signal from the object being observed and focus it on the receiver. The recorded signal already contains scientific information and does not need synthesis. The disadvantage of such antennas is their low resolution. Filled aperture antennas can be divided into several classes based on their surface shape and mounting method.
Paraboloids of rotation
Almost all antennas of this type are installed on Alt-Az mounts and are fully rotatable. Their main advantage is that such radio telescopes can, like optical telescopes, be aimed at an object and guide it. Thus, observations can be carried out at any time as long as the object under study is above the horizon. Typical representatives: Green Bank Radio Telescope, RT-70, Kalyazin Radio Telescope.
Parabolic cylinders
The construction of full-rotation antennas is associated with certain difficulties associated with the huge mass of such structures. Therefore, fixed and semi-mobile systems are built. The cost and complexity of such telescopes increases much more slowly as their size increases. A parabolic cylinder collects rays not at a point, but on a straight line parallel to its generatrix (focal line). Because of this, telescopes of this type have an asymmetrical radiation pattern and different resolution along different axes. Another disadvantage of such telescopes is that, due to limited mobility, only part of the sky is available for observation. Representatives: University of Illinois radio telescope, Indian telescope in Ooty.
The path of rays in the Nance telescope
Antennas with flat reflectors
To work on a parabolic cylinder, it is required that several detectors be placed on the focal line, the signal from which is added taking into account the phases. This is not easy to do on short waves due to large losses in communication lines. Antennas with a flat reflector allow you to get by with only one receiver. Such antennas consist of two parts: a movable flat mirror and a fixed paraboloid. The movable mirror is “pointed” at the object and reflects the rays onto the paraboloid. The paraboloid concentrates the rays at the focal point where the receiver is located. Such a telescope has only part of the sky available for observation. Representatives: Kraus radio telescope, Large radio telescope in Nance.
Earthen bowls
The desire to reduce the cost of construction led astronomers to the idea of using natural relief as a telescope mirror. A representative of this type was the 300-meter. It is located in a karst sinkhole, the bottom of which is paved with aluminum sheets in the shape of a spheroid. The receiver is suspended on special supports above the mirror. The disadvantage of this instrument is that it can only access the sky within 20° of the zenith.
Antenna arrays (common mode antennas)
Such a telescope consists of many elementary irradiators (dipoles or spirals) located at a distance less than the wavelength. Thanks to precise control of the phase of each element, it is possible to achieve high resolution and effective area. The disadvantage of such antennas is that they are manufactured for a strictly defined wavelength. Representatives: BSA radio telescope in Pushchino.
Blank Aperture Antennas
The most important for astronomy purposes are two characteristics of radio telescopes: resolution and sensitivity. In this case, the sensitivity is proportional to the antenna area, and the resolution is maximum size. Thus, the most common circular antennas provide the worst resolution for the same effective area. Therefore, telescopes with a small area but high resolution appeared in radio astronomy. Such antennas are called blank aperture antennas, since they have “holes” in the aperture that exceed the wavelength. To obtain images from such antennas, observations must be carried out in aperture synthesis mode. For aperture synthesis, two synchronously operating antennas are sufficient, located at a certain distance, which is called base. To restore the source image, you need to measure the signal at all possible bases with a certain step up to the maximum.
If there are only two antennas, then you will have to conduct observation, then change the base, conduct observation at the next point, change the base again, etc. This synthesis is called consistent. A classic radio interferometer works on this principle. The disadvantage of sequential synthesis is that it is time-consuming and cannot reveal the variability of radio sources at short times. Therefore it is more often used parallel synthesis. It involves many antennas (receivers) at once, which simultaneously carry out measurements for all the necessary bases. Representatives: Northern Cross in Italy, DKR-1000 radio telescope in Pushchino.
The photo shows the Murchison Radio Astronomy Observatory, which is located in Western Australia. It includes 36 complexes with such mirror antennas operating in the 1.4 GHz range. The diameter of the main mirror of each antenna is 12 meters. Together, these antennas form part of one large radio telescope, Pathfinder. This is the largest radio telescope in existence today.
Dozens of reflective antennas are used for research and observation of the galaxy. They are able to look into such distances that the world's largest optical telescope, Hubble, is not capable of. Together, these antennas work as one large interferometer and form an array capable of collecting electromagnetic waves from the very edge of the universe.
