Astronomy is one of the oldest sciences. From time immemorial, people have followed the movement of the stars across the sky. Astronomical observations of that time helped to navigate the terrain, and were also necessary for the construction of philosophical and religious systems. A lot has changed since then. Astronomy finally freed itself from astrology, accumulated extensive knowledge and technical power. However, astronomical observations made on Earth or in space are still one of the main methods of obtaining data in this science. The methods of collecting information have changed, but the essence of the methodology has remained unchanged.

What are astronomical observations?

There is evidence to suggest that people possessed elementary knowledge about the movement of the Moon and the Sun even in the prehistoric era. The works of Hipparchus and Ptolemy testify that knowledge about the luminaries was also in demand in Antiquity, and much attention was paid to them. For that time and for a long period after, astronomical observations were the study of the night sky and the fixation of what was seen on paper, or, more simply, a sketch.

Until the Renaissance, only the simplest instruments were assistants to scientists in this matter. A significant amount of data became available after the invention of the telescope. As it improved, the accuracy of the information received increased. However, at whatever level of technological progress, astronomical observations are the main way to collect information about celestial objects. Interestingly, this is also one of the areas of scientific activity in which the methods used in the era before scientific progress, that is, observation with the naked eye or with the help of the simplest equipment, have not lost their relevance.

Classification

Today, astronomical observations are a fairly broad category of activities. They can be classified according to several criteria:

• qualifications of the participants;
• the nature of the recorded data;
• location.

In the first case, professional and amateur observations are distinguished. The data obtained in this case is most often the registration of visible light or other electromagnetic radiation, including infrared and ultraviolet. In this case, information can be obtained in some cases only from the surface of our planet or only from space outside the atmosphere: according to the third feature, astronomical observations made on Earth or in space are distinguished.

amateur astronomy

The beauty of the science of the stars and other celestial bodies is that it is one of the few that literally needs active and tireless admirers among non-professionals. A huge number of objects worthy of constant attention, there are a small number of scientists occupied with the most complex issues. Therefore, astronomical observations of the rest of the near space fall on the shoulders of amateurs.

The contribution of people who consider astronomy their hobby to this science is quite tangible. Until the middle of the last decade of the last century, more than half of the comets were discovered by amateurs. Their areas of interest also often include variable stars, observing novae, tracking the coverage of celestial bodies by asteroids. The latter is today the most promising and demanded work. As for New and Supernovae, as a rule, amateur astronomers are the first to notice them.

Options for non-professional observations

Amateur astronomy can be divided into closely related branches:

• Visual astronomy. This includes astronomical observations with binoculars, a telescope, or the naked eye. The main goal of such activities, as a rule, is to enjoy the opportunity to observe the movement of the stars, as well as from the process itself. An interesting branch of this direction is "sidewalk" astronomy: some amateurs take their telescopes out into the street and invite everyone to admire the stars, planets and the Moon.
• Astrophotography. The purpose of this direction is to obtain photographic images of celestial bodies and their elements.
• Telescope building. Sometimes the necessary optical instruments, telescopes and accessories for them, are made by amateurs almost from scratch. In most cases, however, telescope construction consists in supplementing existing equipment with new components.
• Research. Some amateur astronomers seek, in addition to aesthetic pleasure, to get something more material. They are engaged in the study of asteroids, variables, new and supernovae, comets and meteor showers. Periodically, in the process of constant and painstaking observations, discoveries are made. It is this activity of amateur astronomers that makes the greatest contribution to science.

Activities of professionals

Specialist astronomers around the world have more sophisticated equipment than amateurs. The tasks facing them require high accuracy in collecting information, a well-functioning mathematical apparatus for interpretation and forecasting. As a rule, quite complex, often distant objects and phenomena lie at the center of the work of professionals. Often, the study of the expanses of space makes it possible to shed light on certain laws of the universe, to clarify, supplement or refute theoretical constructions regarding its origin, structure and future.

Classification by type of information

Observations in astronomy, as already mentioned, can be associated with the fixation of various radiation. On this basis, the following directions are distinguished:

• optical astronomy studies radiation in the visible range;
• infrared astronomy;
• ultraviolet astronomy;
• radio astronomy;
• x-ray astronomy;
• gamma astronomy.

In addition, the directions of this science and the corresponding observations that are not related to electromagnetic radiation are highlighted. This includes neutrino, studying neutrino radiation from extraterrestrial sources, gravitational-wave and planetary astronomy.

From the surface

Some of the phenomena studied in astronomy are available for research in ground-based laboratories. Astronomical observations on Earth are associated with the study of the trajectories of the movement of celestial bodies, measuring the distance in space to stars, fixing certain types of radiation and radio waves, and so on. Until the beginning of the era of astronautics, astronomers could only be content with information obtained under the conditions of our planet. And this was enough to build a theory of the origin and development of the Universe, to discover many patterns that exist in space.

High above the earth

With the launch of the first satellite, a new era in astronomy began. The data collected by spacecraft is invaluable. They contributed to the deepening of scientists' understanding of the mysteries of the universe.

Astronomical observations in space make it possible to detect all types of radiation, from visible light to gamma and X-rays. Most of them are not available for research from the Earth, because the atmosphere of the planet absorbs them and does not allow them to the surface. An example of discoveries that became possible only after the start of the space age are X-ray pulsars.

Information miners

Astronomical observations in space are carried out using various equipment installed on spacecraft and orbiting satellites. Many studies of this nature are being carried out on the International Space Station. The contribution of optical telescopes launched several times in the last century is invaluable. The famous Hubble stands out among them. For the layman, it is primarily a source of stunningly beautiful photographic images of deep space. However, this is not all that he "can do". With its help, a large amount of information about the structure of many objects, the patterns of their "behavior" was obtained. Hubble and other telescopes are an invaluable source of data necessary for theoretical astronomy, working on the problems of the development of the universe.

