Antennas are not for communication: the world's largest radio telescope. What are radio telescopes used for?

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A radio telescope is a type of telescope and is used to study the electromagnetic radiation of objects. It allows you to study the electromagnetic radiation of astronomical objects in the range of carrier frequencies from tens of MHz to tens of GHz. Using a radio telescope, scientists can receive an 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.

Radiocosmic radiation was first 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. It was only in the early 1940s that Jansky’s work was continued and contributed to the further development of radio astronomy.

A radio telescope consists of an antenna system, a radiometer and recording equipment. A radiometer is a receiving device that measures low-intensity radiation power 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 with which electromagnetic radiation is studied (for example, an infrared telescope, an X-ray telescope, etc.).

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

It is very important that the radio telescope has 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 that is recorded is determined by the relation:
DP=P/(S\sqrt(Dft))
where P is the power of the radio telescope's own noise, 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 (classification is made depending on the wavelength range and purpose):
Full Aperture Antennas: parabolic antennas (used for observation at short waves; installed on rotating 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 fixed vertically located spherical mirror, onto which the source radiation is directed using a flat mirror installed at a certain angle), periscope antennas (small in size vertically 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 (interconnected systems of two radio telescopes for simultaneous observation of a radio source: it has greater resolution, example: Aperture fusion interferometer in Cambridge, England, wavelength 21 cm). Other antenna types: cross (successive fusion Mills cross at Molongo, Australia, wavelength 73.5 cm), ring (successive fusion type instrument at Kalgur, Australia, wavelength 375 cm), compound interferometer (aperture fusion interferometer at Flers , Australia, wavelength 21).

The most accurate in operation are full-rotation parabolic antennas. If they are used, 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 isolates signals from cosmic sources against a 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 using radio telescopes with parabolic mirrors is that when rotated, the mirror is deformed under the influence of gravity. It is because of this that when the diameter increases beyond approximately 150 m, the 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 that are generated by individual rays on the surface of a mirror or other point through which the same ray passes. After this, the records are added taking into account the phase shift.

If the array of antennas is made not continuous, but spaced over a sufficiently large distance, then a large-diameter mirror 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, and not by their diameter. Thus, this system allows you 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 USA, the state of New Mexico. The "Very Large Grille" was created in 1981. The system consists of 27 fully rotating parabolic antennas, which are located along two lines forming the letter “V”. The diameter of each antenna reaches 25 meters. Each antenna can occupy one of 72 positions while moving along the rail tracks. The VLA has the same sensitivity as an antenna with a diameter of 136 kilometers and an angular resolution superior to the best optical systems. It is no coincidence that the VLA was used in the search for water on Mercury, radio coronas around stars and other phenomena.

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

Currently, the prospects for using radio telescopes are that they make it possible to establish communications 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 observe the Huygens lander on Saturn's moon Titan. The ALMA system, consisting of 64 antennas, is being designed. The prospect for the future is the launch of interferometer antennas into space.

A modern radio telescope is a very complex device, consisting mainly of the following main elements: an antenna, a system for moving the antenna in the vertical and horizontal planes, a receiving device, a device for pre-processing the received signal, and an antenna control device. In addition to the above-mentioned elements, a planetary radar also has a transmitting and modulating device, as well as a synchronization system.

Planetary radars with their transmitters turned off are typically used as radio telescopes to observe the radio emissions of planets and other celestial bodies. In this case, the radar receiving device either switches from the narrowband reception mode to the broadband reception mode, or a special radio astronomical receiver - a radiometer - is installed on the telescope.

Let's consider the main devices of radio telescopes and planetary radars (Fig. 5).

Antennas. One of the most complex devices of a modern radio telescope and planetary radar is the antenna system. The antenna collects radio energy from a celestial source and transmits it to the receiving device. The larger the linear dimensions of the antenna, the greater the amount of radio energy collected by the antenna. As the linear dimensions of the antenna increase, its radiation pattern narrows, i.e., the angle within which the antenna effectively receives radio radiation decreases. And thereby the angular resolution of the antenna increases and its gain increases. Therefore, radio astronomers strive to create antennas as large as possible to study radio sources with small angular dimensions.

Radio astronomy antennas can be divided, by analogy with optical telescopes, into two groups - radio reflectors (single antennas) and radio refractors (multi-element antennas). In radio reflectors, the flow of radio emission is collected and focused by a “mirror” system. The focused signal arrives at the feed and is transmitted to the receiving device through a feeder path connecting the antenna to the receiving device. In radio refractors, the flux of radio emission is received by individual antennas and then added up in the feeder path.

The following types of reflector antennas are used in radio astronomy: parabolic, spherical, horn, periscope, variable profile. Refractor antennas include various types of interferometric systems, common-mode antennas, phased arrays, and cross-shaped antennas. The main characteristics of the antennas of some Soviet and foreign telescopes are given in Table. 2.

