mirror telescope. Types of telescopes

We "dug" a little into the issue of the origin of the telescope, and also took a closer look at the refractor telescope, including using a couple of models as an example. Let's take it a step further and talk about reflecting telescopes.

The main difference between a reflector and a refractor telescope is that in a reflector, not a lens, but a mirror is responsible for collecting light and magnifying the image.

A parabolic (mostly, but sometimes it can also be spherical) mirror is located at the bottom of the telescope tube. It collects light and focuses the resulting image on a small auxiliary (secondary) mirror, which already "directs" the image into the eyepiece. In this case, the observer looks through the telescope from the side, and even from the side directly directed to the sky. Such a device can confuse someone, and at first a person who is accustomed to using mainly a refractor will have to suffer a little with the controls.

The very first reflector was invented in 1667 by Sir Isaac Newton, who, apparently, was tired of the chromatic aberrations inherent in all refractors. However, instead of the usual chromatic effect, Newton received other image features that still accompany most reflectors.

And more specifically, the Newton reflector (this name is still used by telescopes of this type) has its own aberrations. Mostly astronomy lovers complain about the so-called "to whom". This effect creates the feeling that the center of the picture and its edges are out of focus with each other - that is, the stars in the center look like they should be, dots, and at the edges they look like comets: smeared, "shaggy and tailed".

In principle, if you are not involved in astrophotography, this feature of reflectors will not bother you much: after all, the object in question, as a rule, is in the center of the picture visible to the observer, which means it will not suffer from the coma effect. And if you are a photographer who wants to start shooting the starry sky, then it is better to take care of finding special correctors in advance to correct this particular aberration.

Coma is far from the only disadvantage of reflectors. These also include:

• the need to periodically adjust the position of the mirror - this process is called "adjustment";
• sensitivity of the device to temperature changes - you cannot take the telescope out of the house to the street in winter and immediately start observing, otherwise the picture will greatly disappoint you;
• decent dimensions - this circumstance somewhat restrains the passion for traveling with a telescope in a backpack;
• sensitivity to bad weather - strong wind can cause "shaking" of the image;
• low protection from dust and other contaminants - in fact, direct access to the central mirror allows dirt to get inside almost unhindered, and you need to wash the mirror surface very carefully, otherwise there is a chance of damaging it;
• the risk of running into low-quality optics in cheap reflectors.

However, all these disadvantages cannot completely defeat the significant advantages:

1. Price. This is, of course, the most positive characteristic reflector. It is simple in design, and the mirror needs less processing than each of the refractor lenses, which, of course, could not but affect the cost of the reflector - and, moreover, in a better way for the buyer. In fact, for the same price, you can find a refractor and a reflector that differ significantly in aperture (again, the reflector wins). Let me remind you: the aperture is the diameter of the main lens (at the refractor) or the main mirror (at the reflector). And as mentioned earlier, a larger aperture is always better. After all, it is on this characteristic that the resolution, contrast, and the maximum distinguishable stellar magnitude depend. And if it's even simpler - the larger the aperture, the better the picture will be.
2. The reflector can be mounted on the lightest type of mount that you can actually make yourself: the Dobson mount is the most compact in terms of dimensions, and besides, it is made of wood, chipboard or plywood. It is clear that in the weight category these materials outperform metal.
3. Excellent performance (as a rule) in terms of aperture ratio - this type of telescope, especially in combination with an equatorial mount, is very good in astrophotography.
4. If the optics are of high quality, then the image in its central part will be practically devoid of any aberrations - and not a single refractor can boast of such an indicator.
5. Great for observing deep space objects.

However, let's already consider some suitable model.

For example, let's take the Celestron PowerSeeker 127 EQ telescope (7500 rubles).

Quite a budget model with an excellent aperture of 127 mm. If you take 7500 rubles. (estimated cost) for the upper money "bar" for the purchase of a telescope, you can find a refractor with a lens diameter of maximum 70 mm. And as has been said more than once, the larger the aperture, the better.

The kit comes with two interchangeable eyepieces for 20 and 4 mm, as well as a triple Barlow lens. In sum, if you look at the specifications attached to the telescope, this optics should give an increase of up to 750 times! However, in practice, you can easily calculate to what extent the device will give you a clear picture. You just need to multiply the aperture value (in mm) by 1.4 - the resulting figure will be exactly the multiplicity, after reaching which the telescope is unlikely to produce a super-clear picture. However, if you multiply the same aperture value by 2, you will find out the absolute qualitative limit of increasing your device. If we talk about this Celestron model, then 127 x 1.4 \u003d 177.8 times, 127 x 2 \u003d 254 times. Total - 254 times will be the most "ceiling" in terms of increase.

