We have already covered the basic X-ray detectors: proportional counters for energies below and scintillation counters for energies up to. The problem is the need to exclude cosmic rays, which also cause ionization within the counters. For this purpose, three methods are used.
The first method is to use anti-coincidence detectors. In this case, the X-ray counters are surrounded by a scintillating substance (either a plastic scintillator or a scintillating liquid) and any events that cause both the counter and the scintillating substance to operate are rejected as caused by a charged particle (Fig. 7.10a).
The second method is to analyze the shape of the electron pulse as a function of time. A fast particle, be it a low-energy cosmic ray particle or a fast electron knocked out of the counter by such a particle, creates an ionized trail that causes a wide pulse at the output. On the other hand, a photon with an energy of about leads to local ionization, and the resulting pulse is short, especially its leading edge. The range of electrons knocked out of argon atoms by cosmic X-rays, for example, is usually less than 0.132 cm. This method of distinguishing between cosmic rays and X-rays is called rise-time or pulse shape discrimination (Fig. 7.10, b and c).
The third method, used for hard x-rays and soft x-rays, involves detectors called layered phosphors. They consist of layers of different scintillating materials that have different efficiencies for detecting photons and charged particles. One component of such a pair could be a detector made of cesium iodide, which is sensitive to photons and used as a standard scintillation photon counter, and the other component could be made of a plastic scintillator, which is not sensitive to photons. Consequently, photons will give a signal only in the first detector, while charged particles passing through
Rice. 7.10. Distinguishing between X-rays (b) and cosmic rays (c) by rise time (or pulse shape).
detector, cause light flashes in both materials. The scintillators used in layered phosphors are selected in such a way that they have different flash times, so a charged particle penetrating the device gives two light flashes, separated by a time interval. A photon causes only one flash, so light flashes can be recorded by one photomultiplier connected to the electronic system, capable of recognizing cosmic rays by characteristic features and eliminating them. Based on the intensity of the light flash caused by a photon, its energy is determined, while for energies characteristic of -radiation, it is possible to achieve an energy resolution of the order of 10% or better.
It is necessary to limit the field of view of the X-ray telescope, which is often done using a mechanical collimator. In the simplest case, the collimator consists of hollow tubes of rectangular cross-section. The radiation pattern of such a collimator has the shape of a triangle, since it can be assumed that X-ray radiation propagates rectilinearly, i.e. in accordance with the laws of geometric optics. The only exception is the case when the beam is incident at a large angle to the normal on the surface of a highly conductive substance, such as copper. Then grazing incidence reflection can occur. For photons with less energy, reflection is observed when the angle between the direction of the beam and the surface of the material is not
Rice. 7.11. Diagram of a simple X-ray telescope. Telescopes of this type were installed on the Uhuru and Ariel-5 satellites.
exceeds several degrees. This reflection process is similar to the deflection of radio waves in an ionized plasma, in which the plasma frequency increases with depth. Although reflection occurs only at very small angles, this is sufficient to develop telescopes with oblique-incidence mirrors that produce an image of the sky in the focal plane (Section 7.3.2).
So, you can assemble a simple X-ray telescope according to the diagram shown in Fig. 7.11. Let us note once again that a significant role is played by modern electronic circuits of amplitude analyzers, discriminators and anti-coincidence circuits, which should be included in such telescopes. Telescopes of this type have worked with great success on board the Uhuru orbital X-ray observatory.
7.3.1. X-ray satellite "Uhuru". The Uhuru X-ray satellite was launched from the coast of Kenya in December 1970. The scientific equipment installed on the satellite included two proportional counters with beryllium windows, each with a useful area of \u200b\u200bThey were directed in opposite directions perpendicular to the axis of rotation and were equipped with mechanical collimators , which limited the field of view (full width at half height) (Fig. 7.12). The period of rotation of the satellite around its axis was 10 minutes. Proportional counters were sensitive in the area
Telescope sensitivity. The sensitivity limit of the telescope was determined by background radiation. There are two types of background radiation.