Astronomical observations - both terrestrial and space - are the only ones for the science of celestial bodies and phenomena. Without them, scientists could only develop various theories without being able to compare them with reality.

Astronomy is based on observations made from the Earth and only since the 60s of our century, carried out from space - from automatic and other space stations, and even from the Moon. The devices made it possible to obtain samples of lunar soil, deliver various instruments, and even land people on the moon. But for the time being, only the celestial bodies closest to the Earth can be explored. Playing the same role as experiments in physics and chemistry, observations in astronomy have a number of features.

First Feature consists in the fact that astronomical observations are in most cases passive in relation to the objects under study. We cannot actively influence celestial bodies, perform experiments (with the exception of rare cases), as is done in physics, biology, and chemistry. Only the use of spacecraft has provided some opportunities in this respect.

In addition, many celestial phenomena proceed so slowly that their observation requires enormous periods; for example, a change in the inclination of the earth's axis to the plane of its orbit becomes noticeable only after hundreds of years. Therefore, for us, some observations made in Babylon and in China thousands of years ago have not lost their significance, and they were, according to modern concepts, very inaccurate.

Second feature astronomical observations is as follows. We observe the position of celestial bodies and their movement from the Earth, which itself is in motion. Therefore, the view of the sky for an earthly observer depends not only on where he is on the Earth, but also on what time of day and year he observes. For example, when we have a winter day, in South America it is a summer night, and vice versa. There are stars visible only in summer or winter.

Third feature astronomical observations is due to the fact that all the luminaries are very far from us, so far that it is impossible to decide either by eye or through a telescope which of them is closer, which is farther. They all seem equally distant to us. Therefore, during observations, angular measurements are usually performed, and already from them conclusions are often drawn about the linear distances and sizes of bodies.

The distance between objects in the sky (for example, stars) is measured by the angle formed by the rays going to the objects from the point of observation. This distance is called angular and is expressed in degrees and its fractions. In this case, it is considered that two stars are not far from each other in the sky, if the directions in which we see them are close to each other (Fig. 1, stars A and B). It is possible that the third star C, in the sky more distant from L, in space to BUT closer than a star AT.

Measurements of the height, the angular distance of an object from the horizon, are performed with special goniometric optical instruments, such as a theodolite. Theodolite is an instrument, the main part of which is a telescope rotating about the vertical and horizontal axes (Fig. 2). Attached to the axes are circles divided into degrees and minutes of arc. In these circles, the direction of the telescope is counted. On ships and airplanes, angular measurements are made with an instrument called a sextant (sextan).

The apparent dimensions of celestial objects can also be expressed in angular units. The diameters of the Sun and the Moon in angular measure are approximately the same - about 0.5 °, and in linear units the Sun is larger than the Moon in diameter by about 400 times, but it is the same number of times farther from the Earth. Therefore, their angular diameters are almost equal for us.

Your observations

For a better assimilation of astronomy, you should start observing celestial phenomena and luminaries as early as possible. Guidelines for observations with the naked eye are given in Appendix VI. Finding the constellations, orienting yourself on the ground using the Polar Star, familiar to you from the course of physical geography, and observing the daily rotation of the sky is conveniently performed using the moving star map attached to the textbook. For an approximate estimate of the angular distances in the sky, it is useful to know that the angular distance between the two stars of the "dipper" Ursa Major is approximately 5 °.

First of all, you need to get acquainted with the view of the starry sky, find planets on it and make sure that they move relative to the stars or the Sun within 1-2 months. (The conditions for the visibility of planets and some celestial phenomena are discussed in the school astronomical calendar for a given year.) Along with this, one should familiarize oneself with the relief of the Moon, with sunspots, and then with other luminaries and phenomena, which are mentioned in Appendix VI . To do this, an introduction to the telescope is given below.

FOREWORD
The book is devoted to the organization, content and methodology of advanced astronomical observations, as well as the simplest mathematical methods for their processing. It begins with a chapter on testing the telescope, the main instrument of observational astronomy. This chapter outlines the main issues related to the simplest theory of the telescope. Teachers will find here a lot of valuable practical advice related to determining the various characteristics of a telescope, checking the quality of its optics, choosing the optimal conditions for observing, as well as the necessary information about the most important telescope accessories and how to handle them when making visual and photographic observations.
The most important part of the book is the second chapter, which considers, on the basis of concrete material, the questions of the organization, content, and methods of conducting astronomical observations. A significant part of the proposed observations - visual observations of the Moon, Sun, planets, eclipses - does not require high qualifications and, with skillful guidance from the teacher, can be mastered in a short time. At the same time, a number of other observations - photographic observations, visual observations of variable stars, program observations of meteor showers, and some others - already require considerable skill, certain theoretical training and additional instruments and equipment.
Of course, not all of the observations listed in this chapter can be implemented in any school. The organization of observations of increased difficulty is most likely available to those schools where there are good traditions of organizing extracurricular activities in astronomy, there is experience in the relevant work and, which is very important, a good material base.
Finally, in the third chapter, based on specific material, the main mathematical methods for processing observations are presented in a simple and visual form: interpolation and extrapolation, approximate representation of empirical functions, and error theory. This chapter is an integral part of the book. It directs both school teachers and students, and, finally, astronomy lovers to a thoughtful, serious attitude towards setting up and conducting astronomical observations, the results of which can acquire a certain significance and value only after they have been subjected to appropriate mathematical processing.
The attention of teachers is drawn to the need to use microcalculators, and in the future - personal computers.
The material of the book can be used in conducting practical classes in astronomy, provided for by the curriculum, as well as in conducting optional classes and in the work of an astronomical circle.
Taking this opportunity, the authors express their deep gratitude to the Deputy Chairman of the Council of Astronomical Circles of the Moscow Planetarium, an employee of the SAI MSU M. Yu. Shevchenko and Associate Professor of the Vladimir Pedagogical Institute, Candidate of Physical and Mathematical Sciences E. P. Razbitnaya for valuable suggestions that contributed to improving the content of the book.
The authors will gratefully accept all critical comments from readers.