The limiting magnitude of distinguishable objects is +13 m.

A reflector with an equatorial mount is very good for observing celestial objects, practically nothing for terrestrial objects. Celestron's equatorial mount comes with subtle movement mechanisms and coordinate circles, all of which will help the beginner cope with the difficult task of pointing and observing at first.

Telescope weight - 7.7 kg, tube length - 508 mm. Much more compact than a refractor with the same aperture - it will be more than a meter long, and the weight indicator will "dive" beyond the mark of 30 kg. Not the best option for hiking, is not it?

A typical representative of reflectors, excellent for observing deep sky objects.

And now let's talk about mirror-lens (catadioptric) telescopes. Sometimes they are also called the combined type.

If in a refractor the lens is based on the use of a lens, in a reflector - on a mirror, then catadioptrics use both lenses and mirror optics in their device. Such lenses are more difficult to manufacture, because their price, of course, will be higher than, say, the cost of a reflector with the same aperture. The second unpleasant feature of this type is that, due to its design, a mirror-lens device cannot provide the observer with the same clear picture as, for example, a refractor.

Another of the "minuses" - mirror-lens telescopes with an optical scheme of Schmidt - Cassegrain, unfortunately, are not without comatic aberration. But Maksutov - Cassegrain can boast of a picture without these "interferences".

Among other things, catadioptrics are the most sensitive to temperature changes - even more reflectors.

However positive points mirror-lens sometimes play a decisive role for many astronomy lovers.

The first is, of course, the size. For example, a refractor with an aperture of 90 mm will be at least 95 cm long (and most likely about a meter). And similar in aperture size Maksutov - Cassegrain - 28 cm long. Significant difference, right? Catadioptrics weigh, respectively, also less than other varieties.

Well, no less significant point - aberrations, more precisely, their almost complete absence. If the optics are of high quality and the manufacturer did not make any serious "mistakes" in the manufacture of the telescope, then the picture will be devoid of all those "irregularities" that both refractors and reflectors certainly accompany to some extent.

For example, consider the Celestron NexStar 90 SLT (16,300 rubles).

As the name implies, the aperture here is 90 mm. This is one of the representatives of a number of Maksuts - Cassegrain, that is, the image obtained with its help will be practically devoid of the usual aberrations.

The set includes two interchangeable eyepieces for 25 mm (50x) and 9 mm (139x), the maximum stellar magnitude of the observed objects is 12.3 m.

Azimuth mount with computer guidance - a similar system is popularly called GoTo. The device already has a database of 4000 objects. The control is simple: select an object from the database and the telescope will automatically "aim" at the region of the sky you need. The choice of an object is made using the remote control, which has the option of updating via the Internet (naturally, when connected to a computer). The possibilities of such control are not limited to just selecting an object: GoTo allows you to point at coordinates, get a brief information about an object; can, upon request, issue the coordinates of the point, which is aimed at this moment. The only thing that can cause difficulties for beginners in astronomy is that before using the telescope, you need to orient yourself on the ground, that is, enter the place and time of observation, and also point the telescope at a couple of stars known to the user. In principle, a convenient system, often saving the time of the observer.

Steel tripod for maximum stability, dovetail mount - the device is installed with a quick and uncomplicated movement. The weight of the telescope is only 5.4 kg.

A great option even for beginners in astronomy. The capabilities of a catadioptric, the convenience of GoTo plus maximum compactness - and now the tool of a real astronomer is at hand (of course, if the price does not scare you away).

Finding the perfect universal telescope is impossible. Every type has its strengths and weaknesses. However, if you know exactly what you are most interested in in the sky, you can choose a device that will reveal its capabilities to the maximum.

A refractor with an aperture of 70-90 mm is suitable for a child as the first telescope (especially in urban conditions): he will be able to examine in detail both the surface of the Moon and the planet solar system, and the Sun. The only note: it is absolutely impossible to view the Sun through a telescope without special filters - you will simply lose your sight, because in this case the telescope acts like an ordinary magnifying glass. Remember what happens to a piece of paper if you point at it Sunbeam through magnifying glass: It will light up quickly. Now imagine that your eye is in place of the piece of paper, and you will instantly feel like experimenting with the Sun.

For high-quality observations of distant space objects (nebulae, globular star clusters, etc.) far from urban illumination, a reflector with an aperture of about 114-150 mm is best suited. Of course, the higher this indicator, the better - you already look at the money there.

Well, if you travel a lot and at the same time want to always have a telescope with you, then the best choice would be some model of Maksutov - Cassegrain or another device from a number of mirror-lens: they are compact and will be easier to carry.