1. The number of counts per second is associated with insufficient exclusion of -quanta and cosmic rays. This value varies from telescope to telescope and for the detectors on board Uhuru it was about
2. Cosmic X-ray background radiation, the brightness of which is very high. This background radiation is isotropic; it is assumed to be of cosmological origin. Dimension in the energy range of the telescope. The sensitivity limit of a telescope is determined statistically. If we take as a criterion for detecting a discrete X-ray source a signal of at least three times
Rice. 7.12. X-ray satellite "Uhuru". a - arrangement of instruments; b - orientation of the X-ray telescope.
greater than the standard deviation associated with noise (in this case statistical noise), then it can be shown that the weakest detectable X-ray point source must have a flux density
where is the solid angle, equal to the angle of view of the telescope, the time of observation of the source. X-ray background radiation in the energy region is equal and has an intensity spectrum approximated by where is measured in We can use these data to show that for a collimator the background radiation of both types is approximately the same, whereas for a smaller field of view only the background due to charged particles is important. Cosmic X-ray background radiation as a source of noise becomes insignificant if the field of view is less than a few degrees.
In normal mode, the satellite scans one strip of the sky over many orbits. Try to calculate the faintest detectable source in one day of observations and compare it with the actual "Uhuru" flux density limit taken from the "Uhuru" catalogs, "Uhuru" in the range How long did it take to scan the entire sky to achieve this level of sensitivity?
Temporal variations. The most remarkable discovery made by Uhuru was pulsating X-ray sources. Telescope
Rice. 7.13. Data logging fragment for the source The histogram shows the number of samples in successive -second bins. A continuous line is a harmonic curve that best approximates the observational results, taking into account the changing sensitivity of the telescope when scanning the source.
with a collimator, it recorded and transmitted data on the X-ray flux to Earth every 0.096 s. The average flux density from the source is equal to and the period is 1.24 s. How much above the noise level was the source when its pulsations were detected? It turns out that during the period the source signal did not greatly exceed the noise level, but the use of Fourier analysis (or power spectrum) methods, if applied to data processing over a longer period of time, allows the discovery of pulsations of much lower intensity. A fragment of the recording is shown in Fig. 7.13.
7.3.2. Einstein X-ray Observatory. The most significant achievements since the Uhuru observations, which caused a revolution in X-ray astronomy, are associated with the flight of the X-ray satellite, also called the Einstein X-ray Observatory. This observatory had a lot of unique equipment on board, including an oblique-incidence telescope that produces images with high angular resolution.
X-rays are reflected only from the surface of conductive materials at large angles of incidence. At reflection energies it occurs if the angle between the surface and the direction of incidence of the radiation is of the order of several degrees; The greater the photon energy, the smaller this angle should be. Therefore, to focus X-rays from a celestial source, you need a parabolic reflector with
Rice. 7.14. Focusing an X-ray beam using a combination of parabolic and hyperbolic oblique incidence mirrors. This combination is used at the Einstein X-ray Observatory.
a very long focal length, and the central part of the reflector may not be used. The focal length of the telescope can be reduced at the expense of the collecting surface area by introducing another collecting mirror, with the preferred configuration being a combination of paraboloid and hyperboloid (Fig. 7.14.) Such a system focuses the X-rays incident only on the annular region shown in the figure. To increase the collecting area, a combination of several mirrors can be used. Such a system was used in the HRI High Disruption Telescope aboard the Einstein Observatory. It made it possible to obtain an image of the celestial sphere in a field of view with a diameter of 25, and the angular destruction was better within a radius of 5 from the center of the field of view.