Hundreds of thousands of antennas around the world are combined into one radio telescope, the Square Kilometer Array.
Similar radio telescopes are deployed around the globe, and many of them are planned to be combined into a single Square Kilometer Array (SKA) system by 2030, with a total receiving area of more than one square kilometer, as you probably guessed from the name. It will include more than two thousand antenna systems located in Africa and half a million complexes from Western Australia. 10 countries are participating in the SKA project: Australia, Canada, China, India, Italy, the Netherlands, New Zealand, South Africa, Sweden and United Kingdom:
No one has ever built anything like this. The SKA radio telescope system will help solve the most pressing mysteries of the universe. It will be able to measure a huge number of pulsars, stellar fragments and other cosmic bodies emitting electromagnetic waves along their magnetic poles. By observing such objects near black holes, new physical laws can be discovered and, perhaps, a unified theory will be developed. quantum mechanics and gravity.
The construction of a unified SKA system begins in stages with smaller components and Pathfinder in Australia will be one of those parts. In addition, the SKA1 system is currently under construction, which will be only a small part of the future Square Kilometer Array, but upon completion it will become the largest radio telescope in the world.
SKA1 will include two parts on different continents in Africa and Australia
SKA1 will consist of two parts: SKA1-mid in southern Africa, and SKA1-low in Australia. SKA1-mid is shown in the figure below and will include 197 reflector antennas with a diameter of 13.5 to 15 meters each:
And the SKA1-low system will be designed to collect low-frequency radio waves that appeared in space billions of years ago, when objects like stars were just beginning to exist. The SKA1-low radio telescope will not use reflective antennas to receive these radio waves. Instead, many smaller turnstile antennas will be installed, designed to collect signals in a wide range of frequencies, including television and FM bands, which coincide with the frequency of the oldest sources in the universe. SKA1-low antennas operate in the range from 50 to 350 MHz, their appearance is shown below:
By 2024, SKA project leaders plan to install more than 131,000 such antennas, grouped in clusters and scattered across the desert for tens of kilometers. One cluster will include 256 such antennas, the signals of which will be combined and transmitted through one fiber-optic communication line. The low-frequency antennas will work together to receive radiation that originated in the universe billions of years ago. And thus, they will help to understand physical processes happening in the distant past.
Operating principle of radio telescopes
Antennas combined into one common array work on the same principle as an optical telescope, only the radio telescope focuses not optical radiation, but received radio waves. The laws of physics dictate that the higher the received wavelength, the larger the diameter of the reflector antenna must be. This is, for example, what a radio telescope looks like without spatial diversity of receiving antenna systems - the operating five-hundred-meter spherical radio telescope FAST in the southwestern province of Guizhou in China. This radio telescope will also become part of the Square Kilometer Array (SKA) project in the future:
But it will not be possible to increase the diameter of the mirror indefinitely, and the implementation of an interferometer as in the photo above is not always and not possible everywhere, so you have to use a large number of geographically dispersed antennas of smaller size. For example, this type of antenna for radio astronomy is the Murchison Widefield Array (MWA). MWA antennas operate in the range from 80 to 300 MHz:
MWA antennas are also part of the SKA1-low system in Australia. They are also able to peer into a dark period of the early universe called the era of reionization. This era existed 13 billion years ago (about a billion years after the Big Bang), when nascent stars and other objects began to heat up a universe filled with hydrogen atoms. What's remarkable is that radio waves emitted by these neutral hydrogen atoms can still be detected. The waves were emitted at a wavelength of 21 cm, but by the time they reached Earth, billions of years of cosmic expansion had passed, stretching them out several more meters.
MWA antennas will be used to detect echoes from the distant past. Astronomers hope that studying this electromagnetic radiation will lead to a deeper understanding of how the early universe formed, and how structures like galaxies formed and changed during this era. Astronomers note that this is one of the main phases during the evolution of the Universe, which is completely unknown to us.
The image below shows sections with MWA antennas. Each section contains 16 antennas, which are interconnected into a single network using optical fiber:
MWA antennas receive radio waves in parts from different directions at the same time. Incoming signals are amplified at the center of each antenna by a pair of low-noise amplifiers and then sent to a nearby beamformer. There, waveguides of different lengths impart a certain delay to the antenna signals. At making the right choice With this delay, the beamformers "tilt" the array's radiation pattern so that radio waves coming from a particular area of the sky reach the antenna at the same time, as if they were being received by one large antenna.
