With the changes of the times, great changes have taken place in the efficacy and application of telescopes. According to the different use effects of these astronomical telescopes, people divide them into refractive telescopes, reflective telescopes and folding reflective telescopes. The use of telescopes has also changed from single to diverse, and they are widely used in military and high-tech biological research.
A telescope with a lens cutting mirror is called a refractive telescope. In the historical evolution, the telescope with concave lens as eyepiece is called galileo telescope. A telescope with a convex lens as an eyepiece is called Kepler telescope. Because the chromatic aberration and spherical aberration of Dan Toujing objects are quite serious, modern refractive telescopes are composed of two or more lens groups. Among them, binoculars made of double lenses are the most widely used. This telescope consists of a convex lens made of crown glass and a concave lens made of flint glass, which are very close together. The combination of these two transparent lenses can completely eliminate the wavelength of the scene and relatively weaken the color difference of the scene position.
The volume and field of view of the double transmission objective lens are relatively small. The relative aperture of a two-lens objective is relatively small, generally between115 and 1/20, and rarely larger than 1/7, and the available field of view is not large. People call a telescope with a double-lens objective less than 8 cm in diameter and capable of sticking two lenses together a double-lens objective. In order to increase the use of relative aperture and field of view, a multi-lens objective group can be used.
Galileo telescope has the advantages of simple structure, less light energy loss, short lens barrel, portability and relatively accurate field of view imaging. However, it has a small expansion factor and a small field of view, and is generally used as a theater mirror and a toy telescope for close viewing. When using Kepler telescope, it is necessary to add a prism group or lens group behind the objective lens to turn the image so that the scene observed by the eyes is positive. The Kepler telescope adopts a double-tube structure with wide front and narrow back, which can form a vertical system with double right-angle prisms. By modifying the original telescope structure, an inverted imaging system can be formed. At the same time, the size and weight of the telescope can be greatly reduced. Its disadvantage is that the lens correction system needs to use a complex set of lenses to invert the imaging image, and the cost is relatively high. The 20×50 three-section telescope invented by the Russians has greatly avoided this situation. It uses an excellent lens image design system to image things.
The refracting telescope used by modern people generally adopts Kepler structure. Because the imaging quality of refractive telescope is better than that of reflective telescope, it has a large field of view and is convenient to use and maintain. Most small and medium-sized astronomical telescopes and many special instruments use refractive systems, but the manufacture of large refractive telescopes is much more difficult than that in reflecting telescope. Because it is very difficult to smelt high-quality large-aperture lenses, and there is a problem of glass absorbing light, large-aperture telescopes all adopt reflection type.
history
16 1 1 year, German astronomer Kepler used two biconvex lenses as objective lens and eyepiece respectively for the first time, which obviously improved the magnification, so later generations called this optical system Kepler telescope. Now people are still using these two kinds of refractive telescopes, and the astronomical telescope adopts Kepler type. It should be pointed out that at that time, because the telescope used a single lens as the objective lens, there was serious chromatic aberration. In order to obtain better observation effect, a lens with small curvature is needed, which will inevitably lead to the lengthening of the mirror body. Since then, astronomers have been trying to develop longer telescopes, but almost all of them have failed.
From 65438 to 0757, Dulong studied the refraction and dispersion phenomena of glass and water, laid the foundation of achromatic theory, and made achromatic lenses with crown glass and flint glass. Since then, the achromatic refraction telescope has completely replaced the long mirror telescope. However, due to the limitation of scientific and technological development at that time, it was difficult to cast large flint glass. When the achromatic telescope was first studied, the largest lens that people could grind was only 10 cm.
At the end of 19, due to the great progress of manufacturing technology, the scientific upsurge of manufacturing large-aperture refractive telescopes followed. Of the eight existing refracting telescopes over 70 cm in the world, seven were built between 1885 and 1897, among which the most representative ones are the Yekeshi telescope with the aperture of 102 cm built in 1897 and the 9/kloc telescope built in 1886.
Refractive telescope has a long focal length, large negative scale and is insensitive to the bending of the lens barrel, so it is most suitable for measuring celestial bodies. But there will always be residual color difference, and at the same time, it absorbs radiation in ultraviolet and infrared bands very strongly. The casting of huge optical glass is also very difficult. When the Yekeshi telescope was built in 1897, the development of refractive telescope reached its peak, and no larger refractive telescope appeared in the next hundred years. This is mainly because it is technically impossible to cast a perfect piece of glass as a lens. At the same time, under the action of gravity, the large-size lens will be seriously deformed, thus losing its sharp focus.
A telescope with concave mirror as the objective lens is a reflective telescope. Can be divided into Newton telescope, seglin telescope and other types. The main advantage of reflective telescope is that there is no chromatic aberration. When the objective lens is a paraboloid, spherical aberration can be eliminated. However, in order to reduce the influence of other aberrations, the available field of view is smaller. The material for making the mirror surface only requires small expansion coefficient, small stress and easy grinding. Generally, polishing mirrors are coated with aluminum film, and the reflectivity of aluminum film is more than 80% in the range of 2000 ~ 9000 angstroms. Therefore, in addition to optical bands, infrared and ultraviolet bands can also be studied by reflecting telescope. The relative aperture of reflecting telescope can be made larger. The relative aperture of the main focal reflecting telescope is about 1/5 ~ 1/2.5, or even larger. Except for Newton telescope, the length of the lens barrel is much shorter than the focal length of the system, and only one surface of the main mirror needs to be machined, which greatly reduces the cost and manufacturing difficulty of the telescope. For the reflective telescope with larger aperture, prime focusing system (or Newton system), Casselin system and folding axis system can be obtained by replacing different secondary mirrors. In this way, the telescope can obtain several different relative apertures and fields of view. At present, apart from the reflective telescope, there is no optical telescope with the aperture above1.34m.. The main scientific research task of launching telescope is to study the physical characteristics of celestial bodies.