An XY detector with the same angular resolution as the telescope should be placed in the focal plane. In HRI, it consists of two microchannel plates mounted one behind the other. These detectors are a series of very thin tubes along which a high potential difference is maintained. An electron that hits one end of the tube begins to accelerate and, colliding with the walls, knocks out additional electrons, which in turn accelerate and also knock out electrons, etc. As in a proportional counter, the goal of this process is to produce an intense electron flash from a single electron. In HRI, the front surface of the first microchannel plate is coated. An X-ray photon incident on the front surface knocks out an electron, resulting in electrons detected at the output of the second plate. This burst of electrons is recorded by a charge detector with mutually perpendicular grids, which makes it possible to accurately measure the coordinates of the X-ray quantum.
To determine the sensitivity of a telescope, you need to know its effective area and the level of background signals from the detector. Since grazing incidence reflection is a function of photon energy and since there is absorption in the detector window material, the effective
Rice. 7.15. Effective area of a high-resolution imaging telescope as a function of energy. The curves show the effect of installing beryllium and aluminum filters in front of the detector.
area is highly dependent on energy (Fig. 7.15). As expected, the maximum effective area corresponds to energies of about and is equal to approximately The response of the detector can be varied by introducing filters into the field of view of the telescope (Fig. 7.15), thus providing coarse energy resolution.
The noise level in the detector, mainly due to charged particles, reaches This means that the source of the Uhuru catalog is at the limit of sensitivity, i.e. a point source with a flux density of the order of Uhuru units in the range can be detected at the 5 o level with an exposure of 50,000 s.
To take full advantage of the high quality of the telescope's mirrors, the spacecraft would have to be stabilized with precision - However, no such attempt has been made. Pointing the telescope is carried out much more roughly, but at any moment its instantaneous orientation relative to standard bright stars is accurately determined. Therefore, as soon as observations are completed, the sky map is reconstructed with the full angular resolution that the telescope has. An example of the quality of images obtained using HRI is shown in Fig. 7.16.
The following instruments were also installed at the Einstein Observatory.
Rice. 7.16. (see scan) X-ray image of a supernova remnant obtained using the high-resolution telescope of the Einstein Observatory. Each image element has an exposure time of 32,519 s.
Rice. 7.17. General layout of instruments on board the Einstein X-ray Observatory.
1 - visor, 2 - front precollimator, 3 - mirror system, 4 - rear precollimator, 5 - diffraction spectrometer, 6 - broadband spectrometer with filters, 7 - focal crystal spectrometer, 8 - high voltage imaging detector, 9 - rear insulating support, 10 - solid-state spectrometer, 11 - multi-channel proportional counter, 12 - electronic equipment units, 13 - optical bench, 14 - front insulating support, 15 - control proportional counter, 16 - thermal collimator of the control proportional counter, 17 - orientation sensor hoods.
a positive number, β is the angle of incidence, the distance between the reflecting crystallographic planes. X-rays pass through the focus and, forming a diverging beam, fall on the crystal. The crystal is curved so that the reflected x-rays are focused onto a position-sensitive proportional detector. At energies, its energy resolution is on the order of 100-1000, and the effective area is about the observatory in one paragraph. The main achievements of the first year of observations are the following: detection of X-ray emission from stars of all luminosity classes, including all main sequence stars, supergiants and white dwarfs; the discovery of more than 80 sources in the Andromeda nebula and the same number in the Magellanic Clouds; High-resolution X-ray images of galaxy clusters, revealing the wide range of different processes that lead to X-ray emission; detection of X-rays from many quasars and active galaxies; registration of sources with a flux density 1000 times weaker than the weakest sources in the Uhuru catalogue. Observations made from the Einstein Observatory have had a significant impact on all areas of astronomy. (A significant part of the first observational results of the Einstein Observatory was published in Astrophys. J., 234, No. 1, Pt. 2, 1979.)
