Characteristics of radio telescopes

Modern radio telescopes make it possible to explore the Universe in such detail, which until recently was beyond the limits of the possible not only in the radio range, but also in traditional visible light astronomy. United in a single network of instruments located on different continents, allow you to look into the very core of radio galaxies, quasars, young star clusters, forming planetary systems. Radio interferometers with extra-long baselines surpassed the largest optical telescopes in terms of "vigilance" by thousands of times. With their help, one 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 "feel" the drift of the continents. Next in line are space radio interferometers, which will allow even deeper insight into the mysteries of the universe.

The earth's atmosphere is not transparent to all types of electromagnetic radiation coming from outer space. It has only two wide "windows of transparency". The center of one of them falls on the optical region, in which the maximum radiation of the Sun lies. As a result of evolution, it was to him that the human eye adapted in terms of sensitivity, which perceives light waves with a length of 350 to 700 nanometers. (In fact, this transparency window is even slightly wider - from about 300 to 1,000 nm, that is, it captures the near ultraviolet and infrared ranges). However, the rainbow streak 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 make observations in new ranges of the spectrum. On the short wavelength side of visible light lie the ultraviolet, x-ray and gamma ranges. On the other side are infrared, submillimeter and radio bands. For each of these ranges, there are astronomical objects that manifest themselves most prominently in it, although in optical radiation they may not represent anything outstanding, so astronomers simply did not notice them until recently.
One of the most interesting and informative ranges of the spectrum for astronomy is radio waves. The radiation recorded by ground-based radio astronomy passes through a second and much wider transparency window 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 a height of about 70 km - reflects into space all radiation at wavelengths longer than 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 explore the Universe in such detail, which until recently was beyond the limits of the possible not only in the radio range, but also in traditional visible light astronomy. United in a single network of instruments located on different continents, allow you to look into the very core of radio galaxies, quasars, young star clusters.
Arecibo in Puerto Rico has the world's largest fixed solid mirror - 305 m. An 800 t structure hangs on cables above the spherical bowl. The mirror is surrounded by a metal mesh around the perimeter, which protects the telescope from radio emission.	The world's largest full-rotation parabolic antenna of the Green Bank Observatory (West Virginia, USA). The 100x110 m mirror was built after a 90 m full-revolving antenna collapsed under its own weight in 1988.

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 the main lobe, which has the form of a cone oriented along the axis of the paraboloid, and several much (by orders of magnitude) weaker side lobes. The "vigilance" 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 petal, 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, which can still be distinguished individually.
A universal rule for telescope construction says that the resolution of an antenna is determined by the ratio of the wavelength to the diameter of the telescope mirror. Therefore, to increase the "vigilance" the telescope must be larger, and the wavelength - smaller. But as luck would have it, radio telescopes work with the longest wavelengths 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 arcseconds. Details about one minute of arc are visible to the naked eye. And 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 snapshot of a section of the sky taken by an amateur camera contains more details than a radio emission map 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, exact 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 the largest possible antennas. And, surprisingly, radio astronomy ended up far ahead of optical in resolution.

The principle of operation of radio telescopes
Fully rotatable parabolic antennas - analogues of optical reflecting telescopes - turned out to be the most flexible in operation from the whole variety of radio astronomy antennas. They can be directed to any point in the sky, follow the radio source - "accumulate the signal", as radio astronomers say - and thereby increase the sensitivity of the telescope, its ability to distinguish much weaker signals from cosmic sources against the background of all kinds of noise. The first large full-turning 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) has dimensions of 100 by 110 m. And this is practically the limit for single mobile radio telescopes. The increase in diameter has three important consequences: two good and one bad. First, the most important thing for us is that the angular resolution increases in proportion to the diameter. Secondly, the sensitivity grows, and 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 mirror telescope (both optical and radio) is approximately proportional to the cube of the diameter of its main mirror.
The main difficulties are associated with the deformation of the mirror under the action of gravity. In order for the telescope mirror to clearly focus radio waves, the deviations of the surface from an ideal parabolic surface should not exceed one tenth of the wavelength. Such accuracy is easily achieved for wavelengths of several meters or decimeters. But at short centimeter and millimeter wavelengths, 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 full-rotation 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 on the whole, 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, construction of a 50-meter antenna for millimeter wave operation is being completed. Perhaps this is the last large single antenna created in the world.
In order to see the details of the structure of radio sources, other approaches are needed, which we have to figure out. Radio waves emitted by an observed object propagate in space, generating periodic changes in the electric and magnetic fields. A parabolic antenna collects the radio waves that have fallen 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 come in phase, they amplify each other, in antiphase, they weaken, up to complete zero. The peculiarity of a parabolic mirror is precisely that all waves from one source come into focus in one phase and amplify each other as much as possible! The functioning of all mirror 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 limits of the telescope's radiation pattern. Unlike optical astronomy, a radio telescope cannot take a photograph of a section 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, one has to scan the radio source point by point. (However, the millimeter radio telescope under construction in Mexico has a radiometer array in focus and is no longer “single-pixel”.)