MWA antennas are divided into groups. Signals from each group are sent to a single receiver, which distributes the signals between different frequency channels and then sends them to the central observatory building via fiber optics. There, using specialized software packages and graphics processing units, the data is correlated, multiplying the signals from each receiver and integrating them over time. This approach creates a single strong signal, as if it were received by one large radio telescope.
Like an optical telescope, the visibility range of such a virtual radio telescope is proportional to its physical size. In particular, for a virtual telescope consisting of a set of reflective or fixed antennas, the maximum resolution of the telescope is determined by its distance between several receiving parts. The greater this distance, the more accurate the resolution.
Today, astronomers are using this property to create virtual telescopes that span entire continents, allowing the telescope's resolution to be increased enough to see the black holes at the center of the Milky Way. But the size of a radio telescope is not the only requirement for obtaining detailed information about a distant object. The quality of resolution also depends on the total number of receiving antennas, the frequency range and the location of the antennas relative to each other.
Data obtained using MWA is sent hundreds of kilometers to the nearest data center with a supercomputer. MWA can send more than 25 terabytes of data per day and this speed will become even higher in the coming years with the release of SKA1-low. And the 131,000 antennas in the SKA1-low radio telescope, working in one common array, will collect more than a terabyte of data every day.
And this is how the problem with power supply of radio telescopes is solved. At the Murchison Radio Astronomy Observatory, power supply to the antenna complexes is provided by solar panels with a capacity of 1.6 megawatts:
Until recently, the observatory's antennas ran on diesel generators, but now, in addition to solar panels, it also has a huge number of lithium-ion battery packs that can store 2.6 megawatt-hours. Some parts of the antenna array will soon receive their own solar panels.
In such ambitious projects, the issue of financing is always quite acute. On this moment The construction budget for SKA1 in South Africa and Australia is approximately €675 million. This is the amount set by the project's 10 member countries: Australia, Canada, China, India, Italy, the Netherlands, New Zealand, South Africa, Sweden and the United Kingdom. But this funding does not cover the full cost of SKA1 that astronomers are hoping for. Therefore, the observatory is trying to attract more countries to a partnership that could increase funding.
Conclusion
Radio telescopes make it possible to observe distant space objects: pulsars, quasars, etc. This is how, for example, using the FAST radio telescope it was possible to detect a radio pulsar in 2016:
After the discovery of the pulsar, it was possible to establish that the pulsar is a thousand times heavier than the Sun and on earth one cubic centimeter of such matter would weigh several million tons. It is difficult to overestimate the importance of the information that can be obtained using such unusual radio telescopes.
Experienced radio operators know: when noise and crackling are sometimes heard in the radio receiver, you should not immediately blame the equipment: it is quite possible that it is the voice... The sun!
People first learned that the Sun has its own “radio station” in the 30s of the last century. The discoverer of cosmic radio waves was the young physicist Karl Jansky. He worked for an American radio company and was assigned to study the direction of arrival of atmospheric short-wave radio interference.
The young researcher designed a special antenna capable of receiving short waves. Armed with this antenna, he began to study the sources of radio interference and their direction. Imagine his surprise when the device stubbornly began to point to... the solar disk! Moreover, these hissing noises were repeated every 24 hours. This indicated that the source of interference may be associated with the Sun (24 hours, as we remember, is the length of a solar day on Earth). But after analyzing the data obtained more carefully, Karl Jansky saw that the radio signal he discovered was repeated every 24 hours, and every 23 hours 56 minutes is the duration of a sidereal day, not a solar one, that is, the period of rotation of the Earth relative to distant stars, and not the Sun . After checking astronomical maps, Karl Jansky discovered that the source of the radiation was an area in the center of our Milky Way galaxy, in the constellation Sagittarius.
Karl Jansky published an article in which he spoke about his discovery, but they did not believe him. But facts are stubborn things. Radio voices have also been discovered in other stars, planets and other celestial objects. This was the beginning new science- radio astronomy. It made it possible to learn a lot about the Universe that people had never suspected before.
Karl Jansky's circular "carousel antenna" - the first radio telescope
The antenna of a modern radio telescope no longer resembles the “clamshell” with which Yansky worked.
Radio telescope RT-32 RAO "Badary"
Located in the Badary Tunkinskogo tract district of the republic Buryatia (Russia).