"Earth and Universe" 1993 No. 5
STAGES OF DEVELOPMENT OF X-RAY ASTRONOMY
The Earth's atmosphere is opaque to X-rays. Therefore, X-ray astronomy was born along with rocket technology: in 1948, with the help of photographic plates raised by a V-2 rocket to an altitude of about 160 km, R. Barnight from the Naval Laboratory (USA) discovered X-ray radiation from the Sun. In 1962, replacing the photographic plate with a Geiger counter, astronomers discovered a second X-ray source, this time beyond the solar system - it was Sco X-1. The naming system adopted in those years was simple: “Sco X-1” means the brightest (1) x-ray (X-ray) source in the constellation Scorpius (Sco). The third object of X-ray astronomy, discovered in 1963, was the famous Crab Nebula in the constellation Taurus (Tau X-1).
In the 1960s, X-ray detectors were mostly carried above the atmosphere on geophysical rockets; their vertical flight lasted only a few minutes, so during this period only about 40 sources were plotted on the X-ray sky maps. But in the 70s, sensitive X-ray detectors began to be placed on artificial Earth satellites, the most famous of which are Uhuru, ANS, Copernicus, OSO-7, SAS-3. This was followed by the launch of large spacecraft - HEAO-1, Einstein, Astron, Granat, Rosat, equipment at the Salyut-4 and -7, Skylab, and Mir stations. Although the work of each of them brought interesting astrophysical information, the most important stages in the development of X-ray astronomy were the launches of the first high-sensitivity X-ray detector, Uhuru, in 1970 and the first X-ray reflecting telescope, Einstein, in 1978 (had high sensitivity and high angular resolution of 2-4"). With their help, X-ray binary stars, X-ray pulsars and flare sources, normal stars with hot coronas, active galactic nuclei and intergalactic gas in galaxy clusters were discovered. In the 80s and early 90s Many powerful instruments were already operating in orbit, but their characteristics remained traditional (Earth and Universe, 1989, No. 5, p. 30.- Ed.).
The next major advance in X-ray astronomy is expected in 1998 with the launch of AXAF's new orbital observatory, the Advanced X-ray Astrophysics Facility.
Back in the 70s, American astronomers conceived the idea of creating four large orbital observatories capable of covering the entire scale of electromagnetic waves, with the exception of radio. In May 1990, HST was launched into orbit - the Hubble Space Telescope, operating in the optical and near ultraviolet ranges (Earth and Universe, 1987, No. 4, p. 49). Then, in April 1991, GRO - Gamma Ray Observatory - was launched. Next in line is the AXAF X-ray observatory, followed by the infrared observatory SIRTF (Space Infrared Telescope Facility).
However, the last two projects are now undergoing significant revision. The fact is that the production of the first observatories was very expensive: HST cost $5.55 billion, and GRO cost $600 million. Moreover, each of the satellites was put into orbit with the help of specially organized expeditions on the Space Shuttle. . Errors in the manufacture of the HST telescope and general economic difficulties forced NASA to reconsider the budget for promising astrophysics projects. First of all, it was decided to abandon the Shuttle or the powerful Titan rocket, which were required to launch heavy observatories. Orbital observatories must become lighter so that they can be launched by cheap, expendable Atlas-type rockets.
For the SIRTF infrared observatory, this means reducing the diameter of the main mirror from 85 to 70 cm, almost halving the size of the satellite, and reducing its minimum lifetime from five to three years. True, recently new, very sensitive detectors of infrared radiation have appeared, which should compensate for the reduction in the area of the telescope mirror. NASA scientists hope that they will be able to launch an infrared observatory before the year 2000.
Even more radical changes are coming to the AXAF project. The observatory was initially conceived as a satellite 17 m long and weighing 15 tons; the wingspan of the solar panels was supposed to be 26 m. Now, instead of one large satellite, it is planned to make two smaller ones: the main one (14 m long and weighing about 6 tons) will house the main X-ray telescope, the second will be equipped with X-ray spectrometers. The launch of the X-ray observatory was originally planned for 1987. Now they say 1998. What do astronomers expect from the AXAF observatory?
IS IT POSSIBLE TO PLAN OPENINGS?