"Team game of radio telescopes"
However, you can do it differently. Instead of bringing all the rays to one point, we can measure and record the electric field oscillations generated by each of them on the surface of the mirror (or at another point through which the same beam 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 according to this principle is called an interferometer, in our case, a radio interferometer.
Interferometers eliminate the need to build huge one-piece antennas. Instead, dozens, hundreds or even thousands of antennas can be placed next to each other and the signals received by them combined. 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 grows, its sensitivity grows much faster than its resolution. Therefore, we quickly find ourselves in a situation where the power of the recorded signal is more than enough, and the angular resolution is sorely lacking. And then the question arises: “Why do we need a solid array of antennas? Can't it be thinned out?" It turned out that it is possible! This idea is called "aperture synthesis" because a much larger diameter mirror is "synthesized" from several separate independent antennas placed over a large area. The resolution of such a "synthetic" instrument is determined not by the diameter of 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 one 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 as early as the 1970s. During this time, a number of large multi-antenna interferometers were created. Some of them have fixed antennas, while others can move along the surface of the earth to make observations in different "configurations". Such interferometers build "synthesized" maps of radio sources with a much higher resolution than single radio telescopes: at centimeter waves it reaches 1 arc second, and this is already comparable to the resolution of optical telescopes when observing through the Earth's atmosphere.

The most famous system of this type - "Very Large Array" (Very Large Array, VLA) - was 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 move away from the center of the interferometer at a distance of up to 21 km. Other systems are also operating today: Westerbork in the Netherlands (14 antennas with a diameter of 25 m), ATCA in Australia (6 antennas of 22 m each), MERLIN in the UK. The latest 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 system of antennas 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 jointly, as if simulating the phenomenon of interference on a computer. This allows the antennas to be spread over arbitrarily long distances. Therefore, the method was called very long baseline radio interferometry (VLBI) and has been successfully used since the early 1970s. The record baseline length achieved in the 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. The United States has created a system that includes 10 radio telescopes with an average diameter of 25 m, located in the continental part of the country, on the Hawaiian and Virgin Islands. In Europe, for VLBI experiments, the 100-meter Bonn telescope and the 32-meter telescope in Medicina (Italy), MERLIN interferometers, Westerbork, and other instruments are regularly combined. This system is called EVN. There is also a global network of radio telescopes for astrometry and geodesy IVS. And recently, Russia began to operate its own interferometric network "Kvazar" of three 32-meter antennas located in the Leningrad region, the North Caucasus and Buryatia. It is important to note that telescopes are not rigidly attached to VLBI networks. They can be used standalone or switched between networks.
Very long baseline interferometry requires very high measurement accuracy: it is necessary to fix 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 fractions of a centimeter. And with the highest accuracy, note the time points at which measurements were taken on each antenna. Atomic frequency standards are used as ultraprecise clocks in VLBI experiments. But do not think that radio interferometers do not have disadvantages. In contrast to a solid parabolic antenna, the directivity pattern of an interferometer has hundreds and thousands of narrow lobes of comparable size instead of one main lobe. Building a source map with such a radiation pattern is like touching a computer keyboard with outstretched 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 an angular resolution of thousandths of an arc second "peered" into the innermost regions of the most powerful "radio beacons" of the Universe - radio galaxies and quasars, which emit in the radio range tens of millions of times more intense than ordinary galaxies. It was possible to "see" how plasma clouds are ejected from the nuclei of galaxies and quasars, to measure the speed of their movement, which turned out to be close to the speed of light. Many interesting things were discovered in our Galaxy. In the vicinity of young stars, sources of maser radio emission have been found (a maser is an analogue of an optical laser, but in the radio range) in the spectral lines of water, hydroxyl (OH) and methanol (CH 3 OH) molecules. On a cosmic scale, the sources are very small - smaller than the solar system. Separate bright spots on the radio maps obtained by interferometers may be the embryos of planets.
Such masers have also 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, for the first time made it possible 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 beyond its power to notice the proper movements of individual objects at intergalactic distances. Finally, interferometric observations have provided new evidence for 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 solar masses, and it cannot be anything other than a black hole.
A number of interesting results have been obtained by the VLBI method in observations in the solar system. Let's start with the most accurate quantitative test of general relativity to date. The interferometer measured the deviation of radio waves in the gravitational field of the Sun with an accuracy of a 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 movement of spacecraft that study other planets. The first time such an experiment was carried out in 1985, when the Soviet vehicles "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 is an amazing fact that is yet to be explained.
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, they tracked how the device moves in the atmosphere of Titan with an accuracy of tens of kilometers! It is not widely known that almost half of the scientific information was lost during the Huygens landing. The probe relayed data through the Cassini station, which took it to Saturn. For reliability, two redundant data transmission channels were provided. However, shortly before landing, it was decided to transmit different information on them. But at the most crucial moment, due to an as yet unexplained failure, one of the receivers on the Cassini did not turn on, and half of the pictures disappeared. And along with them, the data on the wind speed in the atmosphere of Titan, which were transmitted just over the disconnected channel, also disappeared. Fortunately, NASA managed to play it safe - the descent of the Huygens was observed from Earth by a global radio interferometer. This, apparently, will save the missing data on the dynamics of the atmosphere of Titan. 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 Sergey 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 ever larger systems of aperture synthesis - all large instruments being designed are interferometers. So, on the Chajnantor Plateau in Chile, joint efforts of a number of European and American countries began the construction of the ALMA (Atacama Large Millimeter Array) millimeter-wave antenna system. 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 longest distance between ALMA antennas will be 14 km. Due to the very dry climate and high altitude (5100 m), the system will be able to observe at waves shorter than a millimeter. In other places and at a lower altitude, 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.