Most often it is a giant metal bowl with a diameter of several tens or even hundreds of meters.
For example, the large Arecibo radio telescope is located in the crater of an extinct volcano in the Greater Antilles. The slopes of the crater were leveled and covered with metal shields. The result is a huge mirror bowl, with the help of which the radio voices of the stars are captured.
Arecibo Observatory (Puerto Rico).
The Arecibo radio telescope, built in 1963,
second only in size to the Chinese FAST telescope, launched in 2016.
The diameter of the Arecibo radio telescope mirror is almost 305 meters
One of the largest radio telescopes in the world, RATAN-600, is located in our country, near the village of Zelenchukskaya in the Stavropol Territory.
Even having built such a colossus, astronomers did not rest on this. In 1980, through the joint efforts of specialists from the countries of Eastern and Western Europe, as well as China and South Africa a radio telescope was created, the antenna of which turned out to be... half in diameter globe! The most surprising thing is that no new installations were built.
The whole trick is in the original approach that scientists used. Imagine, say, in Crimea and somewhere in Sweden, two radio telescopes are aimed at the same celestial object. At both telescopes, received signals are recorded and transmitted to a computer. Radio astronomers then compare the records and evaluate the information using electronic computers. As a result, it turns out that two telescopes work as one - in a common harness.
Moreover, in this way, not only two, but also a larger number of telescopes can act together. The antenna of such an all-planetary radio telescope turns out to be gigantic, extending over thousands of kilometers. Such networks of radio telescopes are called VLBI networks (stands for Very Long Baseline Radio Interferometry). The VLBI method was invented by the Americans in the 1970s. Nowadays, there are three large networks: QUASAR in Russia, EVN in Europe (Russian radio telescopes also participate in it), and VLBA in the USA.
In the future, scientists aim to create a radio telescope the size of the entire solar system. How? Exactly the same. They want to place one of the radio telescopes on board an automatic interplanetary station and send it somewhere to the outskirts of the solar system, for example, to the orbit of Saturn or Pluto. Other radio telescopes will turn on on Earth. And when the information received is processed using super-powerful computers, it will look as if a supergiant radio telescope was working.
The first step in this direction has already been taken - this is the international project "RadioAstron". The dimensions of this network already exceed the diameter of our planet, because in addition to ground-based radio telescopes, it includes a space radio telescope on the Russian Spektr-R spacecraft, launched into low-Earth orbit in 2011.
Why do scientists need such Gulliver’s “toys”? It turns out that the larger the radio telescope, the equal conditions His “radio ear” is more sensitive. “Teams” of radio telescopes are especially convenient for detecting sources with a complex spatial structure. That is, when not one, but a chorus of radio voices is heard from one place, and you need to figure out who belongs to whom.
In turn, specialists need the accumulated knowledge to better understand the structure of the world. For example, we still don’t know exactly how our solar system was formed. Geological processes on planets, chemical reactions in their depths the appearance of celestial bodies has greatly changed, and now it is not easy to imagine what they were originally like. So it would be important to trace the formation of any other planetary system. Then, by analogy, we could get a clear idea of how ours was formed.
Thus, by jointly “listening” to the gas and dust nebula in the constellation Orion, radio astronomers from five countries were able not only to hear individual radio voices in the general chorus, but also to guess what the “conversation” was about. Most likely, scientists believe, radio telescopes were able to detect protostars (stars whose formation has not yet been completed), perhaps even individual distant systems like the Solar System, right in the midst of construction. So, by watching them, we can learn, apparently, a lot of interesting things about our own.
Radio astronomers also managed to find traces of the Big Bang. Radio astronomers have detected background or relict radio emission in the depths of the Universe, which is nothing more than an echo of the Big Bang. Can you imagine how many billions of years have passed, and the radio echo is still walking across the vastness of the Universe. And scientists managed to hear it.
Thanks to VLBI networks, astronomers have the opportunity to study such mysterious space objects as pulsars, neutron stars, and black holes.
The advent of radio telescopes changed the nature of astronomers' work. As they themselves joke, many have now stopped looking at the stars at night through the “night telescope” of a conventional optical telescope, muttering under their breath the poems of M.V. Lomonosov: “The abyss of stars has opened and is full...” They now work on super-powerful computers, performing complex astronomical calculations, singing words from a romance to the words of M. Yu. Lermontov: “...And the star speaks to the star...”