It turns out that it is possible! Especially if you know what you are looking for. This is exactly the situation in X-ray astronomy now: it is well known what the parameters of an X-ray telescope should be in order to use it to make long-awaited discoveries in the field of cosmology and relativistic astrophysics. However, it was not possible to create such a tool for a long time.
There are two fundamentally different types of X-ray detectors: proportional quantum counters with collimators and X-ray telescopes with a focusing system and image detectors 1 . The first of them was used on Uhuru, the second on Einstein.
1 In reality, many more different types of X-ray detectors have been created, but we want to show the fundamental difference between them.
A proportional counter is a modern version of a Geiger counter, i.e. a gas-filled tube with two electrodes - positive and negative. An X-ray quantum, flying into the tube through a window covered with a thin film, ionizes the gas, and the electrodes collect the resulting ions and electrons. By measuring the resulting current pulse, it is possible to determine the energy of the detected quantum: they are approximately proportional to each other (hence the name of the counter). Proportional counters are capable of recording quanta in a wide energy range - from 1 to 30 eV, and have good spectral resolution, i.e. they determine the energy of a quantum with an accuracy of 15-20%. However, the proportional counter itself is similar to a photographic plate without a lens: it registers quanta coming from all directions. If there is a signal, it means that somewhere in front of the counter there is a source of X-ray radiation, but where exactly is unknown.
To determine the direction to the source, shadow collimators are used, which give free access to the counter only to quanta coming from a certain direction, and obscure the counter from all other quanta. Continuing the analogy with a photographic plate, we can say that by placing it at the bottom of a deep well or a long pipe, we are able to fix the direction of bright sources like the Sun: as soon as they are on the axis of our “collimator”, the plate turns black. However, you cannot image an object with such a tool: its angular resolution is low, and its sensitivity is low. After all, it records all quanta passing through this “collimator” - both quanta from the source and the sky background. And in the X-ray range the sky is quite bright. The situation is reminiscent of daytime observation of stars from the surface of the Earth: only bright sources are visible to the naked eye - the Sun, Moon, Venus - and the stars fade in the radiance of the daytime sky. The collimator is helpless here (remember: the stars are not visible during the day from the bottom of a deep well!), but an optical system - a telescope - can help. It creates an image of a piece of the sky and makes it possible to observe the star separately from the background.
An X-ray lens, if constructed, allows the counter to isolate the source from the background. And if you place many small counters at the focus of an X-ray lens, then they, like grains of photographic emulsion, will build a picture of the X-ray sky, and a “colored” picture if these counters correctly perceive the energy of the incident photons.
Unfortunately, creating an X-ray lens is very difficult: hard quanta penetrate deep into the lens material without being refracted or reflected. Only the lowest-energy X-ray quanta, falling very gently onto a well-polished metal surface, are reflected from it according to the laws of geometric optics. Therefore, an X-ray lens, which is a combination of a paraboloid and a hyperboloid of rotation, is very similar to a slightly conical tube. Usually, in order to intercept more quanta, several lenses of different diameters are made, but with the same focal length, and they are strengthened coaxially like a nesting doll. Then all images are added in the focal plane and mutually enhanced. An X-ray quantum detector placed in this plane records their coordinates and transmits them to a computer, which synthesizes the image.
Effective area and spectral range of the main mirror of the AXAF telescope in comparison with the Einstein Space Observatory telescope
A telescope with a mirror diameter of 60 cm was installed at the Einstein Observatory. However, the effective area of the complex mirror strongly depended on the energy of incoming quanta: for soft X-ray quanta with an energy of 0.25 keV it was 400 cm 2 and decreased to 30 cm 2 for quanta with energy 4 keV. And the telescope was generally unsuitable for recording even harder quanta.
