What are optical telescopes and how to choose them?

Many people do not know what optical telescopes are, and therefore cannot figure out how to choose them, how to analyze classifications and schemes. In addition, those keen on astronomical observations will certainly be happy to know what they are for and who invented the first telescopes. It is useful for them to know the world's largest modern telescopes in the optical range.

Optical telescopes are special devices that collect and focus electromagnetic beams in the visible range. They are designed to increase the brightness and the observed angular size of astronomical objects. From the point of view of physics, the purpose of the device is to increase the amount of light coming from a celestial body, or, as experts say, optical penetration.

It is worth considering that such devices are intended not only for direct personal observation of space, but also for photography. Moreover, it is with professionals that the main part of the work is just photography, and only then they study the images obtained by the system. The key characteristics of telescopes are:

• cross section of the lens;

• its focal length;

• focus and field of view of the eyepiece.

The principle of operation of telescopes is directly related to their structure. Inside is a system of lenses or mirrors. Devices with a single optical glass have not been found for a long time.When an astronomer works with his telescope, he changes the parameters of the eyepiece, leaving the lens unchanged. This allows you to change the magnification. The device includes both collecting and diffusing lenses, on the correct selection and use of which the clarity and accuracy of the picture depend.

It is sometimes said that the very first telescope was developed by Galileo. However, it is not. Until now, the exact developer is unknown, and it is unlikely that it will ever be installed. There is a widespread point of view that the decisive step was taken by the glasses maker John Lippersgey. But, most likely, the creation of the telescope took place in several places at once, independently of each other, because at the beginning of the 17th century, the need for it was palpable.

This is indirectly confirmed by reliably known facts. When filing a patent application, it turned out that several devices of the same kind had already been registered. It is believed that the prototype of the telescope was created by Leonardo da Vinci. Galileo's role was that he developed a reflector telescope, and moreover, he was able to raise the magnification from 3 to 32 times in a few samples.

Today, such indicators will be condescendingly perceived even by amateurs of astronomy. But then Galilean telescopes made it possible to make a number of important discoveries, including highlighting stars in the Milky Way and detecting sunspots. It is curious that the very name "telescope" appeared only in 1611, and it was given by the Greek mathematician Dimisianos.

In the 17th-18th centuries, refractor telescopes were still widely used. This is largely due to the high cost and complexity of reflectors. In the middle of the 19th century, silvered glass mirrors were used. In the last century, an important innovation was mainly the use of huge mirrors. Their creation would have been unthinkable without the development of a powerful industrial base.

This type is also called a refractor. The use of several lenses instead of one allows you to weaken the optical imperfections of each separately. The scheme implies the importance of the focal length, which determines the linear dimensions of distant objects in the focal plane. A set of eyepieces is added to each telescope, suitable for specific cases. Along with the usual refractors, there are also those that are designed for photography (they are called astrographs).

This type of telescope is also called a reflector. The mirror is easier to make. It has a concave parabolic design. The curvature is rather small. A small amount of powdered aluminum is applied to the surface.

The use of a mirror device allows you to confidently observe small details of local space objects - planets and their satellites, rings. Reflectors are suitable for studying nebulae, comets and other extended objects. But there are also telescopes with a lens associated with a complex of mirrors and lenses. It is these models that are the most compact.

The size of a telescope is determined by the size of its optical elements. The largest specimens are placed quite predictably where the state of the atmosphere is optimal for observing space. Topping the list of the largest SALT devices in the southern hemisphere, located in the semi-desert region of South Africa. The main mirror alone has a size of 11x9.8 m. It has been used in practical observations since 2005, supplemented with a special digital camera and a multifunctional spectrograph.

Other modern telescopes include the GTC. In domestic literature and sources, it is often called the Large Canary Telescope. It has been used in practice since 2007. In addition to the optical, it can also work with the infrared range. A number of additional devices are used, and the mirror size is 10.4 m.

“European Extra Large Telescope” is a name that speaks for itself. It is not among the working devices, as commissioning is scheduled for 2024. But this is the largest of those telescopes that have already been built, and the size of the main segment mirror is 39.3 m. The object is located in Chile, on Mount Armasones, at an altitude of just over 3 km above sea level.

The largest telescope in Russia is the so-called "Large azimuth telescope" located near the village of Nizhny Arkhyz. The cross-section of the mirror does not exceed 6 m. It should be taken into account right away that the location of the device itself was recognized as unsuccessful and one cannot count on the most effective observations.

The refractor telescope is a classic. The one that is as close as possible to the traditional "spyglass with legs". The refractive scheme is optimal if you plan to track bright objects such as the moon or binary stars. It is also suitable for daytime observations. But a refractor telescope is not very suitable for observing distant faintly luminous objects. Neither high contrast nor ease of maintenance can reconcile with this disadvantage.

The reflectors already mentioned above are divided into simpler and more expensive subgroups. In the second case, the use of a parabolic mirror is provided. At comparable costs, the reflector will have a larger lens section than the refractor. Therefore, the optical performance will be quite high, as well as the concentration of light. It is the reflex scheme that is recommended for observing various objects outside the solar system.

However, a reflector telescope is more massive than a refractor telescope. You will have to look at it from a certain angle, which will be difficult for an inexperienced astronomer to get used to. Catadioptrics are something intermediate between the two main types. They do not need to be systematically maintained.

However, it is hardly reasonable to limit ourselves to the described circumstances. The cross-section of the objective, also known as the aperture, primarily determines the capabilities of the telescope. It is by this parameter that one can judge the ability to demonstrate small details of objects. Concentration of light is much more important than magnification. It is much easier to make the aperture larger than to use a larger mirror, and for private users this solution is pleasantly light and compact.

In most cases, amateur astronomers opt for telescopes with an aperture of 70 to 130 mm. Along with this, they must study the focal length. It is directly logically tied to the aperture of the lens. The longer the focal length, the better the optics increase, but the aperture decreases at the same time. Therefore, they almost always strive for some balance of parameters.

Increasing to a large extent is not always good. And the point is not only that it degrades other parameters of the telescope. Often, this increases the oversensitivity to vibrations, susceptibility to atmospheric distortion, and so on. By the type of installation, azimuth and equatorial telescopes are distinguished. The former rotate along two axes, and the latter only along one axis, which is much more practical.