What process did the development of astronomical observation telescopes go through?

A telescope is an optical instrument made of concave lenses and convex lenses. It is mainly used to observe the characteristics and conditions of distant target objects. The telescope is made by using the imaging principle of a small hole formed by light passing through a concave lens. It can magnify the scene of an object far away, so that people can clearly observe its specific shape and even the smaller details and shadows of the object. It is more clear and careful, so in ancient times people also called it "clairvoyance". In 1609, Galileo Galilei, a Florentine in Italy, invented a 40-fold double-mirror telescope based on the original telescope and used it for astronomical research. This was the first telescope in history to be used in astronomical research. Practical telescope for scientific research. As the efficiency of this telescope has been greatly increased, people can observe objects in the sky that cannot be seen and distinguished by the naked eye, so this telescope has slowly evolved into an indispensable tool in astronomical observation work.

With the changes of the times, the functions and application methods of telescopes have also undergone great changes. People divide these astronomical telescopes into refractor telescopes, reflection telescopes and catadioptric telescopes according to their different functions. . The purpose of the telescope has also changed from a single use to a variety of uses, and it is widely used in military, high-tech biological research, etc.

Telescopes that use lenses as primary mirrors are called refracting telescopes. In the historical evolution, telescopes made with concave lenses as eyepieces are called Galilean telescopes; telescopes made with convex lenses as eyepieces are called For the Kepler telescope. Because the chromatic aberration and spherical aberration of single-lens objects are quite serious, modern refracting telescopes are made of two or more lens groups. Among them, telescopes made of double lenses are the most common and widely used. This kind of telescope is composed of a convex lens made of a piece of crown glass and a concave lens made of flint glass that are very close to each other. After the combination of the lens and the objective lens, the wavelength of the transmitted scene can be completely eliminated, and the chromatic aberration of the scene position can also be relatively weakened.

The volume and field of view of the double-lens objective lens are relatively small. The relative aperture of a double lens objective is small, generally between 1/15 and 1/20, rarely larger than 1/7, and the usable field of view is not large. Telescopes with a diameter less than 8 cm that can be used to glue two lenses together are called double-lens objectives. To increase the use of relative aperture and field of view, a multi-lens objective lens set can be used.

The Galilean telescope has the good characteristics of simple structure, low light energy loss, short barrel, light weight to carry, and relatively positive field of view imaging. However, its expansion factor is small and the observation field of view is small, so it is generally used as a telescope. See up close with a theater glass and toy telescope. When using the Kepler telescope, a prism group or lens group needs to be added behind the objective lens to convert the image so that the scenery observed by the eyes is an erect image. However, the Kepler telescope uses a binocular structure with a wide front and narrow back. This structure can form a double right-angle prism erect imaging system. This system can correct the inverted imaging system formed in the original telescope structure; it can also Reduce the size and weight of the telescope to a great extent. The disadvantage is that the lens erecting system requires a complex set of lenses to invert the image, which is relatively expensive. However, the 20×50 three-section telescopic classical monocular telescope invented by the Russians greatly avoids this situation. It uses a sophisticated lens erect design system to image things.

Refracting telescopes used by modern people generally adopt the Kepler structure. Since the imaging quality of refracting telescopes is better than that of reflecting telescopes, the field of view is large, it is easy to use, and it is easy to maintain. Small and medium-sized astronomical telescopes and many special instruments mostly use refractive systems. However, large refracting telescopes are much more difficult to manufacture than reflecting telescopes because of the large smelting requirements. It is very difficult to obtain a high-quality lens with a large aperture, and there is a problem with the absorption of light by glass, so large-aperture telescopes are reflective.

History

In 1611, the German astronomer Kepler first used two biconvex lenses as the objective lens and eyepiece respectively, which significantly improved the magnification, so later generations will This optical system is called a Keplerian telescope. Nowadays, people still use these two types of refracting telescopes, and astronomical telescopes use the Keplerian type. It should be pointed out that because the telescopes at that time used a single lens as the objective lens, there was serious chromatic aberration. In order to obtain better observation results, a lens with a very small curvature was needed, which would inevitably lead to the lengthening of the lens body. Since then, astronomers have been trying to develop longer telescopes, but almost always failed.

In 1757, Dulong laid the foundation for the achromatic theory through research on the refraction and dispersion phenomena of glass and water, and created achromatic lenses using crown glass and flint glass. Since then, achromatic refractor telescopes have completely replaced long-bodied telescopes. However, due to the limitations of technological development at that time, it was difficult to cast larger flint glass. When achromatic telescopes were first studied, the largest lens that could be ground was only 10 centimeters.

At the end of the 19th century, due to great progress in manufacturing technology, there was a scientific boom in manufacturing large-aperture refracting telescopes.

Seven of the eight existing refractor telescopes over 70 cm in the world were built between 1885 and 1897. The most representative of them are the 102-cm-diameter Yekaishi Telescope built in 1897 and the 1886 The 91 cm LIKE telescope.