This is very sad, since it is hard quanta that carry unique information. Every astronomer knows how important it is to record the spectral line of a chemical element: its intensity indicates the content of the element, and its position in the spectrum indicates the speed of movement of the source (Doppler effect). However, there are almost no lines in the X-ray spectra; Typically, the spectrum of hot interstellar gas contains only one line of iron with a quantum energy of about 7 keV. Many astrophysicists dream of getting images of “their” objects in it. For example, galaxy researchers could use them to determine the content of heavy elements in the hot coronas of stellar systems and in intergalactic gas; they could measure the speed of galaxy clusters and directly determine the distance to them, which would make it possible to clarify the Hubble constant and the age of the Universe. Unfortunately, the Einstein Observatory telescope is not capable of operating in the 7 keV region: its sensitivity is limited to the range of 0.1 4-4 keV.
The ROSAT X-ray observatory (“Roentgen Satellite”), launched in June 1990, created mainly by German specialists, although it has a higher sensitivity than the Einstein, its operating range is relatively small: 0.1÷2 keV. The angular resolution of ROSAT (4") is approximately the same as that of Einstein (2"÷4").
But the telescope of the AXAF observatory will be able to construct an image in the range of 0.14-10 keV and at the same time give the resolution of a good optical telescope (0.5"). Moreover, taking into account that its composite mirror will have a diameter of 1.2 m , when observing point sources, AXAF will be almost a hundred times more sensitive than Einstein. This means that it will have access to almost a thousand times more space for studying sources of a known type. And how many fundamentally new objects will be discovered? One can only guess. ..
In addition, AXAF will be equipped with a high-resolution crystalline Bragg spectrometer, making it possible to determine the energy of quanta with an accuracy higher than 0.1%. The operating principle of this device is similar to an optical diffraction grating, but since the wavelength of X-ray radiation is very small, the role of a diffraction grating for it in a Bragg spectrograph is played by a natural crystal, the distance between the layers of atoms in which is close to the wavelength of X-ray radiation.
THIRD STAGE OF X-RAY ASTRONOMY
In the book by P.R. Amnuel “The Sky in X-Rays” (M.: Nauka, 1984) an interesting analogy is given between X-ray and optical astronomy. Viewing the X-ray sky from the Uhuru satellite was similar to viewing the night sky with the naked eye. Indeed, the brightest “star” object in the sky - Venus - is 10 thousand times brighter than the faintest 6t star accessible to the eye; This is the same ratio of fluxes from the brightest X-ray source Sco X-1 and the faintest source discovered by Uhuru. The launch of a telescope at Einstein Observatory, 100 times more sensitive than Uhuru, was equivalent to the appearance of a modest, amateur-level optical telescope that could see stars up to 11 m. And another 100 times more sensitive AXAF will be similar to a good professional telescope, for which stars up to 16 m are available.
Each new orbiting observatory makes its own important contribution to astronomy. Even instruments with traditional parameters are capable of collecting a large array of unique information and making many discoveries; An example of this is the Russian observatory “Granat” (Earth and Universe, 1993, No. 1, p. 17.- Red.). It is even more important to create devices with unique characteristics, each of which will give a breakthrough in science. Just one example: before the launch of the GRO observatory, only two pulsars were recorded in the gamma-ray range - Crab and Vela - but now there are about 500 of them! Therefore, astrophysicists are eagerly awaiting the start of operation of new large observatories in orbit.
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.
X-RAY TELESCOPE
Device for studying time and spectrum. St. in the sources of space. x-ray radiation, as well as to determine the coordinates of these sources and construct their images.
Existing radio waves operate in the energy range of e photons, x-rays. radiation from 0.1 to hundreds of keV, i.e. in the wavelength range from 10 nm to hundredths of nm. To carry out astronomical observations in this region of wavelengths, X-rays are raised beyond the Earth's atmosphere on rockets or satellites, since X-rays. radiation is strongly absorbed by the atmosphere. Radiation with e>20 keV can be observed starting from altitudes =30 km from balloons.
R.t. allows:
1) register X-rays with high efficiency. photons;
2) separate events corresponding to the impact of photons of the required energy range from signals caused by the influence of charges. h-ts and gamma photons;
3) determine the direction of arrival of the x-rays. radiation.