Parabolic antennas. The most widely used among reflective antennas are parabolic. These antennas have their analogue in optics - a spotlight with a parabolic reflector, in which light from a “point” source is converted into a parallel beam. In a parabolic antenna, the process goes in the opposite direction - a parallel flow of radio emission is focused by a mirror at the focus of the paraboloid, where it is received by the feed.

Parabolic antennas used in radio astronomy are of impressive size (Fig. 6 and 7). The largest fully rotating parabolic radio telescope on Earth has a mirror with a diameter of 100 m. Its antenna rotates 360° in azimuth and 90° in elevation. The weight of the antenna structure is 3200 tons.

Parabolic antennas can only operate in a limited range of wavelengths: it is absolutely impossible to create a parabolic surface, as a result of which the irregularities of the surface of the paraboloid, when operating at very short wavelengths, begin to worsen the focusing properties of the antenna. This, in turn, leads to a deterioration in the efficiency of the antenna, i.e., as if to reduce the opening area of ​​the antenna collecting the flux of radio emission. And since as the wavelength increases, the radiation pattern of the antenna expands and at a certain wavelength this antenna becomes no longer practical to use for observations (since this reduces its gain), radio astronomers use other types of antennas for longer-wave measurements.

However, even in identical designs of parabolic antennas, the minimum wavelength at which the antenna still operates effectively may be different. This depends on the careful manufacturing of the mirror surface and on the deformation of the mirror when its orientation in space changes, as well as on the action of thermal and wind loads. For example, the mirror with a diameter of 22 m of the RT-22 antenna of the Crimean Astrophysical Observatory is more accurate in its design than the mirror of an antenna of similar dimensions in Pushchino (Physical Institute of the USSR Academy of Sciences).

Parabolic antennas operating in the millimeter wavelength range have a diameter not exceeding 25 m. Larger antennas operate effectively in the centimeter range. The RT-22 antenna of the Crimean Astrophysical Observatory can operate effectively at wavelengths no shorter than 4 mm. The 11 m diameter National Radio Astronomy Observatory antenna on Kitt Peak operates at a maximum wavelength of 1.2 mm. To reduce temperature deformations of the mirror, the antenna of this radio telescope, when not in operation, is located under a dome with a diameter of 30 m (during measurements, the dome is partially opened).

Spherical antennas. There are only a few radio astronomy antennas on the globe that have a spherical mirror. These antennas are also called “earth bowls”, since the spherical reflector in them is located on the surface of the Earth, and the antenna radiation pattern is shifted by moving the feed. The largest antenna of this kind type (with an opening diameter of 305 m) is located on the island of Puerto Rico in South America (Arecibo Observatory).

Antennas with spherical mirrors focus electromagnetic radiation less efficiently than parabolic antennas, but have the advantage that they can survey (scan) the sky within a larger solid angle (without rotating the mirror itself, but only by displacing the reflector from the focus of the mirror). Thus, the antenna in Arecibo allows you to shift the radiation pattern within 20° relative to the zenith in any direction. Its mirror consists of metal shields that line the bottom of an extinct volcano. Cables are stretched on three giant supports along which a special carriage with irradiators and other radio equipment installed on it moves (see the first page of the cover). The antenna can operate effectively up to a wavelength of at least 10 cm (at this wave its radiation pattern is 1.5′ wide). The antenna in Arecibo, before reconstruction, had a spherical surface made of a metal mesh and could only operate effectively in the long-wavelength region of the decimeter range (lambda>50 cm). The Aresib antenna is also used as a planetary radar antenna, operating at a wavelength of 12.5 cm and having an average power of 450 kW.

The Byurakan Astrophysical Observatory operates the shortest wavelength spherical antenna with a fixed mirror, the diameter of which is 5 m. The antenna is a prototype of the future 200-meter bowl designed for the Byurakan Observatory, which, according to calculations, will have a maximum wavelength of 3 cm.

Horn antennas. Unlike mirror (spherical and parabolic) horn antennas consist of a single feed. There are not many radio astronomy antennas of this type on Earth. Due to the fact that their characteristics can be accurately calculated, these antennas are used for precision measurements of radio emission fluxes from certain sources, which are accepted by radio astronomers as reference ones. Using a horn antenna, the flux of radio emission from the Cassiopeia A source was accurately measured and the relict radio emission was discovered. The Cassiopeia A nebula is one of the most powerful sources of radio emission and is widely used by radio astronomers for antenna calibrations as a reference source.

Periscope antennas. Periscopic antennas are also widely used in radio astronomy, the advantage of which is that, despite their relatively large sizes, they have fairly good efficiency. Antennas of this type consist of three elements: a flat mirror that rotates according to the elevation angle; a focusing main mirror (in the form of a spherical or parabolic cylinder) and an irradiator.

