Radio telescope - Magazine "All about Space". Antennas not for communication: the world's largest radio telescope

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.
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 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 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”, since 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 best-known 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 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 base 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. In the United States, 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, 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 Leningrad region, in the North Caucasus and in 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 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 "peep" into the most inner regions 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 - less than 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. Start with at least the most accurate quantitative test to date general theory relativity. 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 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, 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 above sea level (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 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

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 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. Historical 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 California Institute of Technology Observatory in Owens Valley (Puerto Rico). 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 the main settlements to minimize electromagnetic interference from broadcast radios, televisions, radars 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 recording 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, Groat 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 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 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 research objects 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 resolution 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 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.

Bibliography

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17. Radio telescope of the University of Illinois.

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The main purpose of telescopes is to collect as much radiation from a celestial body as possible. This allows you to see dim objects. Secondarily, telescopes are used to view objects at a large angle, or, as they say, to increase. Resolution of small details is the third purpose of telescopes. The amount of light they collect and the available detail resolution is highly dependent on the area of ​​the main part of the telescope - its lens. Lenses are reflex and lens.

lens telescopes.

Lenses, one way or another, are always used in a telescope. But in refracting telescopes, the lens is the main part of the telescope - its lens. Remember that refraction is refraction. A lens lens refracts light rays and collects them at a point called the focus of the lens. At this point, an image of the object of study is built. To view it, use the second lens - the eyepiece. It is placed so that the foci of the eyepiece and objective coincide. Since people have different vision, the eyepiece is made movable so that it is possible to achieve a clear image. We call this sharpening. All telescopes have unpleasant features - aberrations. Aberrations are distortions that result when light passes through the optical system of a telescope. The main aberrations are associated with the imperfection of the lens. Lens telescopes (and telescopes in general) suffer from several aberrations. We will name only two of them. The first is due to the fact that rays of different wavelengths are refracted slightly differently. Because of this, there is one focus for blue rays, and another one for red rays, located further from the lens. Rays of other wavelengths are collected each in its place between these two foci. As a result, we see rainbow-colored images of objects. This aberration is called chromatic. The second strong aberration is the spherical aberration. It is related to the fact that the lens, the surface of which is part of the sphere, in fact, does not collect all the rays at one point. Rays coming at different distances from the center of the lens are collected at different points, due to which the image is fuzzy. This aberration would not exist if the lens had a paraboloid surface, but such a detail is difficult to manufacture. To reduce aberrations, complex, not at all two-lens systems are made. Additional parts are introduced to correct lens aberrations. For a long time holding the championship among the lens telescopes - the telescope of the Yerkes Observatory with a lens of 102 centimeters in diameter.

mirror telescopes.

In simple mirror telescopes, reflecting telescopes, the lens is a spherical mirror that collects light rays and reflects them with the help of an additional mirror towards the eyepiece - the lens at the focus of which the image is built. A reflex is a reflection. SLR telescopes do not suffer from chromatic aberration, since the light in the lens is not refracted. But reflectors have a more pronounced spherical aberration, which, by the way, greatly limits the field of view of the telescope. Mirror telescopes also use complex structures, surfaces of mirrors other than spherical, etc.

Mirror telescopes are easier and cheaper to manufacture. That is why their production has been rapidly developing in recent decades, while new large lens telescopes have not been made for a very long time. The largest reflex telescope has a complex multi-mirror lens equivalent to an entire mirror 11 meters in diameter. The largest monolithic reflex lens has a size of just over 8 meters. The largest optical telescope in Russia is the 6-meter mirror telescope BTA (Large Azimuthal Telescope). The telescope was for a long time the largest in the world.

characteristics of telescopes.

Telescope magnification. The magnification of a telescope is equal to the ratio of the focal lengths of the objective and the eyepiece. If, say, the focal length of the lens is two meters, and the eyepiece is 5 cm, then the magnification of such a telescope will be 40 times. If you change the eyepiece, you can change the magnification. This is what astronomers do, after all, it’s not possible to change, in fact, a huge lens ?!

Exit pupil. The image that the eyepiece builds for the eye can, in the general case, be either larger than the pupil of the eye, or smaller. If the image is larger, then part of the light will not enter the eye, thus, the telescope will not be used at 100%. This image is called the exit pupil and is calculated by the formula: p=D:W, where p is the exit pupil, D is the diameter of the objective, and W is the magnification of the telescope with this eyepiece. Assuming a pupil size of 5 mm, it is easy to calculate the minimum magnification that is reasonable to use with a given telescope objective. We get this limit for a lens of 15 cm: 30 times.

Resolution of telescopes

In view of the fact that light is a wave, and waves are characterized not only by refraction, but also by diffraction, no even the most perfect telescope gives an image of a point star in the form of a point. The ideal image of a star looks like a disk with several concentric (with a common center) rings, which are called diffraction rings. The size of the diffraction disk limits the resolution of the telescope. Everything that covers this disk with itself cannot be seen in this telescope. The angular size of the diffraction disk in arcseconds for a given telescope is determined from a simple relationship: r=14/D, where the diameter D of the objective is measured in centimeters. The fifteen-centimeter telescope mentioned just above has a maximum resolution of just under a second. It follows from the formula that the resolution of a telescope entirely depends on the diameter of its lens. Here is another reason for building the grandest telescopes possible.