In R.T. for the range of 0.1-30 keV, the photon detector is a proportional counter filled with a gas mixture (Ar + CH4, Ar + CO2 or Xe + CO2). X-ray absorption photon by a gas atom is accompanied by the emission of a photoelectron (see PHOTOELECTRON EMISSION), Auger electrons (see Auger EFFECT) and fluorescent photons (see FLUORESCENCE). The photoelectron and Auger electron quickly lose their energy to ionize the gas, and fluorescent photons can also be quickly absorbed by the gas due to the photoelectric effect. In this case, the total number of electron-ion pairs formed is proportional. energy x-ray photon. Thus, the X-ray energy is restored by the current pulse in the anode circuit. photon.
Rice. 1. X-ray a-scheme. telescope with a slit collimator; b - telescope operation in scanning mode.
Under normal conditions, R. t. is irradiated by powerful flows of charges. h-ts and gamma photons decomp. energies, which the X-ray detector records together with X-rays. photons from the radiation source under study. To highlight x-rays. photons from the general background, the anti-coincidence method is used (see COINCIDENCE METHOD). Arrival x-ray photons are also recorded by the shape of the electrical impulse they create. current, since the charger. h-ts give signals that are longer in time than those caused by x-rays. photons.
To determine the direction of the x-ray. The source is a device consisting of a slit collimator and a star sensor rigidly attached to it on the same frame. A collimator (set of plates) limits the X-ray field of view and transmits x-rays. photons traveling only in a small solid angle (=10-15 square degrees). X-ray the photon passing the collimator (Fig. 1,a) is recorded at the top. counter volume. The resulting current pulse is up in the circuit. the anode passes through an anti-coincidence circuit (since there is no prohibiting signal from the lower anode) and is fed to the analyzer to determine the time and energy. characteristic of a photon. The information is then transmitted via telemetry to Earth. At the same time, information from the star sensor about the brightest stars that fall into its field of view is transmitted. This information makes it possible to establish the position of the Rt axes in the production at the moment of photon arrival.
When the RT operates in scanning mode, the direction to the source is determined as the position of the RT, at which the counting speed reaches its maximum. Angle The resolution of RT with a slit collimator or a similar cellular collimator is several tens of arc minutes.
Significantly better angle. resolution (= several tens of seconds) have RT with modulation. collimators (Fig. 2, a). Modular the collimator consists of two (or more) one-dimensional wire grids installed between the detector and the slit collimator, for which the latter is raised above the detector to a height of = 1 m and observations are carried out in either scanning mode (Fig. 1b) or rotation relative to the axis, perpendicular to the mesh plane. The wires in each collimator grid are installed parallel to each other at a distance equal to the diameter of the wire. Therefore, when the source moves across R.’s field of view, shadows from the top. wires slide along the bottom. grid, falling either on the wires, and then the counting rate is maximum, or between them, and then it is minimal (background).
Angle distribution of R.t. counting rate with modulation. collimator (click response function) is shown in Fig. 2, b. For n-grid modulation. collimator angle between adjacent maxima q0=2n-1qr, where qr=d/l - ang. resolution of R. t. In most cases, R. t. with modulation. collimators provide accurate localization of x-rays. sources, sufficient for their identification with celestial objects emitting in other electromagnetic ranges. waves
With modular The encoder technique begins to compete with collimators. aperture allowing to obtain qr 
Rice. 2. a - X-ray device. telescope with modulation collimator; b - angle counting rate distribution.
X-ray source position. radiation in the field of view of the RT is determined by the position of the maximum correlation. functions between the obtained count rate distribution over the detector surface and the screen transmittance function.
In the energy range e>15 keV, crystals are used as R.T. detectors. scintillators NaI (Tl) (see SCINTILLATION COUNTER); to suppress charging background. h-ts of high energies and gamma photons are installed on anti-coincidence with the first crist. scintillators CsI(Tl). To limit the field of view in such RTs, active collimators are used—cylinders of scintillators connected to anti-coincidence with NaI(Tl) scintillators.