Chapter I TESTING TELESCOPES

§ 1. Introduction
Telescopes are the main instruments of every astronomical observatory, including the educational one. With the help of telescopes, students observe the Sun and the phenomena occurring on it, the Moon and its topography, the planets and some of their satellites, the diverse world of stars, open and globular clusters, diffuse nebulae, the Milky Way and galaxies.
Based on direct telescopic observations and on photographs taken with large telescopes, the teacher can create in students vivid natural-scientific ideas about the structure of the world around them and, on this basis, form firm materialistic convictions.
Starting observations at the school astronomical observatory, the teacher should be well aware of the possibilities of telescopic optics, various practical methods for testing it and establishing its main characteristics. The fuller and deeper the teacher's knowledge of telescopes, the better he will be able to organize astronomical observations, the more fruitful the work of students will be and the more convincingly the results of the observations will appear before them.
In particular, it is important for an astronomy teacher to know a brief theory of the telescope, to be familiar with the most common optical systems and telescope installations, and also to have fairly complete information about eyepieces and various telescope accessories. At the same time, he must know the main characteristics, as well as the advantages and disadvantages of small telescopes intended for school and institute educational astronomical observatories, have good skills in handling such telescopes and be able to realistically assess their capabilities when organizing observations.
The effectiveness of the work of an astronomical observatory depends not only on its equipment with various equipment and, in particular, on the optical power of the telescopes available on it, but also on the degree of preparedness of observers. Only a qualified observer, who has good skills in handling the telescope at his disposal and who knows its main characteristics and capabilities, is able to obtain the maximum possible information on this telescope.
Therefore, the teacher faces the important task of preparing activists who are able to make good observations that require endurance, careful execution, great attention and time.
Without the creation of a group of qualified observers, it is impossible to count on the widespread continuous functioning of the school observatory and on its great return in the education and upbringing of all other students.
In this regard, it is not enough for the teacher to know the telescopes themselves and their capabilities, he must also possess a thoughtful and expressive explanation method that does not go far beyond school curricula and textbooks and is based on the knowledge of students obtained in the study of physics, astronomy and mathematics.
At the same time, special attention should be paid to the applied nature of the reported information about telescopes, so that the capabilities of the latter are revealed in the process of carrying out the planned observations and manifest themselves in the results obtained.
Taking into account the above requirements, the first chapter of the book includes theoretical information about telescopes in the amount necessary for making well-thought-out observations, as well as descriptions of rational practical methods for testing and establishing their various characteristics, taking into account the knowledge and capabilities of students.