Radio telescopes of the present and near future time on Earth and in Space
	Project "Radioastron", launched in 2007

The European aperture synthesis system LOFAR will operate at much longer wavelengths - from 1.2 to 10 m. It will be operational within the next three years. This is a very interesting project: in order to reduce the 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 in 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 single out those that came from directions of interest to scientists. In this case, a directivity pattern of the interferometer is formed by purely computational means, 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 quite a feasible task. To solve this problem, Europe's most powerful supercomputer IBM Blue Gene/L with 12,288 processors was installed last year in Holland. Moreover, with appropriate signal processing (requiring even more computer power), LOFAR will be able to simultaneously observe several and even many objects!
But the most ambitious project in the near future is SKA (Square Kilometer Array). The total area of ​​its antennas will be about 1 km2, and the cost of the instrument is estimated at one 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 on a site with a diameter of 5 km, and the rest 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 to accommodate them. However, in September, China dropped out of the number of countries - contenders for the placement of a giant telescope. Now the main struggle will unfold between Australia and South Africa.
The possibilities of increasing the base of ground-based interferometers are practically exhausted. The future is to launch interferometer antennas into space, where there are no restrictions associated with the size of our planet. Such an experiment has already been carried out. In February 1997, the Japanese HALCA satellite was launched, which worked 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 umbrella-shaped antenna 8 m in diameter and operated in an elliptical Earth orbit that provided a base three times the diameter of the Earth. Images of many extragalactic radio sources were obtained with a resolution of thousandths of an arc second. The next phase 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 in a folded state, and after entering orbit, it will turn around. 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 space interferometer antennas. The satellite's orbit will be very elongated, with an apogee of 350,000 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 arc second. This will make it possible to look into the immediate vicinity of the nuclei of radio galaxies and black holes, into the depths of the formation regions of young stars in the Galaxy.

The 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 keen telescope

Russian scientists are also developing a more advanced space radio telescope for operation in the millimeter and submillimeter ranges - the Millimetron. The mirror of this instrument will be cooled with liquid helium to 4 Kelvin (-269°C) to reduce thermal noise and improve sensitivity. Several options for the operation of this interferometer according to the "Space-to-Earth" and "Space-to-Space" schemes (between two telescopes on satellites) are being considered. 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 anti-Sun direction from the Earth (this is 4 times farther than the Moon). In the latter version, at a wavelength of 0.35 mm, the Cosmos-Earth interferometer will provide an angular resolution of up to 45 billion fractions of an arc second!

Use of VLBI for the Earth The method of radio interferometry also has purely practical applications - not in vain, for example, in St. Petersburg, the Institute of Applied Astronomy of the Russian Academy of Sciences deals with 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 arc second, 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 of the Earth's crust with the highest accuracy. For example, it was with the use of VLBI that the motion of the continents was experimentally confirmed. Today, the registration of such movements has already become a routine matter. 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 long-term measured solid-state tides (for the first time recorded by the VLBI method) are noted, but also deflections that occur under the influence of changes in atmospheric pressure, the weight of water in the ocean, and the weight of groundwater. To determine the parameters of the Earth's rotation in the world, daily observations of celestial radio sources are carried out, coordinated by the International VLBI Service for Astrometry and Geodesy IVS. The data obtained are used, in particular, to detect the drift of the orbital planes of satellites of the global positioning system GPS. Without the introduction of appropriate corrections obtained from VLBI observations, the error in determining longitude in the GPS system would be orders of magnitude larger than it is now. In a sense, VLBI plays the same role for GPS navigation that accurate marine chronometers played for star navigation in the 18th century. Accurate knowledge of the parameters of the Earth's rotation is also necessary for the successful navigation of interplanetary space stations. Leonid Petrov, Center for Space Flights. Goddard, NASA

→

A radio telescope is a type of telescope used to study the electromagnetic radiation of objects. It allows studying the electromagnetic radiation of astronomical objects in the carrier frequency range from tens of MHz to tens of GHz. With the help of a radio telescope, scientists can take the object's own radio emission and, based on the data obtained, study its characteristics, such as the coordinates of sources, spatial structure, radiation intensity, as well as spectrum and polarization.

For the first time, radio space radiation was discovered in 1931 by Karl Jansky, an American radio engineer. While studying atmospheric radio interference, Jansky discovered constant radio noise. At that time, the scientist could not exactly explain its origin and identified its source with the Milky Way, namely with its central part, where the center of the galaxy is located. Only in the early 1940s, Jansky's work was continued and contributed to the further development of radio astronomy.

The radio telescope consists of an antenna system, a radiometer, and recording equipment. A radiometer is a receiving device that measures the power of low-intensity radiation in the radio wave range (wavelengths from 0.1 mm to 1000 m). In other words, the radio telescope occupies the lowest frequency position compared to other instruments that study electromagnetic radiation (for example, an infrared telescope, an X-ray telescope, etc.).

The antenna is a device for collecting radio emission from celestial objects. The sonographic characteristics of any antenna are: sensitivity (that is, the minimum possible signal to detect), as well as angular resolution (that is, the ability to separate radiation from several radio sources that are located close to each other).

It is very important that the radio telescope has a high sensitivity and good resolution, since this is what makes it possible to observe smaller spatial details of the objects under study. The minimum flux density DP, which is recorded, is determined by the relationship:
DP=P/(S\sqrt(Dft))
where P is the intrinsic noise power of the radio telescope, S is the effective area of ​​the antenna, Df is the frequency band that is received, t is the signal accumulation time.