In the energy range from 0.1 to several. keV radiation technologies are the most effective, in which radiation incident on a focusing mirror is focused at small angles (Fig. 3). The sensitivity of such a radiation t. is 103 times higher than that of other designs due to its ability to collect radiation with a significant value. area and directed to a small detector, which significantly increases the signal-to-noise ratio. X-ray t., built according to this scheme, gives a two-dimensional image of the x-ray source. radiation similar to conventional optical. telescope.
Rice. 3. X-ray focusing diagram. telescope.
To construct an image in a focusing RT, position-sensitive proportions are used as detectors. cameras, microchannel detectors, and charge-coupled devices (CCDs). Angle the resolution in the first case is determined by ch. arr. spaces. camera resolution and is = 1", microchannel detectors and CCDs give 1-2" (for beams close to the axis). With spectrometric In research, PP detectors and Bragg crystals are used. spectrometers and diffraction position-sensitive gratings detectors. Space X-ray sources radiations are very diverse. X-ray Solar radiation was discovered in 1948 in the USA from a rocket that lifted Geiger counters to the top. layers of the atmosphere. In 1962, the first X-ray source was discovered by the group of R. Giacconi (USA), also from a rocket. radiation outside the Solar System - “Scorpio X-1”, as well as a diffuse X-ray background, apparently extragalactic. origin. By 1966, as a result of experiments on rockets, approx. 30 discrete x-rays. sources. With the launch into orbit of a series of specials. Satellite satellites (“UHURU”, “Ariel”, “SAS-3”, “Vela”, “Copernicus”, “HEAO”, etc.) with R. t. dec. Hundreds of roentgens have been discovered. sources (galactic and extragalactic, extended and compact, stationary and variable). Mn. of these sources have not yet been identified with sources manifesting themselves in optical and other electromagnetic ranges radiation. Among the identified galaxies. objects: close binary star systems, one of the components of which is X-ray. pulsar; single pulsars (Crab, Vela); supernova remnants (extended sources); temporary (transient) sources that sharply increase the luminosity in x-rays. range and again fading over a period of time ranging from several. minutes to several minutes months; so-called B a r s t e r s are powerful flashing X-ray sources. radiation with a characteristic flash time of the order of several. seconds To identified extragalactic. objects include nearby galaxies (Magellan clouds and the Andromeda Nebula), radio galaxies Virgo-A (M87) and Centaurus-A (NGC 5128), quasars (in particular, 3S 273), Seyfert and other galaxies with active nuclei; Galaxy clusters are the most powerful sources of X-rays. radiation in the Universe (in them, hot intergalactic gas with a temperature of 50 million K is responsible for the radiation). The vast majority of space x-ray sources of phenomena objects completely different from those that were known before the beginning of X-rays. astronomy, and above all they are distinguished by their enormous energy release. Luminosity of galactic x-ray sources reaches 1036-1038 erg/s, which is 103-105 times higher than the energy release of the Sun over the entire wavelength range. In extragalactic sources, a luminosity of up to 1045 erg/s was recorded, which indicates the unusual nature of the emission mechanisms manifested here. In close binary star systems, for example, as the main. The mechanism of energy release considers the flow of matter from one component (giant star) to another (neutron star or black hole) - disk accretion, in which matter falling on a star forms a disk near this star, where due to friction warms up and begins to radiate intensely. Among the probable hypotheses for the origin of diffuse X-rays. background, along with the assumption of thermal radiation from hot intergalactic. gas, the inverse Compton effect of electrons on IR photons emitted by active galaxies or on photons of cosmic microwave background radiation is considered. Observational data from the HEAO-B satellite indicate that a significant contribution (>35%) to diffuse X-rays. the background is provided by distant discrete sources, Ch. arr. quasars.
"X-RAY TELESCOPE" in books
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27. TELESCOPE
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