§ 2. Determination of the main characteristics of telescope optics
In order to deeply understand the possibilities of telescope optics, one should first give some optical data on the human eye - the main "tool" of students in most educational astronomical observations. Let us dwell on its characteristics such as extreme sensitivity and visual acuity, illustrating their content on examples of observations of celestial objects.
Under the limiting (threshold) sensitivity of the eye is understood the minimum luminous flux that can still be perceived by an eye fully adapted to the darkness.
Convenient objects for determining the limiting sensitivity of the eye are groups of stars of different magnitudes with carefully measured magnitudes. In a good state of the atmosphere, a cloudless sky on a moonless night far from the city, one can observe stars up to the 6th magnitude. However, this is not the limit. High in the mountains, where the atmosphere is especially clean and transparent, stars up to the 8th magnitude become visible.
An experienced observer must know the limits of his eyes and be able to determine the state of transparency of the atmosphere from observations of the stars. To do this, it is necessary to study well the standard generally accepted in astronomy - the Northern Polar series (Fig. 1, a) and take it as a rule: before carrying out telescopic observations, you first need to determine with the naked eye the stars visible at the limit from this series and establish the state of the atmosphere from them.
Rice. 1. Map of the North Polar Range:
a - for observations with the naked eye; b - with binoculars or a small telescope; c - medium telescope.
The data obtained is recorded in the observation log. All this requires observation, memory, develops the habit of eye assessments and accustoms to accuracy - these qualities are very useful for the observer.
Visual acuity is understood as the ability of the eye to distinguish closely spaced objects or luminous points. Doctors have found that the average sharpness of a normal human eye is 1 minute of arc. These data were obtained by examining bright, well-lit objects and point light sources under laboratory conditions.
When observing stars - much less bright objects - visual acuity is somewhat reduced and is about 3 minutes of arc or more. So, having normal vision, it is easy to notice that near Mizar - the middle star in the handle of the Ursa Major bucket - there is a weak star Alcor. Far from everyone succeeds in establishing the duality of e Lyra with the naked eye. The angular distance between Mizar and Alcor is 1 Г48", and between the components ei and e2 of Lyra - 3"28".
Let us now consider how the telescope expands the possibilities of human vision, and analyze these possibilities.
A telescope is an afocal optical system that converts a beam of parallel beams with a cross section D into a beam of parallel beams with a cross section d. This is clearly seen in the example of the beam path in a refractor (Fig. 2), where the lens intercepts parallel beams coming from a distant star and focuses them to a point in the focal plane. Further, the rays diverge, enter the eyepiece and exit it as a parallel beam of smaller diameter. The beams then enter the eye and are focused to a point at the bottom of the eyeball.
If the diameter of the pupil of the human eye is equal to the diameter of the parallel beam emerging from the eyepiece, then all the rays collected by the objective will enter the eye. Therefore, in this case, the ratio of the areas of the telescope lens and the pupil of the human eye expresses the multiplicity of the increase in the light flux, falling
If we assume that the pupil diameter is 6 mm (in complete darkness it even reaches 7 - 8 mm), then a school refractor with a lens diameter of 60 mm can send 100 times more light energy into the eye than the naked eye perceives. As a result, with such a telescope, stars can become visible, sending us light fluxes 100 times smaller than the light fluxes from stars visible at the limit with the naked eye.
According to Pogson's formula, a hundredfold increase in illumination (luminous flux) corresponds to 5 star magnitudes:
The above formula makes it possible to estimate the penetrating power, which is the most important characteristic of a telescope. The penetrating power is determined by the limiting magnitude (m) of the faintest star that can still be seen with a given telescope under the best atmospheric conditions. Since neither the loss of light during the passage of the optics nor the darkening of the sky background in the field of view of the telescope is taken into account in the above formula, it is approximate.
A more accurate value of the penetrating power of a telescope can be calculated using the following empirical formula, which summarizes the results of observations of stars with instruments of different diameters:
where D is the diameter of the lens, expressed in millimeters.
For orientation purposes, Table 1 shows the approximate values ​​of the penetrating power of telescopes, calculated using the empirical formula (1).
The real penetrating power of the telescope can be determined by observing the stars of the Northern Polar series (Fig. 1.6, c). To do this, guided by table 1 or by the empirical formula (1), set the approximate value of the penetrating power of the telescope. Further, from the given maps (Fig. 1.6, c), stars with somewhat larger and somewhat smaller magnitudes are selected. Carefully copy all the stars of greater brilliance and all selected ones. In this way, a star chart is made, carefully studied, and observations are made. The absence of "extra" stars on the map contributes to the rapid identification of the telescopic picture and the establishment of the stellar magnitudes of the visible stars. Follow-up observations are made on subsequent evenings. If the weather and the transparency of the atmosphere improve, then it becomes possible to see and identify fainter stars.
The magnitude of the faintest star found in this way determines the real penetrating power of the telescope used. The results obtained are recorded in the observation log. From them one can judge the state of the atmosphere and the conditions for observing other luminaries.
The second most important characteristic of a telescope is its resolution b, which is understood as the minimum angle between two stars seen separately. In theoretical optics, it is proved that with an ideal lens in visible light L = 5.5-10
where D is the lens diameter in millimeters. (...)
Rice. 3. Diffraction patterns of close stellar pairs with different angular distances of the components.
It is also instructive to carry out telescopic observations of bright stellar pairs with the lens apertured. As the telescope's inlet is gradually apertured, the diffraction disks of the stars increase, merge, and merge into a single diffraction disk of a larger diameter, but with a much lower brightness.
When conducting such studies, attention should be paid to the quality of telescopic images, which are determined by the state of the atmosphere.
Atmospheric disturbances should be observed with a well-aligned telescope (preferably a reflector), examining diffraction images of bright stars at high magnifications. It is known from optics that with a monochromatic light flux, 83.8% of the energy transmitted through the lens is concentrated in the central diffraction disk, 7.2% in the first ring, 2.8% in the second, 1.5% in the third, and 1.5% in the fourth ring. - 0.9%, etc.
Since the incoming radiation from stars is not monochromatic, but consists of different wavelengths, the diffraction rings are colored and blurred. The clarity of ring images can be improved by using filters, especially narrow-band filters. However, due to the decrease in energy from ring to ring and the increase in their areas, already the third ring becomes inconspicuous.
This should be kept in mind when estimating the state of the atmosphere from visible diffraction patterns of observed stars. When making such observations, you can use the Pickering scale, according to which the best images are rated with a score of 10, and very poor ones with a score of 1.
We give a description of this scale (Fig. 4).
1. Images of stars are undulated and smeared so that their diameters are, on average, twice the size of the third diffraction ring.
2. The image is undulating and slightly out of the third diffraction ring.
3. The image does not go beyond the third diffraction ring. The image brightness increases towards the center.
4. From time to time, the central diffraction disk of the star is visible with short arcs appearing around.
5. The diffraction disk is visible all the time, and short arcs are often visible.
6. The diffraction disk and short arcs are visible all the time.
7. Arcs move around a clearly visible disk.
8. Rings with gaps move around a clearly defined disk,
9. The diffraction ring closest to the disk is motionless.
10. All diffraction rings are stationary.
Points 1 - 3 characterize the poor state of the atmosphere for astronomical observations, 4 - 5 - mediocre, 6 - 7 - good, 8 - 10 - excellent.
The third important characteristic of a telescope is its lens aperture, which is equal to the square of the ratio of the lens diameter
to its focal length (...)