A spherical or ‘parabolic mirror’ focuses the flux of radio emission in the horizontal and vertical planes. Since the linear dimensions of such antennas in the horizontal direction are significantly larger than in the vertical direction, the width of the antenna radiation pattern in the horizontal plane is significantly less than the width of the pattern than in the vertical plane. The shortest wavelength periscope antenna was built at the observatory of the Gorky Radiophysical Institute. It works effectively down to wavelengths of 1 mm. At a wavelength of 4 mm, the radiation pattern width of this antenna is 45″ in the horizontal plane and 8′ in the vertical plane.

Variable profile antennas. Near the village of Zelenchukskaya, Stavropol Territory, the RATAN-600 radio telescope began operating (Fig. 8). The design of its antenna system resembles that of a periscope antenna. However, unlike the latter, the main mirror of this antenna is flat in the vertical plane. Despite its gigantic dimensions (the diameter of the main mirror is 588 m), this antenna can effectively operate up to a wavelength of 8 mm.

Let us now consider the various types of refractor antennas that are effectively used at meter waves.

Common mode antennas. These antennas consist of individual half-wave feeds (dipoles), which make up a fabric with P irradiators in one direction and m irradiators in the orthogonal direction. The distance between the feed in both orthogonal directions is equal to half the wavelength. Using an antenna of this type, consisting of 64 dipoles, the first radar detection of the Moon was carried out at a wavelength of 2.5 m.

In common-mode antennas, the summation of signals from individual feeds is carried out in the feeder path. Moreover, first the signals from the irradiators located in the same row are summed, and then the summation is carried out by floors (or vice versa). The greater the number of feeds in a row, the narrower the antenna radiation pattern in a plane passing along the row of these dipoles. Common mode antennas are narrowband, that is, in practice they can only operate at one wavelength.

The antenna of the USSR Center for Long-Range Space Communications, consisting of 8 parabolic antennas arranged 4 in a row (Fig. 9), has a gain almost 8 times greater than the gain of a separate parabolic antenna. This complex antenna is built on the principle of a common-mode antenna array.

Cross-shaped antennas. A further development of antennas of this type were cross-shaped antennas . They do not use pHt feeds, as in common-mode antennas, and P+ t irradiators. In these antennas P irradiators are located in one direction, and T irradiators in a direction perpendicular to it. By appropriate high frequency phasing, such an antenna has a radiation pattern (in the above planes) similar to that of an antenna consisting of pHt irradiators. However, the gain of such a cross-shaped antenna is less than that of the corresponding common-mode antennas (consisting of pHt irradiators). Often such antennas are called antennas with an unfilled aperture (opening). (In common-mode or filled-aperture antennas (pHt irradiators), to change the direction of the radiation pattern in space, it is necessary to rotate the plane of the irradiators by rotating the movable base.)

In phased arrays and antennas with an unfilled aperture, a change in the direction of the radiation pattern in one of the planes is usually carried out by changing the phase relationships in the feeder path, and in the other plane - by mechanically rotating the antenna system.

The largest cross-shaped antenna in the decameter range is the antenna of the UTR-2 radio telescope of the Kharkov Institute of Radio Engineering and Electronics (Fig. 10). This antenna system consists of 2040 broadband fixed feeds located parallel to the earth's surface and forming two arms - “north-south” and “west-east”.

Interferometers. Antenna interferometers occupy a special place among antenna systems. The simplest radio interferometer consists of two antennas connected by a high-frequency cable; the signals from them are summed up and sent to the receiving device. As in an optical interferometer, the phase difference of the received signals is determined by the difference in the path of the rays, which depends on the distance between the antennas and the direction of arrival of the radio signals (Fig. 11).

Due to the movement of the radio source across the celestial sphere, it is precisely the change in the phase difference of the signals received by the antennas of the radio interferometer that occurs. This leads to the appearance of maxima and minima of interference signals. The movement of the radio source at a certain angle at which the maximum of the interference signal in the radio interferometer replaces the minimum is equivalent to the width of its radiation pattern. However, unlike single antennas, the radio interferometer has a multi-lobe radiation pattern in a plane passing along the base of the interferometer. The wider the distance (base) between the antennas, the narrower the width of the interference lobe. (In a plane orthogonal to the base of the interferometer, the radiation pattern is determined by the dimensions of a single antenna of this interferometer.)

Currently, the creation of highly stable frequency generators has made it possible to implement radio interferometry with independent reception. In this system, high-frequency signals are received by each of two antennas and independently converted to lower frequencies using signals from highly stable atomic frequency standards.

Interferometers with independent reception currently operate with bases larger than the size of a continent and reaching 10,000 km. The angular resolution of such interferometers has reached several ten-thousandths of an arcsecond.

Receivers. One of the main characteristics of a radio telescope and planetary radar is sensitivity - the minimum received signal power that a radio telescope or radar can register. Sensitivity depends on the parameters of the receiving device, the parameters of the antennas and the characteristics of the space surrounding the antenna. In radio astronomy, radio signals received are so weak that in order to register these signals, they have to be amplified many times; At the same time, both useful signals and interference are of a noise nature. This complicates their separation in the receiving device.