In the event that you yourself have not yet decided what exactly you want to study, take a refractor. For the first time, to understand whether such an activity is interesting to you at all, it is quite enough. It is better if the aperture is somewhere around 70-90 mm: smaller sizes are unlikely to bring real pleasure.

And don't forget about the dimensions: many telescopes are extremely inconvenient in terms of hand-carrying, and people who do not have a vehicle should think about this as well.

Sir Isaac Newton (1643-1727) - English scientist

The mirror telescope of the famous English research scientist Isaac Newton is not among the priceless treasures that can cause universal admiration. A telescope is a scientific instrument. But today it is a priceless relic because Newton made it himself. With his help, he enriched science and all mankind with new knowledge about the stars, about the movement of light. The scientific data obtained by him can hardly be overestimated.

Interest in the creation of scientific instruments with which it was possible to conduct research, appeared in Newton in his school years. As a boy, he loved to watch carpenters work, how they build a house, how they make wings for a windmill, how they create wheels for a water mill. He did not just look, he memorized, but sketched at home, created a semblance of drawings, according to which he made working models of windmills and watermills. But he did not just copy, he introduced a certain innovation into each model.

His passion for modeling was noted by teachers at school, relatives and acquaintances of the Newton family drew attention to this. Once he made a watch that operated under the pressure of water flowing from a tank. She fell into the funnel and then turned the wheels. To the surprise of adults, he made a miniature mill for grinding grain. In the role of the engine, he acted as a mouse that rotated the wheel. He achieved this not by training, but by the natural desire of the mouse to feast on it, and hung a bag of grain over it.

Newton was not an inventor. He did not invent any of the devices he created. He took ready-made ones, but made improvements to each one. He needed a telescope to observe the stars, to determine the properties of light, to know its speed, to unravel the mysteries of the universe.

The first telescopes, or spyglasses, appeared in Holland in the 17th century, although the magnifying property of concave glass lenses was known as early as 2500 BC. In 1610, the Italian scientist Galileo Galilei, using an instrument he designed, observed the stars and made the stunning conclusion that the universe is infinite. Before Galileo, many natural phenomena were described speculatively, rarely on the basis of experiments. But Galileo was the first who, on the basis of observations through a telescope, made a conclusion about the movement of stars, about the infinity of the universe. He was compared with Columbus, the discoverer of previously unknown lands. His work has become a role model.

In Holland, Germany, England, scientists began to make their own telescopes. Newton did not escape this temptation. University science at Cambridge demanded new instruments, and the 22-year-old student Newton set about building his own telescope. He polished the lenses himself. It was the hardest job. In his Lectures on Optics, he described the essence of the device he created and its capabilities. Only a few years later he finally managed to realize his ideas in a new telescope.

In 1671, the news that in Cambridge an unknown young inventor had created a special telescope with a reflecting spherical mirror, with which you could zoom in on the sky and watch the stars, reached London. Newton was asked to send the device to the capital. They wanted to demonstrate his action in front of the monarch. On the throne was Charles II, during whose reign England experienced an economic boom. The telescope was meticulously examined by the most prominent scientists of that time, who were members of the Royal Mathematical Society, created in 1662. And everyone recognized the great usefulness of the telescope created at Cambridge. The king agreed with the opinion of scientists, and in the same year, 29-year-old Newton was admitted to the Royal Mathematical Society.

Often an invention The first telescope is attributed to Hans Lipperschley from Holland, 1570-1619, but he was almost certainly not the discoverer. Most likely, his merit is that he was the first to make the new telescope instrument popular and in demand. And also it was he who filed in 1608 an application for a patent for a pair of lenses placed in a tube. He called the device a spyglass. However, his patent was rejected because his device seemed too simple.

Long before him, Thomas Digges, an astronomer, tried to magnify the stars in 1450 using a convex lens and a concave mirror. However, he did not have the patience to refine the device, and the semi-invention was soon safely forgotten. Digges is remembered today for his description of the heliocentric system.

By the end of 1609, small spyglasses, thanks to Lipperschley, had become common throughout France and Italy. In August 1609, Thomas Harriot finalized and improved the invention, which allowed astronomers to see the craters and mountains on the moon.