Refracting telescopes are most suitable for measuring celestial bodies because they have long focal lengths, large film scales, and are not sensitive to tube bending. But it always has residual chromatic aberration, and at the same time it absorbs radiation in the ultraviolet and infrared bands very strongly. It was also very difficult to cast huge optical glass. By the completion of the Yekaishi Telescope in 1897, the development of refracting telescopes reached its peak. In the next hundred years, no larger refracting telescope appeared. This is mainly because it is technically impossible to cast a large piece of perfect glass to make a lens. At the same time, large-sized lenses will be severely deformed under the action of gravity, resulting in the loss of sharp focus.

A telescope that uses a concave reflector as the objective lens is a reflecting telescope. It can be divided into several types such as Newtonian telescope and Cassegrain telescope. The main advantage of a reflecting telescope is that there is no chromatic aberration. When the objective lens adopts a paraboloid, it can also eliminate spherical aberration. However, in order to reduce the influence of other aberrations, the available field of view is smaller. The materials used to make the reflector only require a small expansion coefficient, low stress and ease of grinding. The polished reflector is usually coated with an aluminum film on the surface. The reflectivity of the aluminum film in the 2000-9000 angstrom range is greater than 80%. Therefore, in addition to the optical band, invisible light bands such as infrared and ultraviolet can also be used to reflect the telescope. Research. The relative aperture of a reflecting telescope can be made larger. The relative aperture of a main-focus reflecting telescope is about 1/5 to 1/2.5, or even larger. Besides, except for Newtonian telescopes, the length of the lens tube is much shorter than the focal length of the system. There are many, and only one surface of the primary mirror needs to be processed, thus greatly reducing the cost and manufacturing difficulty of the telescope. A larger-diameter reflecting telescope can obtain the main focus system (or Newtonian system), Cassegrain system and folding axis system by changing different secondary mirrors. In this way, a single telescope can obtain several different relative apertures and fields of view. At present, apart from reflecting telescopes, there are no other optical telescopes with an aperture of 1.34 meters or more. The main scientific research mission of launching telescopes is to study the physical characteristics of celestial bodies.

History

The world’s first reflecting telescope was born in 1668. Newton tried to grind aspherical lenses several times, but failed repeatedly, so he switched to using a spherical mirror as the primary mirror. He ground a 2.5 cm diameter metal into a concave reflector, and placed a reflector at an angle of 45° in front of the focus of the primary mirror, so that the condensed light reflected by the primary mirror passes through the reflector at an angle of 90°. The angular reflection exits the tube and reaches the eyepiece. This system is called a Newtonian reflecting telescope. Although spherical mirrors will produce certain aberrations, the use of reflective mirrors instead of refractors is a successful turn in science.

In 1663, James Gregory proposed a plan: using a concave mirror as a primary mirror and a secondary mirror respectively, placing the secondary mirror outside the focus of the primary mirror, and at the center of the primary mirror. There is a small hole in the center so that the light is reflected twice by the primary mirror and the secondary mirror and then emitted from the small hole to reach the eyepiece. The purpose of this design is to eliminate spherical aberration and chromatic aberration at the same time, which requires a parabolic primary mirror and an ellipsoidal secondary mirror. The suggestion he made was theoretically correct, but due to the limitations of the manufacturing level at the time, some of the requirements it mentioned were unachievable. Therefore, Gregory was unable to obtain a mirror that was useful to him.

In 1672, the Frenchman Cassegrain proposed a third design for a reflecting telescope. The structure was similar to that of the Gregorian telescope. The difference was that the secondary mirror was advanced before the focus of the primary mirror and was convex. Mirror, this is the most commonly used Cassegrain reflecting telescope. This makes the light reflected by the secondary mirror slightly divergent and reduces the magnification, but it eliminates spherical aberration, so that the focal length of the telescope can be very short.

The primary and secondary mirrors of Cassegrain telescopes can come in many different forms, with different optical properties. Because the Cassegrain telescope has a long focal length and a short lens body, the magnification is also large, and the resulting image is clear. It has both a Cassegrain focus, which can be used to study celestial objects in a small field of view, and a Newtonian focus, which can be used to photograph large areas. celestial body. Therefore, Cassegrain telescopes have been widely used.

Herschel was a master of making reflecting telescopes. He was a musician in his early years. Because of his hobby of astronomy, he began to polish telescopes in 1773. He made hundreds of telescopes in his lifetime. The telescope made by Herschel placed the objective lens obliquely in the lens tube, which caused parallel light to converge on one side of the lens tube after reflection.

In the nearly 200 years after the invention of the reflecting telescope, reflective materials have always been an obstacle to its development: the bronze used for casting mirrors is easy to corrode and has to be polished regularly, which requires a lot of money and time, and is durable. A highly corrosive metal, denser than bronze and very expensive. In 1856, German chemist Justus von Liebig developed a method that could coat glass with a thin layer of silver. After being lightly polished, it could reflect light efficiently. This makes it possible to build better and larger reflecting telescopes.

At the end of 1918, the Hooker telescope built under the leadership of Haier was put into use. Its aperture was 254 cm.

