Features of reflector telescopes

It is customary to call a reflector any device whose main function is to reflect... Thus, a reflector telescope was created using this optical phenomenon. Instead of a lens, a concave mirror is located in the objective of the device, which reflects and directs light beams into the eyepiece for viewing or photographing an image. Let's consider the main distinguishing characteristics of a reflector telescope.

A reflector telescope differs from another type of refractor telescope in that a concave mirror made of metal or glass is installed in it instead of a lens system. Such devices are often called just that - "mirror" telescope.

It is quite easy to distinguish a reflex telescope from a refractory one, even without experience in astronomy. The scheme of the second is pretty simple. This is a tube, the diameter of which depends on the diameter of the objective lens located at the end that is facing the observed object. At the other end of the tube there is an eyepiece - a lens of a smaller diameter, through which observation is carried out. The length of the tube of such a device is determined by the focal length of the lens and the strength of the material from which it can be made.

A telescope with a concave mirror looks different, since it has a completely different principle of operation and structure. At the end of the pipe facing the sky, such a device, in general, may not have anything, since the mirror is fixed at the other end.But the eyepiece, as a rule, is on the side at the top of the tube. The path of the rays, in contrast to the refractor, is to one degree or another blocked by a prism or flat mirror located along the central axis of the tube, in which light is collected in order to be reflected into the eyepiece. The structure of the reflector does not require the obligatory use of a pipe, and therefore is devoid of those restrictions that arise in refractors.... All modern large telescopes, including space ones, are arranged according to the following scheme: the tube in them is replaced by a lightweight mesh structure, the purpose of which is to hold all the elements of the optical system.

Amateur astronomers have successfully used both types of telescopes, and they both have their advantages and disadvantages, caused in one case by the refraction of the light flux passing through the lens, in the other - by its reflection from the surface, which can have different curvatures. For observations related to travel and movement of the device, it is better to use a refractor, its design is stronger. Transportation of the reflector is undesirable, as it can cause displacement of structural elements relative to the center line, after which it will be necessary to adjust their position using screws - alignment. Such a telescope can be placed in an amateur observatory.

The use of a concave mirror as a lens is the result of scientific research aimed at reducing distortion caused by lenses (chromatic and spherical aberrations). Research in this direction was carried out in many European countries, English scientists were especially successful in them. In 1663, James Gregory was the first to propose to use a reflective concave mirror instead of a refractive lens (apparently, he invented the first reflector telescope), in 1673 the famous Robert Hooke embodied the described system of an optical device.

However, the great Isaac Newton first created a working telescope with a mirror lens in 1668.

The path of the reflectors was not easy; lens devices, being improved at the same time, gave a clearer and brighter image. Scientists of continental Europe (Germans, French, Italians) made a significant contribution to their development. It seemed that the reflector would remain at the level of an experimental device.

The search went in the direction of improving the coating and the manufacture of mirrors. Subsequently, in order to reduce distortions, various innovations were repeatedly introduced into the system proposed by Newton, which led to the appearance of fundamentally different schemes of reflector telescopes, including hybrid versions, when lenses and mirrors were used in one product. The emergence of new materials and technologies made it possible to create more and more perfect systems, and the absence of the need for a bulky tube in the design of the telescope made it possible to multiply its efficiency.

All reflectors have one thing in common - the use of a concave mirror as a lens... But the further course of the rays collected by the mirror was proposed to be directed into the eyepiece in various ways.

The reflector system developed by Isaac Newton is considered classic. The main mirror has no holes and is relatively easy to manufacture. A flat mirror located near its focus reflects the light flux perpendicular to the center line. The eyepiece is located on the side.

The scheme of Newton's telescope is the simplest in execution and is widely used among amateur astronomers who make their own observation equipment. And companies that produce equipment for amateur astronomy produce such devices in large quantities.

The scheme of a mirror telescope proposed in 1663 turned out to be very successful, since gives a direct image and can be used not only for astronomical observations, but also in terrestrial conditions. A hole is made in the center of the concave mirror, the light reflected from it is directed into the hole by a second, also concave mirror, the eyepiece is placed along the centerline of the telescope, like a refractor or a conventional telescope.

The scheme, developed and implemented by Laurent Cassegrain in the 1770s, resembles Gregory's scheme. The concave mirror also has a hole in the central part. The devices differ in the shape of the second mirror - in the system under consideration it is convex. Telescopes built according to this scheme, with characteristics similar to those of Gregory's devices, are much shorter. The Cassegrain system, improved by the Soviet astronomer Dmitry Maksutov, is now used all over the world to create amateur reflectors.

Another modification of the Cassegrain telescope was the Ritchie-Chretien system developed in the 1920s. Thanks to a different form of mirrors, it was possible to obtain a larger field of view, which turned out to be convenient for observing moving objects (asteroids, comets, planets). And also in this system it was possible to reduce some distortions.

Several attempts have been made to use a concave mirror without a reflector blocking the luminous flux. In the early 70s of the 17th century, William Herschel designed such a reflector telescope, the eyepiece of which did not obstruct the main mirror in any way. This made it possible to significantly increase the power of the device, but gave rise to strong distortions in the form of a coma. In the 1760s, a similar design was developed and implemented by M.V. Lomonosov. At present, devices with such an optical scheme are used for special observations; they are not widely used in amateur astronomy, due to the complexity of the device and adjustment.

The Dietrich Korsch system was developed in the 1970s. It is distinguished by the presence of not two, but three mirrors, which allows you to correct most of the distortions.

The devices of this system are widely used for the manufacture of various optical devices - from binoculars and monoculars to amateur telescopes. Their main advantage is a significant reduction in the length of the device while maintaining the focal length. The mirrors are positioned at an angle to the optical axis without blocking each other.

The Cassegrain system, improved at the beginning of the 20th century by Bernhard Schmidt, became widespread. This is a hybrid scheme, in which, in addition to a concave mirror, a lens objective is used.

In the 20th century, reflector telescopes have firmly ousted refractors from all important astronomical observatories. Along with the development of manufacturing technologies, the diameter of the mirrors installed in telescopes began to grow.

In 1917, the world's largest reflector became the observatory in the United States (Washington state), its mirror reached 100 inches in diameter (2.5 meters). After the Second World War, a device with a 5-meter mirror was manufactured, also installed in California.

The largest azimuth telescope in the Old World remains the Large Azimuth Telescope, created in the USSR in the mid-70s of the last century, mounted in the Karachay-Cherkess Republic at a high-altitude observatory.

The world's largest modern telescope with a solid mirror is installed in the state of Arizona, USA. This is a Large Binocular Telescope. It is equipped with two identical mirrors with a diameter of 8.4 meters. The device was built in 2005.

The largest today are devices with prefabricated segment mirrors: the Large Canary Telescope, the Large South African Telescope, and the Hobby-Eberley Telescope (USA).

Using a mirror telescope is not that difficult. However, unlike a refractor, such a device requires very careful handling. Since the reflector tube is always open, dust can enter it. By settling on the surface of the mirror, it very noticeably reduces its reflectivity.

Moving the reflector is also problematic, as structural elements tend to move under the influence of vibrations. Usually manipulations with mirror telescopes end in laborious adjustment (alignment). The telescope can be adjusted using the adjusting screws, the turns of which cause the mirror to shift; it is impossible to do this quickly without appropriate experience.