Receiving devices of radio telescopes - radiometers, having high sensitivity, also have high stability of their characteristics. Since the sensitivity of the receiver is mainly determined by the characteristics of its high-frequency part, therefore, increased attention is paid to the input nodes of the radiometer. To reduce the noise level of the receiver, “low-noise” high-frequency amplifiers based on traveling wave tubes or tunnel diodes are used in its input devices, and parametric or quantum paramagnetic amplifiers are also used. To obtain even higher sensitivity of the receiver, its high-frequency components are cooled to ultra-low temperatures (liquid nitrogen or liquid helium are used as coolants). A cooling system using liquid helium allows the temperature of the high-frequency receiver units to reach 5-10° K.

To ensure high sensitivity, radio astronomy receivers must have bandwidths of hundreds of megahertz or even several thousand megahertz. However, receivers with such wide bandwidths are not suitable for all studies. Thus, measuring in the radio range the absorption spectra of some gases found in the atmospheres of the Earth and planets (water vapor, oxygen, ozone, etc.) requires maximum bandwidths of the order of 50 MHz. The sensitivity of such a receiver will be relatively low. Therefore, in such measurements, the necessary sensitivity is obtained by increasing the signal accumulation time at the radiometer output.

The permissible signal accumulation time is determined by the measurement scheme and the time of presence of radio emission signals from the celestial body under study in the field of view of the radio telescope antenna. For small accumulation (integration) times, calculated in seconds or tens of seconds, it is usually carried out on the elements of the output filters of the radiometer. For large accumulation times, the integrator functions are performed by a computer.

All the methods described above make it possible to reduce the noise level by hundreds and thousands of times. In this case, the radiometer can measure the intensity of radio emission corresponding to a noise temperature of 0.003-0.01 ° K (with an accumulation time of 1 s). However, not only the receiving device has its own noise, but also the antenna-feeder system, the noise of which depends on many parameters: temperature, antenna efficiency, losses of electromagnetic energy in the feeder path, etc.

In radio astronomy, the intensity of noise signals is usually characterized by noise temperature. This parameter is determined by the noise power in a passband equal to 1 Hz. The higher the efficiency of the antenna, the lower its noise temperature, and therefore, the higher the sensitivity of the radio telescope can be obtained.

Interference with radio reception. Increasing the sensitivity of radio telescopes is limited by external interference of natural origin. Artificial interference is significantly reduced due to the selection of frequency ranges specifically for radio astronomy research, in which the operation of ground-based and space-based radio stations and radio systems for any purpose is prohibited. To reduce the influence of industrial interference, radio telescopes are located away from industrial centers, mainly in mountain pits, since the latter well shield radio telescopes from ground-based industrial interference.

Natural interference comes from radio emissions from the earth's surface and atmosphere, as well as from outer space itself. To reduce the influence of the Earth's background radio emission on the radiometer readings, the radio telescope antenna is designed in such a way that its radiation pattern in the direction to the Earth's surface has a significant attenuation compared to the direction towards the celestial body under study.

Due to the presence in the earth's atmosphere of gases that have molecular absorption lines in the radio range (oxygen, water vapor, ozone, carbon monoxide, etc.), the atmosphere emits noise signals in the millimeter and centimeter ranges and also weakens the received radio emission of celestial bodies in these ranges. The intensity of radio emission from the atmosphere depends significantly on the wavelength - it increases greatly with decreasing wavelength. The radio emission of the atmosphere is especially strong near the resonance lines of the mentioned gases (the most intense lines are the lines of oxygen and water vapor near wavelengths 1.63, 2.5, 5 and 13.5 mm).

To reduce the influence of the atmosphere, radio astronomers choose to observe celestial bodies in areas of the radio range far from resonant lines. These areas, in which atmospheric noise is minimal, are called “windows of transparency” of the atmosphere. In the millimeter range, such “windows” are areas near wavelengths 1.2; 2.1; 3.2 and 8.6 mm. The shorter the “transparency window” is in the shorter wavelength range, the greater the attenuation of the radio signal from the source under study and the higher the level of atmospheric noise. (Radio emission from the atmosphere increases greatly with increasing humidity. The bulk of water vapor is located in the surface layer of the atmosphere at altitudes up to 2-3 km.)

To reduce the influence of the atmosphere on radio astronomy measurements, they try to place radio telescopes in areas with a very dry climate and at high altitudes above sea level. Thus, the requirements for the placement of radio telescopes are in many ways similar to the requirements for the placement of optical telescopes. Therefore, optical telescopes are often located in high-altitude observatories along with radio telescopes.