§ 3. Checking the quality of telescope optics
The practical value of any telescope as an observational instrument is determined not only by its size, but also by the quality of its optics, i.e., the degree of perfection of its optical system and the quality of the lens. An important role is played by the quality of the eyepieces attached to the telescope, as well as the completeness of their set.
The lens is the most critical part of the telescope. Unfortunately, even the most advanced telescopic lenses have a number of drawbacks due to both purely technical reasons and the nature of light. The most important of these are chromatic and spherical aberration, coma and astigmatism. In addition, fast lenses suffer to varying degrees from field curvature and distortion.
The teacher needs to know about the main optical shortcomings of the most commonly used types of telescopes, expressively and clearly demonstrate these shortcomings and be able to reduce them to some extent.
Let us describe successively the most important optical shortcomings of telescopes, consider in what types of small telescopes and to what extent they manifest themselves, and indicate the simplest ways to highlight, display and reduce them.
The main obstacle that prevented the improvement of the refractor telescope for a long time was chromatic (color) aberration, i.e., the inability of a collecting lens to collect all light rays with different wavelengths at one point. Chromatic aberration is caused by the unequal refraction of light rays of different wavelengths (red rays are refracted more weakly than yellow ones, and yellow rays are weaker than blue ones).
Chromatic aberration is especially pronounced in telescopes with single-lens fast lenses. If such a telescope is pointed at a bright star, then at a certain position of the eyepiece
you can see a bright purple speck surrounded by a colored halo with a blurred red outer ring. As the eyepiece extends, the color of the central spot will gradually change to blue, then green, yellow, orange, and finally red. In the latter case, a colored halo with a purple ring border will be visible around the red spot.
If you look at the planet through such a telescope, the picture will be very blurry, with iridescent stains.
Two-lens lenses that are largely free of chromatic aberration are called achromatic. The relative aperture of a refractor with an achromatic lens is usually 715 or more (for school refracting telescopes, it leaves 7o, which somewhat degrades the image quality).
However, an achromatic lens is not completely free from chromatic aberration and converges well only rays of certain wavelengths. In this regard, the objectives are achromatized in accordance with their purpose; visual - in relation to the rays that act most strongly on the eye, photographic - for the rays that act most strongly on the photographic emulsion. In particular, the lenses of school refractors are visual in their purpose.
The presence of residual chromatic aberration in school refractors can be judged on the basis of observations with very high magnifications of diffraction images of bright stars, quickly changing the following filters: yellow-green, red, blue. It is possible to ensure a quick change of light filters by using disk or sliding frames, described in
§ 20 of the book "School Astronomical Observatory"1. The changes in the diffraction patterns observed in this case indicate that not all rays are equally focused.
The elimination of chromatic aberration is more successfully solved in three-lens apochromatic objectives. However, it has not yet been possible to completely destroy it in any lens objectives.
A reflex lens does not refract light rays. Therefore, these lenses are completely free from chromatic aberration. In this way, reflex lenses compare favorably with lenses.
Another major disadvantage of telescopic lenses is spherical aberration. It manifests itself in the fact that monochromatic rays traveling parallel to the optical axis are focused at different distances from the lens, depending on which zone they have passed through. So, in a single lens, the rays that have passed near its center are focused furthest, and the closest - those that have passed through the edge zone.
This can be easily seen if a telescope with a single-lens objective is directed at a bright star and observed with two diaphragms: one of them should highlight the flux passing through the central zone, and the second, made in the form of a ring, should transmit the rays of the edge zone. Observations should be carried out with light filters, if possible, with narrow bandwidths. When using the first aperture, a sharp image of the star is obtained at a slightly larger extension of the eyepiece than when using the second aperture, which confirms the presence of spherical aberration.
In complex lenses, spherical aberration, together with chromatic aberration, is reduced to the required limit by selecting lenses of a certain thickness, curvature, and types of glass used.
[ The remnants of uncorrected spherical aberration in complex lens telescopic objectives can be detected using (the apertures described above, observing diffraction patterns from bright stars at high magnifications. When studying visual lenses, yellow-green filters should be used, and when studying photographic lenses, blue.
! There is no spherical aberration in mirror parabolic (more precisely, paraboloidal) lenses, since the lenses | reduce to one point the entire beam of rays traveling parallel to the optical axis. Spherical mirrors have spherical aberration, and it is the greater, the larger and brighter the mirror itself.
For small mirrors with small luminosity (with a relative aperture of less than 1: 8), the spherical surface differs little from the paraboloidal one - as a result, the spherical aberration is small.
The presence of residual spherical aberration can be detected by the method described above, using different diaphragms. Although mirror lenses are free from chromatic aberration, filters should be used to better diagnose spherical aberration, because the color of the observed diffraction patterns at different apertures is not the same, which can lead to misunderstandings.
Let us now consider the aberrations that arise when rays pass obliquely to the optical axis of the objective. These include: coma, astigmatism, field curvature, distortion.
With visual observations, one should follow the first two aberrations - coma and astigmatism, and study them practically by observing the stars.
The coma manifests itself in the fact that the image of the star away from the optical axis of the objective takes the form of a blurry asymmetric spot with a displaced core and a characteristic tail (Fig. 6). Astigmatism, on the other hand, consists in the fact that the lens collects an inclined beam of light from the star not into one common focus, but into two mutually perpendicular segments AB and CD, located in different planes and at different distances from the lens (Fig. 7).
Rice. 6. Formation of coma in oblique rays. The circle outlines the field near the optical axis, where the coma is insignificant.
With good alignment in the telescope tube of a low-aperture objective and with a small field of view of the eyepiece, it is difficult to notice both aberrations mentioned above. They can be clearly seen if, for the purpose of training, the telescope is somewhat misaligned by turning the lens through a certain angle. Such an operation is useful for all observers, and especially for those who build their telescopes, because sooner or later they are bound to face alignment issues, and it will be much better if they act consciously.
To misalign the reflector, simply loosen and tighten the two opposite screws holding the mirror.
In a refractor, this is more difficult to do. In order not to spoil the thread, you should glue a transition ring truncated at an angle from cardboard and insert it with one side into the telescope tube, and put the lens on the other.
If you look at the stars through a misaligned telescope, they will all appear tailed. The reason for this is coma (Fig. 6). If, however, a diaphragm with a small central hole is put on the telescope inlet and the eyepiece is moved back and forth, then one can see how the stars are stretched into bright segments AB, then turn into ellipses of different compression, circles, and again into segments CD and ellipses (Fig. 7).
Coma and astigmatism are eliminated by turning the lens. As it is easy to understand, the axis of rotation during adjustment will be perpendicular to the direction. If the tail lengthens when the mirror adjusting screw is turned, then the screw must be rotated in the opposite direction. The final fine-tuning during adjustment should be carried out with a short-focus eyepiece at high magnifications so that the diffraction rings are clearly visible.
If the telescope lens is of high quality and the optics are aligned correctly, then the out-of-focus images of the star, when viewed through a refractor, will look like a small light disk surrounded by a system of colored concentric diffraction rings (Fig. 8, al). In this case, the patterns of prefocal and extrafocal images will be exactly the same (Fig. 8, a 2, 3).
Out-of-focus images of a star will have the same appearance when viewed through a reflector, only instead of a central bright disk, a dark spot will be seen, which is a shadow from an auxiliary mirror or a diagonal total reflection prism.
The inaccuracy of the telescope alignment will affect the concentricity of the diffraction rings, and they themselves will take an elongated shape (Fig. 8, b 1, 2, 3, 4). When focusing, the star will appear not as a sharply defined bright disk, but as a slightly blurred bright spot with a weak tail thrown to the side (coma effect). If the indicated effect is caused by a really inaccurate adjustment of the telescope, then the matter can be easily corrected, it is enough just to change its position somewhat in the desired direction by acting with the adjusting screws of the lens (mirror) frame. It is much worse if the reason lies in the astigmatism of the lens itself or (in the case of a Newton reflector) in the poor quality of the auxiliary diagonal mirror. In this case, the drawback can be eliminated only by grinding and repolishing the defective optical surfaces.
From out-of-focus images of a star, other shortcomings of the telescopic lens, if any, can be easily detected. For example, the difference in the sizes of the corresponding diffraction rings of the prefocal and extrafocal images of a star indicates the presence of spherical aberration, and the difference in their chromaticity indicates significant chromatism (for linear
call lens); the uneven distribution density of the rings and their different intensities indicate the zoning of the lens, and the irregular shape of the rings indicates local more or less significant deviations of the optical surface from the ideal.
If all the listed disadvantages revealed by the pattern of out-of-focus images of a star are small, then they can be put up with. Specular lenses of amateur telescopes that have successfully passed the Foucault shadow test, as a rule, have an impeccable optical surface and withstand tests on out-of-focus star images perfectly.
Calculations and practice show that with perfect alignment of the optics, coma and astigmatism have little effect on visual observations when low-aperture objectives (less than 1:10) are used. This applies equally to photographic observations, when luminaries with relatively small angular sizes (planets, the Sun, the Moon) are photographed with the same lenses.
Coma and astigmatism greatly spoil images when photographing large areas of the starry sky with parabolic mirrors or two-lens lenses. Distortion increases sharply with fast lenses.
The table below gives an idea of ​​the growth of coma and astigmatism depending on the angular deviations from the optical axis for parabolic reflectors of different luminosity.
Rice. 9. Curvature of the field of view and images of stars in its focal plane (with correction of all other aberrations).
tism, but there is a curvature of the field. If you photograph a large area of ​​the starry sky with such a lens and at the same time focus on the central zone, then as you retreat to the edges of the field, the sharpness of the images of stars will deteriorate. And vice versa, if focusing is performed on the stars located at the edges of the field, then the sharpness of the images of stars will deteriorate in the center.
In order to obtain a photograph sharp across the entire field with such a lens, the film must be bent in accordance with the curvature of the field of sharp images of the lens itself.
The curvature of the field is also eliminated with the help of a plano-convex Piazzi-Smith lens, which turns the curved wave front into a flat one.
The curvature of the field can be most simply reduced by aperture of the lens. It is known from the practice of photographing that with a decrease in the aperture, the depth of field increases - as a result, clear images of stars are obtained over the entire field of a flat plate. However, it should be remembered that aperture reduction greatly reduces the optical power of the telescope, and in order for faint stars to appear on the plate, the exposure time must be significantly increased.
Distortion manifests itself in the fact that the lens builds an image that is not proportional to the original, but with some deviations from it. As a result, when photographing a square, its image may turn out with sides concave inward or convex outward (pincushion and barrel distortion).
Examining any lens for distortion is very simple: to do this, you need to greatly aperture it so that only a very small central part remains uncovered. Coma, astigmatism and curvature of the field with such a diaphragm will be eliminated and distortion can be observed in its purest form
If you take pictures of rectangular grilles, window openings, doors with such a lens, then, by examining the negatives, it is easy to establish the type of distortion inherent in this lens.
The distortion of the finished lens cannot be eliminated or reduced. It is taken into account in the study of photographs, especially when carrying out astrometric work.