Relative hole. The ratio of the lens diameter to its focal length is called aperture ratio. This parameter determines the luminosity of the telescope, i.e., roughly speaking, its ability to display objects as bright. Lenses with a relative aperture of 1:2 - 1:6 are called fast lenses. They are used to photograph objects that are weak in brightness, such as nebulae.

Telescope without an eye.

One of the most unreliable parts of a telescope has always been the observer's eye. Each person has his own eye, with its own characteristics. One eye sees more, the other less. Each eye sees colors differently. The human eye and its memory are not able to preserve the whole picture offered for contemplation by a telescope. Therefore, as soon as it became possible, astronomers began to replace the eye with instruments. If you connect a camera instead of an eyepiece, then the image obtained by the lens can be captured on a photographic plate or film. The photographic plate is capable of accumulating light radiation, and this is its undeniable and important advantage over the human eye. Long-exposure photographs are able to display incomparably more than a person can see through the same telescope. And of course, the photo will remain as a document, which can be repeatedly referred to later. Even more modern means are CCD cameras with polar charge coupling. These are light-sensitive microcircuits that replace a photographic plate and transmit the accumulated information to a computer, after which they can do new picture. The spectra of stars and other objects are studied using spectrographs and spectrometers attached to the telescope. Not a single eye is able to distinguish colors and measure the distances between lines in the spectrum so clearly, as these devices easily do, which also save the image of the spectrum and its characteristics for subsequent studies. Finally, no one can look through two telescopes with one eye at the same time. Modern systems of two or more telescopes, united by one computer and spaced apart, sometimes at distances of tens of meters, make it possible to achieve amazingly high resolutions. Such systems are called interferometers. An example of a system of 4 telescopes - VLT. It is no coincidence that we have combined four types of telescopes into one subsection. The Earth's atmosphere is reluctant to let the corresponding wavelengths of electromagnetic waves through, so telescopes for studying the sky in these ranges tend to be taken out into space. It is with the development of astronautics that the development of the ultraviolet, x-ray, gamma and infrared branches of astronomy is directly related.

radio telescopes.

The most common objective of a radio telescope is a metal bowl of a paraboloid shape. The signal collected by it is received by an antenna located at the focus of the lens. The antenna is connected to a computer, which usually processes all the information, building images in conditional colors. A radio telescope, like a radio receiver, can only receive a certain wavelength at a time. In the book of B. A. Vorontsov-Velyaminov “Essays on the Universe” there is a very interesting illustration that is directly related to the subject of our conversation. In one observatory, guests were invited to come to the table and take a piece of paper from it. A person took a piece of paper and read something like this on the back: “By taking this piece of paper, you have expended more energy than all the radio telescopes of the world have received during the entire existence of radio astronomy.” If you have read this section (and you should), then you must remember that radio waves have the longest wavelengths of all types of electromagnetic radiation. This means that the photons corresponding to radio waves carry very little energy. To collect an acceptable amount of information about the luminaries in the radio beams, astronomers build huge telescopes. Hundreds of meters - this is the not-so-surprising milestone for lens diameters that has been reached modern science. Fortunately, everything in the world is interconnected. The construction of giant radio telescopes is not accompanied by the same difficulties in processing the surface of the lens, which are inevitable in the construction of optical telescopes. Permissible surface errors are proportional to the wavelength, therefore, sometimes, the metal bowls of radio telescopes are not a smooth surface, but simply a grating, and this does not affect the reception quality in any way. The long wavelength also allows the construction of grandiose interferometer systems. Sometimes telescopes from different continents participate in such projects. The projects include space-scale interferometers. If they come true, radio astronomy will reach unprecedented limits in the resolution of celestial objects. In addition to collecting the energy emitted by celestial bodies, radio telescopes can “illuminate” the surface of the bodies of the solar system with radio beams. A signal sent from, say, the Earth to the Moon will bounce off the surface of our satellite and be received by the same telescope that sent the signal. This research method is called radar. With the help of radar, you can learn a lot. For the first time, astronomers learned that Mercury rotates around its axis in this way. Distance to objects, speed of their movement and rotation, their relief, some data on chemical composition surfaces - these are the important information that can be clarified by radar methods. The most grandiose example of such studies is the complete mapping of the surface of Venus, carried out by AMS "Magellan" at the turn of the 80s and 90s. As you may know, this planet hides its surface from the human eye behind dense atmosphere. Radio waves, on the other hand, pass through clouds unhindered. Now we know about the relief of Venus better than about the relief of the Earth (!), because on Earth the cover of the oceans prevents us from studying most of the solid surface of our planet. Alas, the speed of propagation of radio waves is great, but not unlimited. In addition, with the remoteness of the radio telescope from the object, the scattering of the sent and reflected signal increases. At a distance of Jupiter-Earth, the signal is already difficult to receive. Radar - by astronomical standards, a melee weapon.