Galileo Galilei and telescope

The big breakthrough came when the Italian mathematician Galileo Galilei learned of a Dutchman's attempt to patent the lens tube. Inspired by the discovery, Halley decided to make such a device for himself. In August 1609, it was Galileo who made the world's first full-fledged telescope. At first, it was just a spotting scope - combination spectacle lenses, today it would be called a refractor. Before Galileo, most likely, few people guessed to use this entertaining tube for the benefit of astronomy. Thanks to the device, Galileo himself discovered mountains and craters on the Moon, proved the sphericity of the Moon, discovered the four satellites of Jupiter, the rings of Saturn, and made many other useful discoveries.

To today's person, the Galileo telescope will not seem special; any ten-year-old child can easily assemble a much better instrument using modern lenses. But the Galileo telescope was the only real working telescope for that day with 20x magnification, but with a small field of view, a slightly blurred image, and other shortcomings. It was Galileo who discovered the age of the refractor in astronomy - the 17th century.

17th century in the history of stargazing

Time and the development of science made it possible to create more powerful telescopes, which made it possible to see much more. Astronomers have started using lenses with longer focal lengths. The telescopes themselves turned into large unliftable tubes in size and, of course, were not convenient to use. Then tripods were invented for them. Telescopes were gradually improved and refined. However, its maximum diameter did not exceed a few centimeters - it was not possible to produce large lenses.

By 1656 Christian Huyens made a telescope that magnifies 100 times the observed objects, its size was more than 7 meters, the aperture was about 150 mm. This telescope is already considered to be at the level of today's amateur telescopes for beginners. By the 1670s, a 45-meter telescope had already been built, which further enlarged objects and gave a greater angle of view.

Isaac Newton and the invention of the reflector

But even an ordinary wind could serve as an obstacle to obtaining a clear and high-quality image. The telescope began to grow in length. The discoverers, trying to get the most out of this device, relied on the optical law they discovered - a decrease in the chromatic aberration of a lens occurs with an increase in its focal length. To remove chromatic noise, the researchers made telescopes of the most incredible length. These tubes, which were then called telescopes, reached 70 meters in length and caused a lot of inconvenience in working with them and adjusting them. The shortcomings of refractors forced great minds to look for solutions to improve telescopes. The answer and a new way was found: the collection and focusing of rays began to be carried out using a concave mirror. The refractor was reborn into a reflector, completely freed from chromatism.

This merit belongs entirely to Isaac Newton, it was he who managed to give new life telescopes with a mirror. His first reflector was only four centimeters in diameter. And he made the first mirror for a telescope with a diameter of 30 mm from an alloy of copper, tin and arsenic in 1704. The image became clear. By the way, his first telescope is still carefully kept in the Astronomical Museum in London.

But also long time opticians never managed to make full-fledged mirrors for reflectors. The year of birth of a new type of telescope is considered to be 1720, when the British built the first functional reflector with a diameter of 15 centimeters. It was a breakthrough. In Europe, there was a demand for portable, almost compact telescopes two meters long. About 40-meter pipes of refractors began to be forgotten.

By the end of the 18th century, compact, handy telescopes had replaced bulky reflectors. Metal mirrors also turned out to be not very practical - expensive to manufacture, as well as dimming with time. By 1758, with the invention of two new types of glass: light - crowns and heavy - flint, it became possible to create two-lens lenses. What safely and took advantage of the scientist J. Dollond, who made a two-lens lens, later called the dollar lens.

Herschel and Ross telescopes

After the invention of achromatic lenses, the victory of the refractor was absolute, it remained only to improve lens telescopes. Forgot about concave mirrors. It was possible to revive them to life by the hands of amateur astronomers. William Herschel, English musician who discovered the planet Uranus in 1781. His discovery has not been equaled in astronomy since ancient times. Moreover, Uranus was discovered with the help of a small homemade reflector. Success prompted Herschel to start making larger reflectors. Herschel himself in the workshop fused mirrors made of copper and tin. The main work of his life is a large telescope with a mirror with a diameter of 122 cm. This is the diameter of his largest telescope. The discoveries were not long in coming, thanks to this telescope, Herschel discovered the sixth and seventh satellites of the planet Saturn. Another, no less famous, amateur astronomer, the English landowner Lord Ross, invented a reflector with a mirror with a diameter of 182 centimeters. Thanks to the telescope, he discovered a number of unknown spiral nebulae. The telescopes of Herschel and Ross had many shortcomings. Mirror metal lenses were too heavy, reflected only a small part of the light falling on them, and dimmed. A new perfect material for mirrors was required. This material was glass. The French physicist Leon Foucault in 1856 tried to insert a silver-plated glass mirror into a reflector. And the experience was a success. Already in the 90s, an amateur astronomer from England built a reflector for photographic observations with a glass mirror 152 centimeters in diameter. Another breakthrough in telescope construction was obvious.