§ 4. Eyepieces and limiting magnifications of the telescope
The eyepiece set is a necessary addition to the telescope. Earlier we have already clarified (§ 2) the purpose of the eyepiece in a magnifying telescopic system. Now it is necessary to dwell on the main characteristics and design features of various eyepieces. Leaving aside the Galilean eyepiece from one diverging lens, which has not been used in astronomical practice for a long time, let us turn immediately to special astronomical eyepieces.
Historically, the first astronomical eyepiece, which immediately replaced the Galilean eyepiece, was the Kepler eyepiece from a single short-focus lens. Possessing a much larger field of view in comparison with Galileo's eyepiece, in combination with the long-focus refractors common at that time, it produced fairly clear and slightly colored images. However, later the Kepler eyepiece was superseded by the more advanced Huygens and Ramsden eyepieces, which are still found today. The most commonly used astronomical eyepieces at present are the Kellner achromatic eyepiece and the Abbe orthoscopic eyepiece. Figure 11 shows the arrangement of these eyepieces.
The Huygens and Ramsden eyepieces are most simply arranged. Each of them is composed of two plano-convex converging lenses. The front one (facing the objective) is called the field lens, and the back one (facing the observer's eye) is called the eye lens. In the Huygens eyepiece (Fig. 12), both lenses face the objective with their convex surfaces, and if f \ and / 2 are the focal lengths of the lenses, and d is the distance between them, then the relationship must be satisfied: (...)