Antennas used in radio telescopes can be divided into several main types (the classification is made depending on the wavelength range and purpose):
Full aperture antennas: parabolic antennas (used for observation at short waves; mounted on rotary devices), radio telescope with spherical mirrors (wave range up to 3 cm, fixed antenna; movement in space of the antenna beam is carried out by irradiating different parts of the mirror), Kraus radio telescope (wavelength 10 cm ; a stationary vertically located spherical mirror, to which the radiation of the source is directed using a flat mirror set at a certain angle), periscope antennas (small in the vertical direction and large in the horizontal direction);
Blank aperture antennas(two types depending on the image reproduction method: sequential synthesis, aperture synthesis - see below). The simplest instrument of this type is a simple radio interferometer (an interconnected system of two radio telescopes for simultaneous observation of a radio source: has a higher resolution, example: Aperture Synthesis Interferometer in Cambridge, England, wavelength 21 cm). Other types of antennas: cross (Mills cross with serial synthesis in Molongo, Australia, wavelength 73.5 cm), ring (serial synthesis type instrument in Kalgoor, Australia, wavelength 375 cm), composite interferometer (aperture synthesis interferometer in Flers , Australia, wavelength 21).

The most accurate in operation are full-rotation parabolic antennas. In the case of their use, the sensitivity of the telescope is enhanced due to the fact that such an antenna can be directed to any point in the sky, accumulating a signal from a radio source. Such a telescope singles out the signals of cosmic sources against the background of various noises. The mirror reflects radio waves, which are focused and captured by the irradiator. The irradiator is a half-wave dipole that receives radiation of a given wavelength. The main problem with the use of radio telescopes with parabolic mirrors is that the mirror deforms under the action of gravity during rotation. It is because of this that in the case of an increase in diameter above about 150 m, deviations in measurements increase. However, there are very large radio telescopes that have been operating successfully for many years.

Sometimes, for more successful observations, several radio telescopes are used, installed at a certain distance from each other. Such a system is called a radio interferometer (see above). The principle of its operation is to measure and record the oscillations of the electromagnetic field, which are generated by individual beams on the surface of a mirror or another point through which the same beam passes. After that, the records are added taking into account the phase shift.

If the antenna array is made not continuous, but spaced apart at a sufficiently large distance, then a mirror of large diameter will be obtained. Such a system works on the principle of "aperture synthesis". In this case, the resolution is determined by the distance between the antennas, not their diameter. Thus, this system allows not to build huge antennas, but to get by with at least three, located at certain intervals. One of the most famous systems of this kind is VLA (Very Large Array). This array is located in the US state of New Mexico. The "Very Large Grid" was created in 1981. The system consists of 27 full rotatable parabolic antennas, which are located along two lines forming the letter “V”. The diameter of each antenna reaches 25 meters. Each antenna can take one of 72 positions, moving along the rail tracks. The VLA is as sensitive as an antenna with a diameter of 136 kilometers and surpasses the best optical systems in terms of angular resolution. It is no coincidence that the VLA was used in the search for water on Mercury, radio coronas around stars and other phenomena.

By their design, radio telescopes are most often open. Although in some cases, in order to protect the mirror from weather events (temperature changes and wind loads), the telescope is placed inside a dome: solid (Highstack Observatory, 37-m radio telescope) or with a sliding window (11-m radio telescope at Kitt Peak, USA).

Currently, the prospects for the use of radio telescopes lie in the fact that they allow you to establish communication between antennas located in different countries and even on different continents. Such systems are called very long baseline radio interferometers (VLBI). A network of 18 telescopes was used in 2004 to monitor the Huygens landing on Saturn's moon Titan. The design of the ALMA system, consisting of 64 antennas, is underway. The prospect for the future is the launch of interferometer antennas into space.

The principle of operation of a radio telescope

2.1.1 A 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 registration and processing.

The designs of antennas of radio telescopes 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 self-recording electrical measuring device. On modern radio telescopes, the analog signal from the output of the radiometer is converted into digital and recorded on a hard disk in the form of one or several files.

To direct antennas into the region of the sky under study, they are usually mounted on azimuth mounts that provide rotation in azimuth and altitude (full-rotation antennas). There are also antennas that allow only limited rotations, and even completely stationary. The direction of reception in antennas of the latter type (usually very large) is achieved by moving the feeds, which perceive the radio emission reflected from the antenna.

2.1.2 The principle of operation of a radio telescope is more similar to that of a photometer than an optical telescope. A radio telescope cannot build an image directly, it only measures the energy of radiation coming from the direction in which the telescope "looks". Thus, to obtain an image of an extended source, the radio telescope must measure its brightness at each point.

Due to the diffraction of radio waves by the aperture of the telescope, the measurement of the direction to a point source occurs with some error, which is determined by the antenna 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, 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 follow the path of increasing the aperture. Increasing the aperture also improves another important characteristic - sensitivity. A radio telescope must have a high sensitivity in order to reliably detect the faintest possible sources. Sensitivity is determined by the level of flux density fluctuations:

,

where is the intrinsic noise power of the radio telescope, is the effective area (collecting surface) of the antenna, 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.

A telescope is a unique optical instrument designed to observe celestial bodies. The use of instruments allows us to consider a variety of objects, not only those that are located near us, but also those that are thousands of light years away from our planet. So what is a telescope and who invented it?

First inventor

Telescopic devices appeared in the seventeenth century. However, to this day there is a debate about who invented the telescope first - Galileo or Lippershey. These disputes are related to the fact that both scientists at about the same time were developing optical devices.