This breakthrough was not without the participation of Russian scientists. I'M IN. Bruce became famous for developing special metal mirrors for telescopes. Lomonosov and Herschel, independently of each other, invented a completely new design of the telescope, in which the primary mirror tilts without the secondary one, thereby reducing light loss.

The German optician Fraunhofer put the production and quality of lenses on the conveyor. And today there is a telescope with a whole, working Fraunhofer lens in the Tartu Observatory. But the refractors of German optics were also not without a flaw - chromatism.

Rise of refractor astronomy

The two-mirror system in a telescope was proposed by the Frenchman Cassegrain. Cassegrain could not realize his idea in full due to the lack of technical feasibility of inventing the necessary mirrors, but today his drawings have been implemented. It is Newton's and Cassegrain's telescopes that are considered the first "modern" telescopes, invented at the end of the 19th century. By the way, cosmic Hubble telescope It works just like the Cassegrain telescope. And Newton's fundamental principle using a single concave mirror has been used at the Special Astrophysical Observatory in Russia since 1974. The heyday of refractor astronomy occurred in the 19th century, when the diameter of achromatic lenses gradually grew. If in 1824 the diameter was another 24 centimeters, then in 1866 its size doubled, in 1885 the diameter began to be 76 centimeters (Pulkovo observatory in Russia), and by 1897 the Ierk refractor was invented. It can be calculated that for 75 years the lens objective has increased at a rate of one centimeter per year.

By the end of the 19th century, they invented new method lens production. Glass surfaces began to be treated with a silver film, which was applied to a glass mirror by exposing grape sugar to silver nitrate salts. These groundbreaking lenses reflected up to 95% of the light, as opposed to the antique bronze lenses that only reflected 60% of the light. L. Foucault created reflectors with parabolic mirrors by changing the shape of the surface of the mirrors. In the late 19th century, Crossley, an amateur astronomer, turned his attention to aluminum mirrors. He bought a concave glass parabolic mirror with a diameter of 91 cm was immediately inserted into the telescope. Today, telescopes with such huge mirrors are installed in modern observatories. While the growth of the refractor slowed, the development of the reflecting telescope gained momentum. From 1908 to 1935, various observatories around the world built more than a dozen reflectors with a lens larger than the Ierk one. The largest telescope is installed at the Mount Wnlson Observatory, its diameter is 256 centimeters. And even this limit will soon be surpassed twice. An American giant reflector has been mounted in California, today its age is more than twenty years.

Recent history of telescopes

More than 40 years ago, in 1976, Soviet scientists built a 6-meter BTA telescope - the Large Azimuthal Telescope. Until the end of the 20th century, the ARB was considered the world's largest telescope. The inventors of the BTA were innovators in original technical solutions, such as alt-azimuth installation with computer guidance. Today, these innovations are used in almost all giant telescopes. At the beginning of the 21st century, the BTA was pushed into the second ten largest telescopes in the world. And the gradual degradation of the mirror from time to time - today its quality has fallen by 30% from the original - turns it only into a historical monument to science.

To a new generation telescopes include two large 10-meter twin telescopes KECK I and KECK II for optical infrared observations. They were installed in 1994 and 1996 in the USA. They were collected thanks to the help of the W. Keck Foundation, after which they are named. He provided over $140,000 for their construction. These telescopes are the size of an eight-story building and weigh more than 300 tons each, but they work with the highest precision. Principle of operation - the main mirror with a diameter of 10 meters, consisting of 36 hexagonal segments, working as one reflective mirror. These telescopes were installed in one of the best places on Earth for astronomical observations - in Hawaii, on the slope of the extinct volcano Manua Kea 4,200 m high. By 2002, these two telescopes, located at a distance of 85 m from each other, began to operate in the interferometer mode, giving the same angular resolution as an 85-meter telescope.

And in June 2019, NASA plans to launch a unique infrared telescope (JWST) with a 6.5-meter mirror into orbit.

The history of the telescope has come a long way - from Italian glaziers to today's giant satellite telescopes. Modern large observatories have long been computerized. However amateur telescopes and many apparatuses, such as the Hubble, are still based on the principles of operation invented by Galileo.

Irina Kalina, 04/15/2014
Update: Tatyana Sidorova, 02.11.2018
Reprinting without an active link is prohibited!

Amateur astronomers mainly use telescopes of two traditional types for observations. These are telescopes refractors, in which lenses and telescopes are used to build an image - reflectors, where a mirror serves for these purposes.
Sometimes they use to build an image. catadioptric systems, which are combinations of several lenses and mirrors ( mirror-lens telescope).