KOHETS FRAGMEHTA TEXTBOOK

Followed the movement of the stars in the sky. Astronomical observations of that time helped to navigate the terrain, and were also necessary for the construction of philosophical and religious systems. A lot has changed since then. Astronomy finally freed itself from astrology, accumulated extensive knowledge and technical power. However, astronomical observations made on Earth or in space are still one of the main methods of obtaining data in this science. The methods of collecting information have changed, but the essence of the methodology has remained unchanged.

What are astronomical observations?

There is evidence to suggest that people possessed elementary knowledge about the movement of the Moon and the Sun even in the prehistoric era. The works of Hipparchus and Ptolemy testify that knowledge about the luminaries was also in demand in Antiquity, and much attention was paid to them. For that time and for a long period after, astronomical observations were the study of the night sky and the fixation of what was seen on paper, or, more simply, a sketch.

Until the Renaissance, only the simplest instruments were assistants to scientists in this matter. A significant amount of data became available after the invention of the telescope. As it improved, the accuracy of the information received increased. However, at whatever level of technological progress, astronomical observations are the main way to collect information about celestial objects. Interestingly, this is also one of the areas of scientific activity in which the methods used in the era before scientific progress, that is, observation with the naked eye or with the help of the simplest equipment, have not lost their relevance.

Classification

Today, astronomical observations are a fairly broad category of activities. They can be classified according to several criteria:

• qualifications of the participants;
• the nature of the recorded data;
• location.

In the first case, professional and amateur observations are distinguished. The data obtained in this case is most often the registration of visible light or other electromagnetic radiation, including infrared and ultraviolet. In this case, information can be obtained in some cases only from the surface of our planet or only from space outside the atmosphere: according to the third feature, astronomical observations made on Earth or in space are distinguished.

amateur astronomy

The beauty of the science of the stars and other celestial bodies is that it is one of the few that literally needs active and tireless admirers among non-professionals. A huge number of objects worthy of constant attention, there are a small number of scientists occupied with the most complex issues. Therefore, astronomical observations of the rest of the near space fall on the shoulders of amateurs.

The contribution of people who consider astronomy their hobby to this science is quite tangible. Until the middle of the last decade of the last century, more than half of the comets were discovered by amateurs. Their areas of interest also often include variable stars, observing novae, tracking the coverage of celestial bodies by asteroids. The latter is today the most promising and demanded work. As for New and Supernovae, as a rule, amateur astronomers are the first to notice them.

Options for non-professional observations

Amateur astronomy can be divided into closely related branches:

• Visual astronomy. This includes astronomical observations with binoculars, a telescope, or the naked eye. The main goal of such activities, as a rule, is to enjoy the opportunity to observe the movement of the stars, as well as from the process itself. An interesting branch of this direction is "sidewalk" astronomy: some amateurs take their telescopes out into the street and invite everyone to admire the stars, planets and the Moon.
• Astrophotography. The purpose of this direction is to obtain photographic images of celestial bodies and their elements.
• Telescope building. Sometimes the necessary optical instruments, telescopes and accessories for them, are made by amateurs almost from scratch. In most cases, however, telescope construction consists in supplementing existing equipment with new components.
• Research. Some amateur astronomers seek, in addition to aesthetic pleasure, to get something more material. They are engaged in the study of asteroids, variables, new and supernovae, comets and meteor showers. Periodically, in the process of constant and painstaking observations, discoveries are made. It is this activity of amateur astronomers that makes the greatest contribution to science.

Activities of professionals

Specialist astronomers around the world have more sophisticated equipment than amateurs. The tasks facing them require high accuracy in collecting information, a well-functioning mathematical apparatus for interpretation and forecasting. As a rule, quite complex, often distant objects and phenomena lie at the center of the work of professionals. Often, the study of the expanses of space makes it possible to shed light on certain laws of the universe, to clarify, supplement or refute theoretical constructions regarding its origin, structure and future.

Classification by type of information

Observations in astronomy, as already mentioned, can be associated with the fixation of various radiation. On this basis, the following directions are distinguished:

• optical astronomy studies radiation in the visible range;
• infrared astronomy;
• ultraviolet astronomy;
• radio astronomy;
• x-ray astronomy;
• gamma astronomy.

In addition, the directions of this science and the corresponding observations that are not related to electromagnetic radiation are highlighted. This includes neutrino, studying neutrino radiation from extraterrestrial sources, gravitational-wave and planetary astronomy.

From the surface

Some of the phenomena studied in astronomy are available for research in ground-based laboratories. Astronomical observations on Earth are associated with the study of trajectories of movement by measuring the distance in space to stars, fixing certain types of radiation and radio waves, and so on. Until the beginning of the era of astronautics, astronomers could only be content with information obtained under the conditions of our planet. And this was enough to build a theory of the origin and development of the Universe, to discover many patterns that exist in space.

High above the earth

With the launch of the first satellite, a new era in astronomy began. The data collected is invaluable. They contributed to the deepening of scientists' understanding of the mysteries of the universe.