In 1608, Lippershey developed eyeglasses for the nobility, allowing them to see distant objects up close. At this time, military negotiations were underway. The army quickly appreciated the benefits of the development and suggested that Lippershey not assign copyright to the device, but modify it so that it could be viewed with two eyes. The scientist agreed.

The new development of the scientist could not be kept secret: information about it was published in local print media. Journalists of that time called the device a spotting scope. It used two lenses, which made it possible to enlarge objects and objects. From 1609, pipes with a threefold increase were sold with might and main in Paris. Since this year, any information about Lippershey disappears from history, and information about another scientist and his new discoveries appears.

Around the same time, the Italian Galileo was engaged in grinding lenses. In 1609, he presented to society a new development - a telescope with a threefold increase. Galileo's telescope had a higher image quality than Lippershey's tubes. It was the brainchild of the Italian scientist who received the name "telescope".

In the seventeenth century, telescopes were made by Dutch scientists, but they had poor image quality. And only Galileo managed to develop such a technique for grinding lenses, which made it possible to clearly enlarge objects. He was able to get a twenty-fold increase, which was a real breakthrough in science in those days. Based on this, it is impossible to say who invented the telescope: if, according to the official version, it was Galileo who presented the world with a device that he called a telescope, and if you look at the version of the development of an optical device for magnifying objects, then Lippershey was the first.

First observations of the sky

After the advent of the first telescope, unique discoveries were made. Galileo applied his development to tracking celestial bodies. He was the first to see and sketch lunar craters, spots on the Sun, and also considered the stars of the Milky Way, satellites of Jupiter. Galileo's telescope made it possible to see the rings of Saturn. For your information, there is still a telescope in the world that works on the same principle as Galileo's device. It is located at the York Observatory. The device has a diameter of 102 centimeters and regularly serves scientists to track celestial bodies.

Modern telescopes

Over the centuries, scientists have constantly changed the devices of telescopes, developed new models, and improved the magnification factor. As a result, it was possible to create small and large telescopes with different purposes.

Small ones are usually used for home observations of space objects, as well as for observing nearby space bodies. Large devices allow you to view and take pictures of celestial bodies located thousands of light years from Earth.

Types of telescopes

There are several types of telescopes:

1. Mirrored.
2. Lens.
3. catadioptric.

Galilean refractors are classified as lens refractors. Reflective type devices are referred to as mirror devices. What is a catadioptric telescope? This is a unique modern development that combines a lens and a mirror device.

Lens telescopes

Telescopes play an important role in astronomy: they allow you to see comets, planets, stars and other space objects. One of the first developments were lens devices.

Every telescope has a lens. This is the main part of any device. It refracts rays of light and gathers them at a point called a focus. It is in it that the image of the object is built. An eyepiece is used to view the image.

The lens is placed so that the eyepiece and focus match. In modern models, movable eyepieces are used for convenient observation through a telescope. They help to adjust the sharpness of the image.

All telescopes have aberration - a distortion of the object in question. Lens telescopes have several distortions: chromatic (red and blue rays are distorted) and spherical aberration.

Mirror models

Mirror telescopes are called reflectors. A spherical mirror is mounted on them, which collects the light beam and reflects it with the help of a mirror onto the eyepiece. Chromatic aberration is not characteristic of mirror models, since light is not refracted. However, mirror instruments exhibit spherical aberration, which limits the field of view of the telescope.

Graphic telescopes use complex structures, mirrors with complex surfaces that differ from spherical ones.

Despite the complexity of the design, mirror models are easier to develop than lens counterparts. Therefore, this type is more common. The largest diameter of a mirror-type telescope is more than seventeen meters. On the territory of Russia, the largest device has a diameter of six meters. For many years it was considered the largest in the world.

Telescope Specifications

Many people buy optical devices for observing space bodies. When choosing a device, it is important to know not only what a telescope is, but also what characteristics it has.

1. Increase. The focal length of the eyepiece and the object is the magnification of the telescope. If the focal length of the lens is two meters, and the eyepiece is five centimeters, then such a device will have a magnification of forty times. If the eyepiece is replaced, the magnification will be different.
2. Permission. As you know, light is characterized by refraction and diffraction. Ideally, any image of a star looks like a disk with several concentric rings, called diffraction rings. The dimensions of the disks are limited only by the capabilities of the telescope.

Telescopes without eyes

And what is a telescope without an eye, what is it used for? As you know, each person's eyes perceive the image differently. One eye can see more and the other less. So that scientists can see everything they need to see, they use telescopes without eyes. These devices transmit the image to the monitor screens, through which everyone sees the image exactly as it is, without distortion. For small telescopes, for this purpose, cameras have been developed that are connected to devices and take pictures of the sky.

The most modern methods of space vision is the use of CCD cameras. These are special light-sensitive microcircuits that collect information from the telescope and transfer it to a computer. The data received from them is so clear that it is impossible to imagine what other devices could receive such information. After all, the human eye cannot distinguish all shades with such high clarity, as modern cameras do.

Spectrographs are used to measure the distances between stars and other objects. They are connected to telescopes.

A modern astronomical telescope is not one device, but several at once. The received data from several devices are processed and displayed on monitors in the form of images. Moreover, after processing, scientists receive images of very high definition. It is impossible to see the same clear images of space with the eyes through a telescope.

radio telescopes

Astronomers use huge radio telescopes for their scientific developments. Most often they look like huge metal bowls with a parabolic shape. Antennas collect the received signal and process the received information into images. Radio telescopes can only receive one wave of signals.

infrared models

A striking example of an infrared telescope is the Hubble apparatus, although it can be optical at the same time. In many ways, the design of infrared telescopes is similar to the design of optical mirror models. Heat rays are reflected by a conventional telescopic lens and focused at one point, where the device that measures heat is located. The resulting heat rays are passed through thermal filters. Only then does the photo take place.