When we think of observing the starry sky, we imagine something like this. Reality, I say right away, is different from the photo

The main part of any telescope that builds an image is lens. From its characteristics - apertures D, focal length/and focal relation f / D - depends on the range of observations that this telescope allows.

Of course, telescopes with wide apertures (large objective diameters) are preferable because they have a large light-collecting surface, high resolution, and high magnification. However, large aperture telescopes, whatever type they are, are more expensive and bulky.

Collecting and resolving power of telescopes

The most important characteristic of both a telescope and binoculars is aperture(D)- lens diameter.

The aperture defines the dimensions of the collecting surface, the area of ​​which is proportional to the square of the diameter. The larger the collecting surface of the device, the weaker the object it allows to observe. Thus, the maximum stellar magnitude of an object that can be observed in a given telescope depends on the square of the objective diameter.

The next important characteristic of the telescope is resolution, i.e., the ability to distinguish the smallest formations on the disks of planets or double stars.

If the diameter of the lens is measured in millimeters, then the resolution, expressed in seconds of arc, is determined by the value of 138/D.

For telephoto lenses with a focal ratio greater than f/12*, the resolution is slightly higher and is determined by the formula 116/D.

The slightly lower resolution of reflectors and catadioptric telescopes compared to refracting telescopes with the same objective diameter is partly due to the shielding of the central part of the light beam that has passed through the objective. Image quality, especially with reflecting telescopes, can also be severely affected by air currents that occur in the telescope tube.

Telescopes refractors

The lens of a refractor telescope is an achromatic system glued together from several lenses, which collects rays of different wavelengths into one focus.

Usually the focal ratios of amateur refractors are less than f/10 or f/12, since shorter focal length achromatic lenses are very expensive. Therefore, refractors are best used for observations that require large focal ratios, fairly high magnifications, and a limited field of view.

For serious observations it is necessary to use telescopes with an aperture of at least 75 mm.

Of course, it is possible to carry out observations with telescopes with smaller apertures, however, it should be remembered, especially for beginners, that such observations are associated with great difficulties; for this reason, observations with good binoculars may be more effective than with a telescope with a small aperture.

Unlike telescopes of other types, refractors do not have losses due to partial screening of the light beam by intermediate mirrors; nevertheless, refractors with objectives less than 100 mm in diameter are usually used for observations.

Less common are large refractors with apertures over 150 mm, since they are quite expensive and bulky.

Telescopes reflectors

Most amateur reflecting telescopes have f/6 to f/8 focal ratios; compared to refractors, they are more convenient for observations that require a wider field of view and lower magnification.

Reflecting telescopes come in different types. In the practice of amateur observations, two types of reflectors are most often used: Newtonian systems and Cassegrain systems.

In a Newtonian telescope, the secondary mirror is flat, so the focal length and focal ratio of the lens are constant. In a Cassegrain telescope, the secondary mirror is convex, which significantly increases the overall focal length of the telescope and thereby changes its effective focal ratio. For this reason, Cassegrain reflectors find use in observations of the same type as refracting telescopes.

The biggest advantage of reflectors is their low cost. For the same aperture, they are significantly cheaper than any other type of telescope. In addition, you can make the necessary mirror for the reflector lens on your own or, in extreme cases, just buy it, and the tube of such a telescope is easy to assemble at home.

Almost all amateur telescopes with a large collecting surface (objective diameters over 200 mm) are reflectors. The minimum objective diameter of reflectors commonly used for general observations is about 150 mm; such a reflector is no more expensive than a refractor with a lens diameter of 75 mm. Since the reflector has a large collecting surface, fainter objects can be observed through it, but it is not as compact as a refractor.

Smaller reflectors with small focal ratios are intermediate in their characteristics between binoculars and conventional reflectors; besides, they are quite compact.

However, reflectors also have disadvantages. The most significant of them is the need from time to time to update reflective coatings and align optical elements. In the absence of expensive optical glass that hermetically closes the reflector tube, it is necessary to cover each telescope mirror with a lid or cover to prevent dust from penetrating.

When observing, the eyepiece in a Newtonian telescope can be in an uncomfortable position; to avoid this, it is necessary to provide for the possibility of rotating the telescope tube.

If the reflector tube is not hermetically sealed with an optical window, then cold outside air, penetrating into it, creates air currents degrading the image. Very effective tool This disadvantage can be overcome by the use of large heat-insulating pipes, but more often, "pipes" of a skeletal structure are used for this purpose.