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 receive the object's own radio emission and, based on the obtained data, 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 further development 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. Sonny 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), a radio telescope with spherical mirrors (wave range up to 3 cm, fixed antenna; movement in space of the antenna beam is carried out by irradiation different parts mirrors), Kraus radio telescope (wavelength 10 cm; stationary vertically located spherical mirror, to which the radiation of the source is directed using a flat mirror installed at a certain angle), periscope antennas (small vertically and large horizontally);
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 (sequential synthesis type instrument in Kalgoor, Australia, wavelength 375 cm), compound interferometer (interferometer with aperture synthesis 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 vibrations electromagnetic field, which are generated by individual rays on the surface of a mirror or another point through which the same ray 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).

At present, the prospects for the use of radio telescopes lie in the fact that they make it possible 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 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 band. The diameter of the main mirror of each antenna is 12 meters. Together, these antennas are part of one large Pathfinder radio telescope. It is the largest radio telescope in existence today.

Dozens of mirror antennas are used for research and observation of the galaxy. They can see far beyond the world's largest optical telescope, Hubble. 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 Square Kilometer Array

Similar radio telescopes are deployed throughout the globe, and many of them are planned to be combined into a single Square Kilometer Array (SKA) system by 2030, with a total reception 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 participate 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 it. The SKA radio telescope system will solve the most pressing mysteries of the universe. He will be able to measure a huge number of pulsars, stellar fragments and other cosmic bodies that emit 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 is starting in stages with smaller components and the 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 when completed 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 reflector antennas to receive these radio waves. Instead, many smaller turnstile antennas will be installed, designed to collect signals over 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 pictured below:

By 2024, SKA project leaders plan to install more than 131,000 of these antennas, grouped into 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 pick up radiation that originated in the universe billions of years ago. And thus, help to understand physical processes taking place in the distant past.

The principle of operation of radio telescopes

The antennas, combined into one common array, work on the same principle as an optical telescope, only the radio telescope focuses not the optical radiation, but the received radio waves. The laws of physics dictate such requirements that the higher the received wavelength, the larger the diameter of the reflector antenna should be. This is how, for example, a radio telescope without spatial diversity of receiving antenna systems looks like - the operating five-hundred-meter FAST spherical radio telescope 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 work to increase the mirror diameter to infinity, and the implementation of the interferometer as in the photo above is not always and everywhere possible, so you have to use a large number of smaller geographically separated antennas. An example of such antennas 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 the dark period of the early universe, called the epoch of reionization. This epoch existed 13 billion years ago (about a billion years after the Big Bang), when only nascent stars and other objects began to heat the universe filled with hydrogen atoms. Remarkably, the radio waves emitted by these neutral hydrogen atoms can still be detected. The waves were emitted with a wavelength of 21 cm, but by the time they reached the Earth, billions of years of cosmic expansion had passed, stretching them for several more meters.

The MWA antennas will be used to detect echoes of the distant past. Astronomers hope that studying this electromagnetic radiation will provide 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 a section with MWA antennas. Each section contains 16 antennas, which are combined 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 routed to a nearby beamformer. There, waveguides of different lengths give the antenna signals a certain delay. By properly choosing this delay, the beamformers "tilt" the radiation pattern of the array so that radio waves arriving from a certain area of ​​the sky reach the antenna at the same time, as if they were received by one large antenna.

MWA antennas are divided into groups. The signals from each group are sent to one receiver, which distributes the signals to different frequency channels and then sends them to the observatory's central building via optical fiber. There, using specialized software packages and graphics processing units, the data is correlated by multiplying the signals from each receiver and integrating them over time. This approach allows you to create 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 reflector or fixed antennas, the maximum resolution of the telescope is determined by its distance between several receiving parts. The larger this distance, the more accurate the resolution.

Today, astronomers are using this property to build virtual telescopes that span entire continents, allowing them to increase the resolution of the telescope well enough to see the black holes at the center. milky way. But the size of a radio telescope is not the only requirement for obtaining detailed information about a distant object. Resolution quality also depends on total receiving antennas, frequency range and the location of the antennas relative to each other.

The data obtained with MWA is sent hundreds of kilometers away to the nearest data center with a supercomputer. MWA can send more than 25 terabytes of data per day, and in the coming years with the release of SKA1-low, this speed will become even higher. And 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 the power supply of radio telescopes is solved. At the Murchison Radio Astronomy Observatory, antenna complexes are powered by solar panels with a capacity of 1.6 megawatts:

Until recently, the observatory's antennas were powered by 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. At the moment, the budget for the construction of SKA1 in South Africa and Australia is about 675 million euros. This is the amount set by the 10 project member countries: Australia, Canada, China, India, Italy, the Netherlands, New Zealand, South Africa, Sweden and the United Kingdom. But that funding doesn't cover the full cost of SKA1 that astronomers are hoping for. Therefore, the observatory is trying to get more countries involved in a partnership that could increase funding.

Conclusion

Radio telescopes allow you to observe distant space objects: pulsars, quasars, etc. For example, using the FAST radio telescope, a radio pulsar was detected 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 with the help of such unusual radio telescopes.