The results of observing cosmic radio emission are also influenced by moisture concentrated in clouds and falling as precipitation. Atmospheric noise due to these components increases significantly with decreasing wavelength (at waves shorter than 3-5 cm). Therefore, radio astronomers try to carry out measurements in cloudless weather.

In addition to the radio emission of the atmosphere and the surface of the Earth, the factor limiting the sensitivity of the radio telescope is the cosmic radiation of the Galaxy and Metagalaxy. In the decimeter, centimeter and millimeter wavelength ranges, the Metagalaxy radiates like an absolutely black body heated to a temperature of 2.7° K. This radiation is distributed isotropically in space. The intensity of radiation from the interstellar medium in the Galaxy depends on the direction of observation (the intensity of radiation in the direction of the Milky Way is especially high). Radiation of galactic origin also increases with wavelength at waves longer than 30 cm. Therefore, observing the radio emission of celestial bodies at waves longer than 50 cm is a very difficult task, which is also aggravated by the increasing influence of the earth’s ionosphere at meter wavelengths.

Transmitters. To measure the reflection characteristics of planets, the average power of planetary radar transmitters must be hundreds of kilowatts. Currently, only a few such radars have been created.

Planetary radar transmitters operate either without modulation or use some type of modulation. The choice of transmitter radiation mode depends on the research objectives. Thus, measuring the effective scattering area and the “Doppler” spectrum of the signal reflected from the planet does not require modulation and is usually carried out with a monochromatic emitted signal. At the same time, planetary ranging and radar mapping require a modulated signal.

Pulse modulation of the transmitter (used in lunar exploration) cannot provide a large average radiation power, and therefore it is practically not used in planetary exploration. Frequency and phase modulation methods are used in almost all transmitters of the largest planetary radars. Thus, the planetary radar of the USSR Center for Deep Space Communications uses the linear frequency modulation method to measure range, and the planetary radar of the Massachusetts Institute of Technology uses the “pseudo-noise phase shift keying” method.

Planetary radar transmitters must have very high radiation frequency stability (the relative instability of the transmitter should be on the order of 10 -9). Therefore, they are built according to the scheme: stabilized low-power generator + power amplifier.

The main characteristics of transmitters used in foreign planetary radars, as well as individual characteristics of these radars, are given in Table. 3 (see p. 38).

Devices for pointing antennas and processing received signals. A modern radio telescope is unthinkable without a computer. Usually it even uses two computers. One of them operates in the guidance and tracking circuit of the radiation source under study. It produces signals proportional to the current azimuth and elevation angle of the source, which then enter the antenna drive control units. The same computer also monitors the correct execution of control commands by the antenna drives by analyzing signals from the rotation angle sensors of these drives.

Antenna systems of radio telescopes can change the position of the radiation pattern in both one and two planes. Typically, the position of the antenna radiation pattern is changed by mechanically moving the antenna or feed in the corresponding plane. (An exception is phased array antennas, in which the direction of reception of radio emission is changed by changing the phase relationships in the feeder path.)

Antennas with one degree of freedom are usually installed along the meridian and change their position according to the elevation angle, and the radio emission of the source is measured as it passes through the geographic meridian on which the radio telescope is located. A large number of radio telescopes operate on this principle. Full rotation antennas are usually mirror type antennas.

In addition to the generally accepted azimuth-elevation guidance system, some radio telescopes use an equatorial system, in which the radio telescope antenna can be rotated relative to an axis parallel to the Earth's rotation axis (along the parallel), as well as in the orthogonal plane. Such an antenna pointing system requires simpler algorithms to control the position of the radiation pattern in space.

Antenna control systems, in addition to pointing and tracking the selected source, allow for surveying (scanning) the sky in a certain vicinity around the source. This mode is used to measure the distribution of radio emission intensity across the planet's disk.

The second computer on modern radio telescopes is used for primary processing of measurement results. The input signal for this computer is the current coordinates and voltage values ​​at the output of the radiometer, proportional to the intensity of radio emission from the studied and calibration sources. Using these data, the computer calculates the distribution of radio emission intensity depending on the coordinates, i.e., builds a map of radio brightness temperatures of the source under study.

To calibrate the intensity of received signals, a comparison of radio emission from the source under study with certain standards, which can be either primary or secondary, is used. The primary standardization method, the so-called “artificial moon” method, was developed by the Soviet scientist V. S. Troitsky. In this measurement method, the primary standard is the radio emission of an absorbing disk installed in front of the radio telescope antenna. Using the “artificial moon” method, a large cycle of precision measurements of radio emission from the Moon and other sources was carried out at the Gorky Radiophysical Institute.

Radio emission signals from some discrete sources (for example, radio sources in the constellations Cassiopeia, Cygnus, Virgo, Taurus, as well as some quasars) are usually used as secondary standards. Sometimes the radio emission of Jupiter is used as a secondary standard.