Unfortunately, in the latter case, there are other problems associated with warm air flows from the observer himself (so try to wear more heat-insulating clothing when observing!). In addition, this increases the dew on the optical elements. therefore great importance acquires the correct design of the observatory itself.

catadioptric system of telescopes ( mirror-lens telescope)

Among catadioptric telescopes, the most widely used are telescopes Maksutov system and Schmidt-Cassegrain system.

At a given focal length, they are more portable and easy to observe, especially when paired with a variety of devices that track complex movement. celestial bodies. Naturally, such telescopes are much more expensive than both refractors and reflectors of the same size.

Catadioptric telescopes have large focal ratios: f/10, f/12 and even f/15, so they can be used to perform the same tasks as refractors and Cassegrain reflectors.

How to test a telescope before buying

A number of studies of the quality of telescope optics can be carried out independently, but it should be remembered that ideal optical systems do not exist. Any optical system distorts images, such distortions are called aberrations.

In the manufacture of a telescope, aberrations are sought to be minimized. Specific requirements for the amount of allowable aberrations depend on the nature of the research for which this telescope is intended. For example, when studying planets and photographing celestial objects, the requirements for the value of permissible aberrations are higher than when observing.

Chromatic aberration, characteristic to some extent for, refractors and telescopes of some other types, is expressed in the coloring of the image of celestial bodies. It is especially noticeable at sharp boundaries between light and dark areas, for example, on the limbs of the Moon, etc. Reflecting telescopes do not create this type of aberration.

Availability distortion(distortions in the image of the relative position of the stars) can be checked by observing the image of a straight line or a rectangular brickwork in the wall of a house.

Check how your telescope is imaging a point source. If possible, this is best done at night, examining the image of the stars. Such checks can be carried out during the day, observing "artificial stars" ( sunlight reflected by a distant hot air balloon) or any other point light source.

Yes, although it sounds trite, it would not be out of place to remind you that a telescope is an accurate and very sensitive instrument. Check it carefully before buying, disappointment from a low-quality "toy" will discourage all the desire to study the starry sky

In a good telescope, the image of a star is exactly in focus and has the shape of a perfectly round diffraction disk. These images should be in the shape of a perfect circle, not only in focus, but also out of it. Their elongation indicates the presence astigmatism or deformation of the optical elements of the telescope, which may occur due to improper mounting.

The curvature of the field is indicated by the defocusing of the image of the star as it moves from the center to the edge of the telescope's field of view. The curvature of the field is inherent in most telescopes, but this defect mainly affects photographic observations. Another aberration, coma, appears as the image of a star is pulled out (it takes the form of a comet) at the edge of the field of view. Coma is also present in most telescopes, but is more noticeable in reflectors than in refractors.

Inspections of the mechanical components of telescopes and their mounting are mainly general character. For good work, it is necessary to achieve structural rigidity of both the telescope tube itself and its mount. This is best achieved by rigidly mounting the axes of the telescope - each mounted on two fairly spaced supports.

The rotation around the axes must be smooth, and on equatorial installations, both axes must be provided with locking screws. All drives, the focusing frame of the eyepieces and other mechanisms for adjusting the telescope must operate without play.

Today there are many types of telescopes, but few people know what exactly Newton's reflector- not only one of the most common designs, but also one of the most important in historical terms. It was thanks to Newton's reflector that the most important discoveries were made, and indeed astronomy as a science received a powerful impetus to development.

The Newtonian reflector is a reflex telescope by design, that is, the role of the lens in it is performed by a concave mirror. This gives several advantages at once, if we compare such a design with another - a telescope - a refractor, that is, a lens:

• A mirror is much easier to make than a lens, especially since a quality lens objective requires several high quality lenses. You only need one mirror.
• The requirements for glass for a mirror are much lower - the main thing is that it can withstand mechanical loads from its weight and temperature fluctuations. The lens requires high-quality optical glass, without any defects. For a mirror, the transparency of glass, the presence of small defects in its thickness, does not matter.
• With an equal objective diameter, the Newtonian reflector is much more compact than the refractor. For example, a refractor tube with a 150 mm lens would be over 2 meters long, and would be very expensive, not to mention the astronomical cost of such a lens and a powerful mount. A reflector of this diameter is half as long, much smaller, and the cost of a mirror is quite affordable.
• A mirror telescope gives the best image, because in the refractor there is a refraction of light, and in the reflector it is only a reflection. Therefore, the reflector is practically free from many aberrations, for example, chromatic ones - when a colored border appears around the object, and gives a sharper and better image.
• A mirror can reflect light of almost any length, including ultraviolet, which turns out to be important for observations and photography. In a refractor, light passes through a lens, and most of the spectrum is simply lost, including ultraviolet.
• Such a telescope has a large luminosity, which allows you to take clearer and better photographs.
• Due to the design of the Newton reflector, the eyepiece is located on the side, which makes it possible to carry out observations with great convenience. The refractor can be equipped with an inverting prism, but this is an extra obstacle in the way of light, increasing its losses, and this convenience is relative.
• Structurally, the telescope consists of a main mirror of a spherical or parabolic shape, and a secondary flat mirror, which simply brings the focused beam out of the tube where the eyepiece is located for observation.