Ultraviolet telescopes

Film may be exposed to ultraviolet light when photographed. In some part of the ultraviolet range, it is possible to receive images without processing and exposure. And in some cases it is necessary that the rays of light pass through a special design - a filter. Their use helps to highlight the radiation of certain areas.

There are other types of telescopes, each of which has its own purpose and special characteristics. These are models such as X-ray and gamma-ray telescopes. According to their purpose, all existing models can be divided into amateur and professional. And this is not the whole classification of devices for tracking celestial bodies.

Taganrog State Pedagogical Institute named after A.P. Chekhov"

Radio astronomy. radio telescopes.

Main characteristics.

Completed by a student

Faculty of Physics and Mathematics

51 groups: Mazur V.G.

Taganrog

Introduction

radio astronomy 1. Comparison with optical astronomy…………………………. 2. Ranges of registered radio emission……………….. 3. Historical background………………………………………….. Radio telescopes…………………………………………………. 4. Working principle ……………………………………………….. 5. Radio interferometers…………………………………………. 6. The first radio telescopes ………………………………………. 7. Classification of radio telescopes………………………………… a) Antennas with a filled aperture…………………………… b) Paraboloids of revolution………………………………………… c) Parabolic cylinders……………………………………… d) Antennas with flat reflectors…………………………… e) Earth bowls…………………………………………………. f) Antenna arrays (common-mode antennas)…………………… g) Unfilled aperture antennas………………………… Conclusion Bibliography

Introduction

Radio astronomy is a branch of astronomy that studies space objects by analyzing the radio emission coming from them. Many cosmic bodies emit radio waves that reach the Earth: these are, in particular, the outer layers of the Sun and the atmospheres of planets, clouds of interstellar gas. Radio emission is accompanied by such phenomena as the interaction of turbulent gas flows and shock waves in the interstellar medium, the rapid rotation of neutron stars with a strong magnetic field, "explosive" processes in the nuclei of galaxies and quasars, solar flares, etc. The radio signals of natural objects arriving at the Earth have the nature of noise . These signals are received and amplified by special electronic equipment and then recorded in analog or digital form. Often, radio astronomy is more sensitive and long-range than optical.

A radio telescope is an astronomical instrument for receiving the own radio emission of celestial objects (in the Solar System, the Galaxy and the Metagalaxy) and studying their characteristics, such as: coordinates, spatial structure, radiation intensity, spectrum and polarization.

RADIO ASTRONOMY

§1. Comparison with optical astronomy

Of all types of cosmic electromagnetic radiation, only visible light, near (short-wave) infrared radiation and part of the radio wave spectrum pass through its atmosphere through its atmosphere, practically unabated. On the one hand, radio waves, which have a much longer wavelength than optical radiation, easily pass through the cloudy atmospheres of planets and clouds of interstellar dust, which are opaque to light. On the other hand, only the shortest radio waves pass through regions of ionized gas that are transparent to light around stars and in interstellar space. Weak space signals are picked up by radio astronomers using radio telescopes, the main elements of which are antennas. Usually these are metal reflectors in the form of a paraboloid. At the focus of the reflector, where the radiation is concentrated, a collecting device is placed in the form of a horn or dipole, which diverts the collected radio emission energy to the receiving equipment. Reflectors with a diameter of up to 100 m are made movable and full-circle; they can aim at an object in any part of the sky and follow it. Larger reflectors (up to 300 m in diameter) are motionless, in the form of a huge spherical bowl, and pointing at the object occurs due to the rotation of the Earth and the movement of the irradiator in the focus of the antenna. Even larger reflectors usually look like part of a paraboloid. The larger the reflector, the more detailed the observed radio pattern. Often, to improve it, one object is observed synchronously by two radio telescopes or their entire system containing several dozen antennas, sometimes separated by thousands of kilometers.

§2. Ranges of registered radio emission

Radio waves from a few millimeters to 30 m long pass through the earth's atmosphere; in the frequency range from 10 MHz to 200 GHz. Thus, radio astronomers are dealing with frequencies that are noticeably higher than, for example, the broadcast radio range of medium or short waves. However, with the advent of VHF and television broadcasting in the frequency range of 50-1000 MHz, as well as radars (radar) in the range of 3-30 GHz, radio astronomers have problems: powerful signals from terrestrial transmitters in these ranges interfere with the reception of weak space signals. Therefore, through international agreements, radio astronomers have been allocated several frequency bands for space observation in which signal transmission is prohibited.

§3. History reference

Radio astronomy as a science began in 1931, when K. Yansky from the Bell Telephone Company began to study radio interference and discovered that they come from the central part of the Milky Way. The first radio telescope was built in 1937-1938 by radio engineer G. Reber, who independently made a 9-meter reflector in his garden from iron sheets, in principle the same as the current giant parabolic antennas. Reber compiled the first radio map of the sky and found that the entire Milky Way radiates at a wavelength of 1.5 m, but its central part radiates most strongly. In February 1942, J. Hay noticed that in the meter range the Sun interferes with radars when flashes occur on it; radio emission of the Sun in the centimeter range in 1942-1943 was discovered by J. Southworth. The systematic development of radio astronomy began after World War II. In Great Britain, the large observatory Jodrell Bank (University of Manchester) and the station of the Cavendish Laboratory (Cambridge) were created. The Radiophysical Laboratory (Sydney) has set up several stations in Australia. Dutch radio astronomers began to study clouds of interstellar hydrogen. In the USSR, radio telescopes were built near Serpukhov, in Pulkovo, in the Crimea. The largest radio observatories in the United States are the National Radio Astronomy Observatories in Green Bank (West Virginia) and Charlottesville (Virginia), the Cornell University Observatory in Arecibo (Puerto Rico), the Caltech Observatory in Owens Valley (West Virginia). California), MIT Lincoln Laboratory and Harvard University Oak Ridge Observatory (Massachusetts), UC Berkeley Hat Creek Observatory (CA), University of Massachusetts Five College Radio Astronomy Observatory (Massachusetts).