I continue the story about the New Year’s trip to the “land of telescopes” that I started (the largest optical telescope in Eurasia with a diameter of the main monolithic mirror of 6 m). This time we will talk about two of its relatives - the RATAN-600 and RTF-32 radio telescopes. The first is listed in the Guinness Book of Records, and the second is part of the only radio interferometric complex “Kvazar” constantly operating in Russia. By the way, the Kvazar complex now plays an important role in the operation of the GLONASS system. Let's talk about everything in more detail and as accessible as possible!

Now let's have some fun! :)

For science, the main advantages of the telescope are multi-frequency (range from 0.6 to 35 GHz) and a large aberration-free field (which allows almost instantaneous measurement of radio spectra of cosmic sources in a wide range of frequencies), high resolution and high sensitivity in brightness temperature (which allow for research extended structures, such as fluctuations of microwave background radiation on small angular scales, unattainable even with specialized spacecraft and ground-based instruments).

The telescope consists of two main reflectors:

1. Circular reflector (on the right and along the entire image).
This is the largest part of the radio telescope, it consists of 895 rectangular reflecting elements measuring 11.4 by 2 meters, located in a circle with a diameter of 576 meters. They can move in three degrees of freedom. The circular reflector is divided into 4 independent sectors, named after the parts of the world: north, south, west, east. The total area is 12,000 m². The reflective elements of each sector are aligned in a parabola, forming a reflective and focusing strip of the antenna. A special feed is located at the focus of such a strip.

2. Flat reflector (left).
The flat reflector consists of 124 flat elements with a height of 8.5 meters and a total length of 400 meters. The elements can rotate about a horizontal axis located near ground level. To carry out some measurements, the reflector can be removed by aligning its surface with the plane of the ground. The reflector is used as a periscopic mirror. During operation, the flux of radio emission hitting the flat reflector is directed towards the southern sector of the circular reflector. Having reflected from a circular reflector, the radio wave is focused on the irradiator, which is installed on ring rails. By installing the irradiator in a given position and rearranging the mirror, you can direct the radio telescope to a given point in the sky. A source tracking mode is also possible, in which the irradiator continuously moves, and the mirror is also rearranged.

12. View of the flat reflector from the reverse side. The mechanisms that set the plates in motion are visible.

13. The radio telescope has five receiving irradiator cabins installed on railway platforms with radio receiving equipment and observers. Some resemble an armored train, others resemble alien ships. In the photo we see two such cabins. As planned, the platforms can move along one of 12 radial paths, which provides a set of fixed azimuths in 30° increments. The relocation of the irradiators between the tracks should have been carried out using the central turntable (in the center of the photo)... This was intended, but then this was abandoned (and that’s enough) and the turntable is not used, and part of the rails have been dismantled.

14. At the end of 1985, an additional conical reflector-irradiator was installed. The basis is a conical secondary mirror, under which the irradiator is located. It allows you to receive radiation from the entire circular reflector, while achieving the maximum resolution of the radio telescope. However, in this mode, only radio sources can be observed whose direction deviates from the zenith by no more than ±5 degrees. This irradiator most often appears in illustrations related to the telescope, probably because of its alien appearance :)

15. It’s also good to remove the general radio telescope from the top platform of this irradiator. Well, in general, I’m glad that there is an opportunity to climb :) There was no such opportunity on the RTF-32.

By the way, there was a curiosity that led to the formation of a persistent local “urban legend”. When the first observations were made at RATAN, in order to avoid interference from vehicles, traffic along the village of Zelenchukskaya near RATAN was stopped. The closed nature of the telescope and the lack of sufficient information about this structure, close to the village and impressive in its size, gave rise to various myths among the local population - that RATAN allegedly “irradiates”. Perhaps this rumor was also facilitated by the name “irradiators” - although in fact they emit absolutely nothing, but only receive a signal.

16. Cabin No. 1 is in position, observations will begin in a few minutes, but for now we are invited to go inside this “armored train.”

14. Our guide and observer’s workplace.

What tasks are set for RATAN?
- detection of a large number of space sources of radio emission, identifying them with space objects;
- study of radio emission from stars;
- study of quasars and radio galaxies;
- study of solar system bodies;
- studies of areas of increased radio emission on the Sun, their structure, magnetic fields;
- detection of artificial signals of extraterrestrial origin (SETI);
- research of cosmic microwave background radiation.

The telescope explores astronomical objects over the entire range of distances in the Universe: from the closest ones - the Sun, solar wind, planets and their satellites in the solar system, to the most distant star systems - radio galaxies, quasars and the cosmic microwave background. Over 20 scientific programs of both domestic and foreign applicants are being carried out at the radio telescope.
According to the project "Genetic Code of the Universe" on RATAN-600 all components of background radiation are studied at all angular scales. Daily observations of the Sun with a radio telescope provide unique information, complemented by other instruments, about the properties of solar plasma in the altitude range from the chromosphere to the lower corona, that is, those regions of the Sun’s atmosphere where powerful solar flares originate. This information makes it possible to predict outbreaks of solar activity that affect the well-being of people and the operation of energy systems on the planet. Currently, the RATAN-600 observational data archive contains more than half a million records of radio objects.