The main mirror is located on a special platform equipped with adjustment screws to adjust its inclination. The secondary flat mirror is located on braces near the front end of the tube. Thus, only two reflections occur in the telescope.

The eyepiece is equipped with a focuser for smooth adjustment of sharpness.

The Newtonian reflector is a fairly cheap telescope compared to a refractor of the same diameter as the lens. The difference in price can reach several times, and in larger models there are no analogues at all. For example, models with a lens diameter of 50-80 mm can be considered the most popular refractors, with a diameter of 90 mm they have a rather significant price.

At the same time, a Newtonian reflector with a mirror diameter of 110 - 150 mm is quite accessible to almost any astronomy enthusiast. Many amateurs also have 200 mm models in their arsenal, which already belong to the professional class. A refractor of this diameter can only be found in an observatory, they are not for sale.

The history of the appearance of the Newton reflector

As the name implies, a telescope of this design was first created by the famous English scientist Isaac Newton, known for his work in mathematics, physics, astronomy, and other sciences. Created but not invented. The idea of ​​such a design belongs to the Scottish scientist - mathematician and astronomer James Gregory, who proposed it in 1663, but did not translate it into a real telescope.

Newton created the first telescope of this design in 1668, but it was unsuccessful. The second model was better and produced an excellent image at 40x magnification.

This was a big breakthrough in astronomy, especially considering that at that time they used refractors - lens telescopes of a primitive design, or even spyglasses. Of course, such tools did not give a high-quality image, and their magnification was small, although many discoveries were made with them.

Be that as it may, in 1671-1672, Newton demonstrated his telescope in front of the king himself and in the Royal Society, which caused a lot of enthusiasm. Newton became famous and was made a Fellow of the Royal Society. Subsequently, the reflecting telescope became the main astronomical instrument and made it possible to make many important discoveries.

Since then, little has changed, although many other telescope designs have appeared, including reflectors. However, the Newtonian reflector, as the simplest and at the same time effective tool, enjoys the well-deserved love of amateur astronomers around the world, and many have designed their first DIY Newton's reflector.

What is better to observe in a Newtonian reflector

Almost everything can be observed in a telescope of this design, but it will be inconvenient for ground-based observations, as it gives an inverted image - for astronomical purposes this is completely irrelevant.

Thanks to large diameter mirrors compared to refractors and less loss of light, the reflector allows you to better see faint objects - nebulae, galaxies, planets. Also for these reasons, it is more effective when photographing.

Of course, the moon can be perfectly observed through a reflector, and it will give excellent detail on its surface.

How to make a Newton's reflector with your own hands

Now Newton's reflector can be easily bought in a store, and for relatively little money you can get a very different configuration that will allow you to see many space objects.

However, if desired and perseverance, you can make a Newton's reflector with your own hands. This is, of course, a painstaking matter, but you can get at your disposal a fairly powerful telescope, the cost of which in the store is tens, or even hundreds of thousands of rubles. For example, quite successfully, with some experience, amateurs created 200 and 250-mm telescopes for home observatories.

Creating high-quality optics and mechanics requires not only materials, but also knowledge. Therefore, for those who wish to make a Newton's reflector on their own, we recommend the book by Navashin M.S. "Telescope of an amateur astronomer" and the book by L.L. Sikoruk "Telescopes for Astronomy Lovers". In them you can find not only a lot of theory, but also practical step by step instructions to build a telescope. By the way, in the book by Sikoruk L.L. other, more complex systems are also considered, which can also be created independently.

Why is it needed now, when you can buy everything in the store? The reasons can be different - from simple savings to purely practical interest. In the end, a telescope created by one's own hands, according to one's own requirements, may turn out to be no worse than a purchased one, and the acquired skills will definitely not be superfluous.

Where to buy Newton's reflector

It is not difficult to buy a Newton reflector now. This is a very popular design, which is produced in many variants by almost all telescope manufacturers. In cities, in optics stores, you can certainly find such models in abundance.

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You can buy a Newton's reflector and through the Internet. Here are models of this design of almost any size and any manufacturer. It will not be a problem to choose the right model according to its characteristics or price, and you can order it directly on the website.