RADIOTELESCOPES

The radio telescope occupies the initial, in terms of frequency range, position among astronomical instruments for studying electromagnetic radiation. Higher frequency telescopes are thermal, visible, ultraviolet, x-ray and gamma radiation.

Radio telescopes should preferably be located far from major population centers to minimize electromagnetic interference from broadcast radio stations, television, radar, and other emitting devices. Placing a radio observatory in a valley or lowland protects it even better from the influence of technogenic electromagnetic noise.

A 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 registration and processing.

The designs of antennas of radio telescopes 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 self-recording electrical measuring instrument. On modern radio telescopes, the analog signal from the output of the radiometer is converted into digital and recorded on a hard disk in the form of one or several files.

To direct antennas into the region of the sky under study, they are usually mounted on azimuth mounts that provide rotation in azimuth and altitude (full-rotation antennas). There are also antennas that allow only limited rotations, and even completely stationary. The direction of reception in antennas of the latter type (usually very large) is achieved by moving the feeds, which perceive the radio emission reflected from the antenna.

§4. Principle of operation

The principle of operation of a radio telescope is more similar to that of a photometer than an optical telescope. A radio telescope cannot build an image directly, it only measures the energy of radiation coming from the direction in which the telescope "looks". Thus, to obtain an image of an extended source, the radio telescope must measure its brightness at each point.

Due to the diffraction of radio waves by the aperture of the telescope, the measurement of the direction to a point source occurs with some error, which is determined by the antenna 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, either reduce the wavelength or increase the aperture. However, the use of short wavelengths increases the requirements for the quality of the mirror surface (see the Rayleigh criterion). Therefore, they usually follow the path of increasing the aperture. Increasing the aperture also improves another important characteristic - sensitivity. A radio telescope must have a high sensitivity in order to reliably detect the faintest possible sources. Sensitivity is determined by the level of flux density fluctuations:

,

where is the intrinsic noise power of the radio telescope, is the effective area (collecting surface) of the antenna, 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.

§5. Radio interferometers

In addition to increasing the aperture diameter, there is another way to increase the resolution (or narrow the radiation pattern). If we 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 restore information about the source with an 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 time stamps and stored on a carrier. 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.

§6. The first radio telescopes

Home - Karl Jansky

A copy of the radio telescopeJansky

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 noise, he built a vertically polarized unidirectional antenna such as Bruce's canvas. The dimensions of the structure were 30.5 m in length and 3.7 m in height. The work was carried out on a wave of 14.6 m (20.5 MHz). The antenna was connected to a sensitive receiver, at the output of which was a recorder with a long time constant .

Emission recording obtained by Jansky on February 24, 1932. The maxima (arrows) repeat after 20 min. is the period of a full rotation of the antenna.

In December 1932, Jansky already reported on the first results obtained with his setup. The article reported the discovery of "... a constant hiss of unknown origin", which "... is difficult to distinguish from the hiss caused by the noises of the equipment itself. The direction of arrival of hissing interference changes gradually during the day, making a complete rotation in 24 hours. In his next two papers, in October 1933 and October 1935, Karl Jansky gradually comes to the conclusion that the source of his new interference is the central region of our galaxy. Moreover, the greatest response is obtained when the antenna is directed to the center of the Milky Way.

Jansky recognized that advances in radio astronomy would require larger, sharper antennas that could be easily orientated in various 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 US.

Rebirth - Grout Reber

Meridian radio telescopeGroot Rebera

In 1937, Grote Reber, a radio engineer from Weton (USA, Illinois), became interested in the work of Jansky 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 in elevation , and the change in the position of the lobe of the diagram 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-shaped beam 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 receivedGroat Reberin 1944

Improving his equipment, Reber undertook a systematic survey of the sky and in 1944 published the first radio charts 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 constellation Sagittarius, Cygnus A, Cassiopeia A, Canis Major and Puppis. Reber's maps are quite good even compared to modern meter wavelength maps.

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 flourishing of radio astronomy, which led to the development of millimeter and submillimeter wavelengths, which made it possible to achieve much higher resolutions.

§7. Classification of radio telescopes

A wide range of wavelengths, a variety of objects of research in radio astronomy, the rapid pace of development of radio physics and radio telescope construction, 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 most simple and familiar to use. Filled aperture antennas simply collect the signal from the observed object and focus it on the receiver. The recorded signal already carries scientific information and does not need to be synthesized. The disadvantage of such antennas is the low resolution. Blank aperture antennas can be divided into several classes according to their surface shape and mounting method.