Astronomers used this telescope to reveal for the first time the true size of the Milky Way and our location in it. We are proud that Hubble's theory of cosmic expansion was the result of observations using the Hooker telescope.

In the 20th century, from the end of the 1920s to the 1930s, the success of the Hooker telescope inspired astronomers to build larger reflecting telescopes. In 1948, the United States built a telescope with an aperture of 508 cm. In order to commemorate the outstanding telescope manufacturer Haier, it was named the Haier Telescope. It took more than 20 years from the design to the completion of the Hale Telescope. Although it can see farther and have stronger resolving power than the Hooker Telescope, it has not given mankind an updated understanding of the universe. As Asimov said: "The Hale Telescope, like the Yerkes Telescope half a century ago, seems to herald the end of a particular type of telescope." Later, in 1976, the former Soviet Union built a 600-centimeter telescope, but its role was not as good as the Hale telescope, which once again verified Asimov's words.

Reflecting telescopes have many advantages. For example, they have no chromatic aberration and can record various information about celestial bodies in a wide range of visible light. Compared with refracting telescopes, they are easier to make. But at the same time, it also has many shortcomings. If the aperture is large, the field of view will be relatively small, the clarity and brightness of the image data obtained are not very high, and the objective lens of the refractor needs to be regularly coated.

After World War II, reflecting telescopes developed rapidly in astronomical observations. In 1950, a 5.08-meter-diameter Hale reflecting telescope was installed on Palomar Mountain. In 1969, a 6-meter-diameter reflector was installed on Mount Pastukhov in the northern Caucasus of the former Soviet Union. In 1990, NASA launched the Hubble Space Telescope into orbit. However, due to a mirror failure, the Hubble Telescope did not begin to fully function until astronauts completed space repairs and replaced the lens in 1993. Hubble takes pictures without being affected by the Earth's atmosphere, so the pictures it takes are 10 times clearer than similar telescopes on Earth. In 1993, the United States built the 10-meter Keck Telescope on Mauna Kea, Hawaii. Its mirror is made up of 36 1.8-meter reflectors. In 2001, the European Southern Observatory in Chile completed the development of the "Very Large Telescope" (VLT), which consists of four telescopes with an aperture of 8 meters and has a light-gathering capacity equivalent to that of a 16-meter reflecting telescope. Now, a group of telescopes under construction have begun to attack the white giant brothers on Mauna Kea. These new competitors include the 30-meter California Extremely Large Telescope (CELT), the 20-meter Giant Magellan Telescope (GMT) and the 100-meter Overwhelming Large Telescope (OWL for short). Scientists pointed out that the new telescopes developed can not only take pictures with better quality than Hubble space pictures, but also collect more light. Clearer and more reliable space image data will allow people to better understand the initial state of stars and cosmic gas when galaxies were formed 10 billion years ago, and to clearly observe planets around distant stars.

The spherical reflector in the catadioptric telescope is used for imaging, while the refractor can be used to correct aberrations. At the same time, it can avoid difficult large-scale aspheric surface processing and obtain good image quality. The more widely used one is the Schmidt telescope. It places a Schmidt correction plate at the center of the spherical mirror. One of its surfaces is a flat surface and the other is a slightly deformed aspherical surface, making the central part of the beam slightly convergent and the peripheral part slightly divergent, which just corrects spherical aberration and coma.

There is also a Maksutov telescope, which adds a meniscus lens in front of the spherical reflector. By selecting the appropriate parameters and position of the meniscus lens, spherical aberration and coma can be corrected at the same time. And derivatives of these two telescopes, such as super Schmidt telescope, Baker-Nunn camera, etc. Catadioptric telescopes are characterized by large relative aperture, even greater than 1, strong light power, wide field of view, and excellent image quality. It is suitable for sky survey photography and observation of nebulae, comets, meteors and other celestial bodies. The reflector of the catadioptric telescope is protected by a secondary mirror and is not easily invaded by dust and other pollutants.

History

The world’s first catadioptric telescope appeared in 1814.

In 1931, the German optician Schmidt used an aspherical thin lens similar to a parallel plate as a correction mirror, and cooperated with a spherical reflector to create a lens that could eliminate spherical aberration and off-axis aberration. Schmidt-type catadioptric telescope has strong light power, large field of view and small aberration. It is suitable for taking photos of large areas of the sky. The photo effect of faint nebulae is very outstanding. Today the Schmidt telescope is an important tool for astronomical observation.

In 1940, Maksutov produced a new type of refractory telescope. Maksutov used a meniscus-shaped lens as a correction lens, turning its two surfaces into two spherical surfaces with different curvatures. The difference is not big, but the curvature and thickness are very large.

All its surfaces are spherical, which is easier to grind than the correction plate of the Schmidt telescope. The lens barrel is also shorter, but the field of view is smaller than that of the Schmidt telescope. The clarity and brightness are smaller, but the magnification factor is smaller. Large, and the requirements for glass are also higher.

Refractory telescopes absorb the advantages of refraction and reflection telescopes respectively, so they are very suitable for amateur astronomical observations and are also the best choice for astronomy enthusiasts.