15. And this is what radiometers, measuring and recording equipment look like. Some have remained from the time of the first observations, and some have already been replaced with modern equipment. One thing can be said - the radio telescope lives and develops, also being an experimental platform for engineers.

16. This concludes our excursion to RATAN-600: the radio telescope is loaded with observations and it is impossible to distract the people working there.

So, RATAN-600 is still the world's largest reflector mirror and the main radio telescope in Russia, operating in the central "transparency window" of the earth's atmosphere in the wavelength range 1-50 cm. No other radio telescope in the world has such frequency overlap with the possibility carrying out simultaneous observations at all frequencies. Thanks to him and the nearby BTA, astronomers around the world know the names of the villages of the Zelenchuk and Karachay-Cherkess Republics.

17. I took a photo at the top of the “UFO”, as a souvenir :)

P.S. I hope I didn't bore you too much with technical details?

A telescope is a unique optical instrument designed for observing celestial bodies. The use of instruments allows us to examine a variety of objects, not only those that are located close to us, but also those that are located thousands of light years 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 debate about who invented the telescope first - Galileo or Lippershei. These disputes are related to the fact that both scientists were developing optical devices at approximately the same time.

In 1608, Lippershey developed glasses for the nobility to allow them to see distant objects up close. At this time, military negotiations were conducted. 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 looked at with both eyes. The scientist agreed.

The scientist’s new development 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 that allowed objects and objects to be magnified. Since 1609, trumpets with threefold magnification were sold in full swing in Paris. From this year, any information about Lippershey disappears from history, and information about another scientist and his new discoveries appears.

Around the same years, the Italian Galileo was engaged in grinding lenses. In 1609, he presented to society a new development - a telescope with threefold magnification. The Galileo telescope had higher image quality than the Lippershey telescope. It was the brainchild of the Italian scientist that 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 a lens grinding technique that made it possible to clearly enlarge objects. He was able to obtain 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, then it was Galileo who introduced the world to 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 appearance of the first telescope, unique discoveries were made. Galileo used his development to track celestial bodies. He was the first to see and sketch lunar craters, spots on the Sun, and also examined the stars of the Milky Way and the 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 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 design 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 cosmic bodies. Large devices make it possible to view and take photographs 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 considered lens refractors. Mirror devices include reflex 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 light rays and collects them at a point called focus. It is in it that the image of the object is constructed. To view the picture, use an eyepiece.

The lens is placed so that the eyepiece and focus coincide. Modern models use movable eyepieces for convenient observation through a telescope. They help adjust the sharpness of the image.

All telescopes have aberration - 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 installed on them, which collects the light beam and reflects it using a mirror onto the eyepiece. Chromatic aberration is not typical for 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. In Russia, the largest device has a diameter of six meters. For many years it was considered the largest in the world.

Telescope characteristics

Many people buy optical devices for observing cosmic 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 factor 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 forty-fold magnification. 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 disk sizes are limited only by the capabilities of the telescope.

Telescopes without eyes

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

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

To measure the distances between stars and other objects, special instruments are used - spectrographs. They are connected to telescopes.

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

Radio telescopes

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

Infrared models

A striking example of an infrared telescope is the Hubble apparatus, although it can also be optical. 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 heat-measuring device is located. The resulting heat rays are passed through thermal filters. Only after this does photography take place.

Ultraviolet telescopes

When photographing, film can be exposed to ultraviolet rays. In some parts of the ultraviolet range it is possible to receive images without processing or exposure. And in some cases it is necessary for the light rays to pass through a special structure - a filter. Their use helps 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 entire classification of devices for tracking celestial bodies.

The main purpose of telescopes is to collect as much radiation from a celestial body as possible. This allows you to see dim objects. Secondly, telescopes are used to view objects from a large angle or, as they say, to magnify. Resolving small details is the third purpose of telescopes. The amount of light they collect and the available resolution of detail strongly depends on the area of ​​the main part of the telescope - its lens. Lenses come in mirror and lens types.

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 objective. Let us remember that refraction is refraction. A lens lens refracts light rays and collects them at a point called the focal point of the lens. At this point, an image of the object of study is constructed. To view it, use a second lens - an eyepiece. It is placed so that the focuses of the eyepiece and lens coincide. Since people's vision is different, 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 occur 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. Let's name just 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 for red rays, located further from the lens. Rays of other wavelengths are collected each in their own 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 spherical aberration. It is due to the fact that a lens, the surface of which is part of a sphere, does not actually collect all the rays at one point. Rays coming at different distances from the center of the lens are collected at different points, which is why the image turns out unclear. This aberration would not exist if the lens had a paraboloid surface, but such a part is difficult to manufacture. To reduce aberrations, complex, not two-lens systems are made. Additional parts are introduced to correct lens aberrations. Long holding the lead among lens telescopes is the Yerkes Observatory telescope with a lens 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. Reflex is reflection. Mirror 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, mirror surfaces other than spherical, and so on.