Paraboloids of revolution

Almost all antennas of this type are mounted on Alt-azimuth mounts and are fully rotatable. Their main advantage is that such radio telescopes, like optical ones, can be aimed at an object and guide it. Thus, observations can be carried out at any time while the object under study is above the horizon. Typical representatives: Green Bank radio telescope, RT-70, Kalyazinsky 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-movable systems are built. The cost and complexity of such telescopes grows much more slowly as they grow in size. 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 asymmetric radiation pattern and different resolutions along different axes. Another disadvantage of such telescopes is that, due to limited mobility, only part of the sky is available to them for observation. Representatives: University of Illinois Radio Telescope, Ooty Indian Telescope.

The course of rays in the Nanse 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. On short waves, this is not easy to do because of the large losses in the 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 "points" to the object and reflects the rays onto the paraboloid. The paraboloid concentrates the rays at the focal point where the receiver is located. Only part of the sky is available for observations with such a telescope. Representatives: Kraus radio telescope, Large radio telescope in Nanse.

earthen bowls

The desire to reduce the cost of construction led astronomers to the idea of ​​using natural relief as a telescope mirror. The representative of this type was the 300-meter Arecibo radio telescope. It is located in a sinkhole, the bottom of which is paved with spheroid-shaped aluminum sheets. the receiver on special supports is suspended above the mirror. The disadvantage of this tool is that the sky area within 20° from the zenith is available to it.

Antenna arrays (common-mode antennas)

Such a telescope consists of many elementary feeds (dipoles or spirals) located at a distance less than the wavelength. By precisely controlling 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 the purposes of astronomy are two characteristics of radio telescopes: resolution and sensitivity. In this case, the sensitivity is proportional to the area of ​​the antenna, and the resolution is proportional to the maximum size. Thus, the most common circular antennas give the worst resolution for the same effective area. Therefore, telescopes with small

Telescope DKR-1000, with unfilled aperture

area, but high resolution. Such antennas are called empty aperture antennas, since they have "holes" in the aperture that exceed the wavelength. To obtain an image from such antennas, observations must be carried out in the 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, it is necessary to measure the signal at all possible bases with some step up to the maximum .

If there are only two antennas, then you will have to observe, then change the base, observe at the next point, change the base again, etc. This synthesis is called consistent. The classical radio interferometer works according to this principle. The disadvantage of sequential synthesis is that it is time consuming and cannot reveal the variability of radio sources over short times. Therefore, it is more commonly 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, radio telescope DKR-1000 in Pushchino.

Large arrays such as VLA are often referred to as sequential synthesis. However, due to the large number of antennas, almost all bases are already present, and additional permutations are usually not required.

List of radio telescopes.

Location	Antenna type	The size	Minimum operating wavelength
USA, Green Bank	Parabolic segment with active surface
Russia, Kalyazin Radio Astronomy Observatory	Parabolic reflector
Russia, Bear Lakes	Parabolic reflector
Japan, Nobeyama	Parabolic reflector
Italy, Medicine	Parabolic reflector
Spain, Granada	Parabolic reflector
Puerto Rico, Puerto Rico, Arecibo	spherical reflector
Russia, Badary, Siberian solar radio telescope	Antenna array 128x128 elements (cross-shaped radio interferometer)
France, Nancy	two-mirror
India, Ooty	parabolic cylinder
Italy, Medicine, "Northern Cross"	"T" of two parabolic cylinders

Bibliography

1. Space physics: small. ents., 1986, p. 533

2. Kaplan S. A. How radio astronomy originated // Elementary radio astronomy. - M.: Nauka, 1966. - S. 12. - 276 p.

3. 1 2 Kraus D.D. 1.2. A Brief History of the First Years of Radio Astronomy // Radio Astronomy / Ed. V. V. Zheleznyakova. - M.: Soviet radio, 1973. - S. 14-21. - 456 p.

4. Great Soviet encyclopedia. - USSR: Soviet Encyclopedia, 1978.

5. Electromagnetic radiation. Wikipedia.

6. Radio telescope // Space Physics: Little Encyclopedia / Ed. R. A. Sunyaeva. - 2nd ed. - M.: Sov. Encyclopedia, 1986. - S. 560. - 783 p. - ISBN 524(03)

7. P.I.Bakulin, E.V.Kononovich, V.I.Moroz General astronomy course. - M.: Nauka, 1970.

8. 1 2 3 4 John D. Kraus. Radio astronomy. - M.: Soviet radio, 1973.

9. Jansky K.G. Directional Studies of Atmospherics at Hight Frequencies. - Proc. IRE, 1932. - T. 20. - S. 1920-1932.

10. Jansky K.G. Electrical disturbances apparently of extraterrestrial origin.. - Proc. IRE, 1933. - T. 21. - S. 1387-1398.

11. Jansky K.G. A note on the source of interstellar interference.. - Proc. IRE, 1935. - T. 23. - S. 1158-1163.

12. Reber G. Cosmic Static. - Astrophys. J., June, 1940. - T. 91. - S. 621-624.

13. Reber G. Cosmic Static. - Proc. IRE, February, 1940. - V. 28. - S. 68-70.

14. 1 2 Reber G. Cosmic Static. - Astrophys. J., November, 1944. - T. 100. - S. 279-287.

15. Reber G. Cosmic Static. - Proc. IRE, August, 1942. - T. 30. - S. 367-378.

16. 1 2 N.A.Esepkina, D.V.Korolkov, Yu.N.Pariyskiy. Radio telescopes and radiometers. - M.: Nauka, 1973.

17. Radio telescope of the University of Illinois.

18. 1 2 L. M. Gindilis "SETI: The Search for Extraterrestrial Intelligence"