Astronomical observations in space make it possible to detect all types of radiation, from visible light to gamma and X-rays. Most of them are not available for research from the Earth, because the atmosphere of the planet absorbs them and does not allow them to the surface. X-ray pulsars are an example of discoveries that became possible only after that.

Information miners

Astronomical observations in space are carried out using various equipment installed on spacecraft and orbiting satellites. Many studies of this nature are carried out on the invaluable contribution of optical telescopes launched several times in the last century. The famous Hubble stands out among them. For the layman, it is primarily a source of stunningly beautiful photographic images of deep space. However, this is not all that he "can do". With its help, a large amount of information about the structure of many objects, the patterns of their "behavior" was obtained. Hubble and other telescopes are an invaluable source of data necessary for theoretical astronomy, working on the problems of the development of the universe.

Astronomical observations - both terrestrial and space - are the only ones for the science of celestial bodies and phenomena. Without them, scientists could only develop various theories without being able to compare them with reality.

Astronomy is a science that studies celestial objects and the Universe in which we live.

Remark 1

Since astronomy as a science does not have the opportunity to conduct an experiment, the main source of information is the information that researchers receive during observation.

In this regard, a field called observational astronomy is singled out in astronomy.

The essence of observational astronomy is to obtain the necessary information about objects in space using instruments such as telescopes and other equipment.

Observations in astronomy make it possible, in particular, to track patterns in the properties of certain objects under study. The obtained results of the study of some objects can be extended to other objects with similar properties.

Sections of observational astronomy

In observational astronomy, the division into sections is associated with the division of the electromagnetic spectrum into ranges.

Optical astronomy - contributes to observations in the visible part of the spectrum. At the same time, mirrors, lenses, and solid-state detectors are used in observation devices.

Remark 2

In this case, the region of visible radiation lies in the middle of the range of the studied waves. The wavelength of visible radiation is in the range from 400 nm to 700 nm.

Infrared astronomy is based on the search and study of infrared radiation. In this case, the wavelength exceeds the limiting value for observations with silicon detectors: about 1 μm. To study the selected objects in this part of the range, researchers mainly use telescopes - reflectors.

Radio astronomy is based on observations of radiation with a wavelength from millimeters to tens of millimeters. By the principle of their operation, receivers using radio emission are comparable to those receivers that are used in broadcasting radio programs. However, radio receivers are more sensitive.

X-ray astronomy, gamma-ray astronomy and ultraviolet astronomy are included in high energy astronomy.

Observation methods in astronomy

Obtaining the desired data is possible when astronomers register electromagnetic radiation. In addition, researchers conduct observations of neutrinos, cosmic rays or gravitational waves.

Optical and radio astronomy uses ground-based observatories in its activities. The reason for this is that at the wavelengths of these ranges, the atmosphere of our planet has a relative transparency.

Observatories are mostly located at high altitudes. This is due to the reduction in absorption and distortion that the atmosphere creates.

Remark 3

Note that a number of infrared waves are significantly absorbed by water molecules. Because of this, observatories are often built in dry places at high altitude or in space.

Balloons or space observatories are mainly used in the fields of x-ray, gamma-ray and ultraviolet astronomy, and with a few exceptions, in far-IR astronomy. At the same time, observing air showers, you can detect the gamma radiation that created them. Note that the study of cosmic rays is currently a rapidly developing area of ​​astronomical science.

Objects located close to the Sun and to the Earth can be seen and measured when they are observed against the background of other objects. Such observations were used to build models of the orbits of the planets, as well as to determine their relative masses and gravitational perturbations. The result was the discovery of Uranus, Neptune and Pluto.

Radio astronomy - the development of this field of astronomy was the result of the discovery of radio emission. Further development of this area led to the discovery of such a phenomenon as cosmic background radiation.

Neutrino astronomy - this area of ​​astronomical science uses neutrino detectors in its arsenal, located mainly underground. Neutrino astronomy tools help to obtain information about processes that researchers cannot observe with telescopes. An example is the processes occurring in the core of our Sun.

Gravitational wave receivers have the ability to record traces of even such phenomena as the collision of such massive objects as neutron stars and black holes.

Automatic spacecraft are actively used in astronomical observations of the planets of the solar system. The geology and meteorology of the planets are being studied especially actively with their help.

Conditions for conducting astronomical observations.

For better observation of astronomical objects, the following conditions are important:

1. Research is carried out mainly in the visible part of the spectrum using optical telescopes.
2. Observations are mainly carried out at night, since the quality of the data obtained by researchers depends on the transparency of the air and visibility conditions. In turn, the visibility conditions depend on turbulence and the presence of heat flows in the air.
3. The absence of a full moon gives an advantage in observing astronomical objects. If the full moon is in the sky, then this gives additional illumination and complicates the observation of faint objects.
4. For an optical telescope, the most suitable place for observation is open space. In outer space, it is possible to make observations that do not depend on the vagaries of the atmosphere, for lack of such in space. The disadvantage of this method of observation is the high financial cost of such studies.

After space, the most suitable place for observing outer space are the peaks of the mountains. Mountain peaks have a large number of cloudless days and have quality visibility conditions associated with good atmospheric quality.

Example 1

An example of such observatories are the mountain peaks of the islands of Mauna Kea and La Palma.

The level of darkness at night also plays a big role in astronomical observations. Artificial illumination created by human activity interferes with high-quality observation of faint astronomical objects. However, the use of plafonds around street lamps helps to help the problem. As a result, the amount of light reaching the earth's surface increases, and the radiation directed towards the sky decreases.

5. The influence of the atmosphere on the quality of observations can be great. To obtain a better image, telescopes with additional image blur correction are used. To improve the quality, adaptive optics, speckle interferometry, aperture synthesis, or placing telescopes in space are also used.