Mirror telescopes are easier and cheaper to make. 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 reflecting telescope has a complex multi-mirror lens, equivalent to an entire mirror with a diameter of 11 meters. The largest monolithic SLR lens measures just over 8 meters. The largest optical telescope in Russia is the 6-meter reflecting telescope BTA (Big Azimuth 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 lens and 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, you really can’t change a huge lens?!

Exit pupil. The image that the eyepiece creates for the eye can, in general, be either larger than the eye pupil or smaller. If the image is larger, then some of the light will not reach 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 lens, and W is the magnification of the telescope with a given eyepiece. If we take the size of the eye pupil to be 5 mm, then it is easy to calculate the minimum magnification that is reasonable to use with a given telescope lens. Let's get this limit for a 15 cm lens: 30x.

Telescope resolution

Since light is a wave, and waves are characterized not only by refraction, but also by diffraction, no even the most advanced telescope can image a point star in the form of a point. An 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 cannot be seen with this telescope. The angular size of the diffraction disk in arcseconds for a given telescope is determined from a simple ratio: r=14/D, where the diameter D of the lens 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 depends entirely on the diameter of its lens. This is another reason for building telescopes as big as possible.

Relative hole. The ratio of the diameter of the lens to its focal length is called the relative aperture. This parameter determines the aperture ratio 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 faint 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 his memory are not able to preserve the entire 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, 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 can display incomparably more than a person can see through the same telescope. And of course, the photograph will remain as a document that can be referred to repeatedly in the future. An even more modern means is CCD cameras - polar-charge-coupled devices. These are photosensitive microcircuits that replace a photographic plate and transfer the accumulated information to a computer, after which they can take a new picture. The spectra of stars and other objects are studied using spectrographs and spectrometers attached to the telescope. No eye is capable of distinguishing colors so clearly and measuring the distances between lines in the spectrum, as the above-mentioned devices easily do, which also save the image of the spectrum and its characteristics for subsequent studies. Finally, no person can look through two telescopes at the same time with one eye. Modern systems of two or more telescopes, united by one computer and spaced, 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 is VLT. It is no coincidence that we have combined four types of telescopes into one subsection. The Earth's atmosphere reluctantly transmits the corresponding wavelengths of electromagnetic waves, so telescopes to study the sky in these ranges tend to be taken into space. The development of ultraviolet, x-ray, gamma and infrared branches of astronomy is directly related to the development of astronautics.

Radio telescopes.

The lens of a radio telescope is most often a paraboloid-shaped metal bowl. 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, constructing images in false colors. A radio telescope, like a radio receiver, can only receive a certain wavelength at a time. In the book by B. A. Vorontsov-Velyaminov “Essays on the Universe” there is a very interesting illustration that is directly related to the subject of our conversation. At one observatory, guests were asked to come to a table and take a piece of paper from it. The person took a piece of paper and on the back read something like the following: “By taking this piece of paper, you spent more energy than was received by all the radio telescopes in the world during the entire existence of radio astronomy.” If you read this section (and you should), then you may 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 stars in radio rays, astronomers build huge telescopes. Hundreds of meters – this is the not-so-surprising milestone for lens diameters that has been achieved by modern science. Fortunately, everything in the world is interconnected. The construction of giant radio telescopes does not involve the same difficulties in processing the lens surface that are inevitable in the construction of optical telescopes. The permissible errors of the surface 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 quality of reception in any way. The long wavelength also makes it possible to build grand 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 resolving celestial objects. In addition to collecting energy emitted by celestial bodies, radio telescopes can “illuminate” the surface of solar system bodies with radio rays. A signal sent, say, from the Earth to the Moon, will be reflected from the surface of our satellite and will be received by the same telescope that sent the signal. This research method is called radar. You can learn a lot using radar. For the first time, astronomers learned that Mercury rotates around its axis in exactly this way. The distance to objects, the speed of their movement and rotation, their topography, some data on the chemical composition of the surface - these are the important information that can be determined by radar methods. The most ambitious example of such research is the complete mapping of the surface of Venus, carried out by the Magellan spacecraft at the turn of the 80s and 90s. As you may know, this planet hides its surface from the human eye behind a dense atmosphere. Radio waves pass through clouds without hindrance. Now we know about the topography of Venus better than about the topography of the Earth (!), because on Earth the blanket of oceans prevents the study of most of the solid surface of our planet. Alas, the speed of propagation of radio waves is high, but not limitless. In addition, with the distance of the radio telescope from the object, the dispersion of the sent and reflected signal increases. At the Jupiter-Earth distance, it is already difficult to receive a signal. Radar is, by astronomical standards, a melee weapon.