history
The first reflecting telescope in the world was born in 1668. Newton used to grind the aspheric mirror several times, but failed repeatedly, so he used the spherical mirror as the main mirror. He grinds out a concave mirror with a diameter of 2.5cm, and places a reflector with an angle of 45o in front of the focus of the main mirror, so that the concentrated light reflected by the main mirror can be reflected from the lens barrel to the eyepiece at an angle of 90o. This system is called Newton reflecting telescope. Although spherical mirror will produce some aberrations, it is a turning point of scientific success to replace refractive mirror with reflector.
1663, James? Gregory is proposing a scheme: concave mirror is used as the primary mirror and the secondary mirror respectively, and the secondary mirror is placed out of the focal point of the primary mirror, and a small hole is left in the center of the primary mirror, so that the light can be reflected twice by the primary mirror and the secondary mirror and then exit from the small hole and 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. His suggestion was correct in theory, but some of the requirements mentioned in it could not be realized due to the limitation of manufacturing level at that time, so Gregory could not get useful mirrors for him.
1672, Frenchman seglin put forward the third design scheme of reflective telescope. The structure is similar to Gregory telescope, except that the secondary mirror is convex in front of the focus of the primary mirror, which is the most commonly used Cassegrain reflective telescope. This makes the light reflected by the secondary mirror slightly divergent, reducing the magnification, but eliminating the spherical aberration, so that the telescope can also make the focal length very short.
The primary mirror and secondary mirror of seglin telescope can have many different forms, and their optical properties are also different. Because of the long focal length, short mirror body and large magnification of seglin telescope, the images obtained are clear; Seglin focus can be used to study celestial bodies with small field of view, while Newton focus can be configured to shoot large-area celestial bodies. Therefore, seglin telescope has been widely used.
Herschel is a master of making reflecting telescope. He was a musician in his early years. Because he loves astronomy, he began to grind telescopes from 1773 and made hundreds of telescopes in his life. In the telescope made by Herschel, the objective lens is obliquely placed in the lens barrel, so that the parallel light is reflected and converged on one side of the lens barrel.
In the nearly 200 years after the invention of reflective telescope, reflective materials have been an obstacle to its development: bronze casting mirrors is easy to corrode and must be polished regularly, which requires a lot of money and time, while metals with good corrosion resistance are denser and more expensive than bronze. 1856 german chemist Eustace? Feng? Justus von Liebig developed a method to coat a thin layer of silver on glass, which can reflect light efficiently after polishing. In this way, it is possible to make a better and larger reflective telescope.
19 18 At the end of this year, the Hooke telescope built by Haier was put into use, with a diameter of 254 cm. Astronomers used this telescope to reveal for the first time the true size of the Milky Way and our position in it. It is proud that Hubble's theory of cosmic expansion is the result of observation with Hooke telescope.
In the late 1920s and 1930s, the success of Hooke Telescope inspired astronomers to build larger reflective telescopes. 1948, the United States built a telescope with a diameter of 508 cm. In order to commemorate Haier, an outstanding telescope manufacturer, it was named Haier Telescope. Haier telescope has been designed and manufactured for more than 20 years. Although it has a farther field of vision and stronger resolution than Hooke telescope, it has not given mankind an updated understanding of the universe. As Asimov said, "Haier telescope, like Yekeshi telescope half a century ago, seems to indicate that a certain type of telescope has almost come to an end." Later 1976, the former Soviet Union built a 600 cm telescope, which was not as good as Haier telescope, and once again verified Asimov's words.
Reflective telescope has many advantages, for example, it has no chromatic aberration, can record all kinds of information of celestial bodies in a wide range of visible light, and is easier to make than refractive telescope. But at the same time, it also has many shortcomings. If the aperture is large, the field of view will be small, and the clarity and brightness of the obtained image data are not very high. The objective lens of the refractor needs to be coated regularly.
After World War II, reflective telescopes developed rapidly in astronomical observation. 1950, a Haier reflective telescope with a diameter of 5.08m was installed on the Paloma Mountain. 1969, a reflector with a diameter of 6 meters was installed on Pastuhov Mountain in the North Caucasus of the former Soviet Union. 1990, NASA put the Hubble Space Telescope into orbit. However, due to mirror failure, the Hubble Space Telescope did not fully play its role until 1993, when astronauts completed space repair and replaced the lens. Hubble telescope is not affected by the earth's atmosphere when taking pictures, so the resolution of the pictures it takes is 10 times higher than that of similar telescopes on earth. 1993, the Keck telescope with a diameter of 10 meter was built in Monaque Mountain, Hawaii, and its mirrors were composed of 36 mirrors with a diameter of 1.8 meter. 200 1 the European southern observatory in Chile has developed the "VLT", which consists of four 8-meter-diameter telescopes, and its light-gathering capacity is equivalent to one 16-meter reflective telescope. Now, a number of telescopes under construction have begun to attack the White Giant brothers on Mount Monaque. These new competitors include the 30-meter Californian Telescope (California? Very big? Telescope (CELT), the 20-meter-diameter Large Magellanic Telescope (Giant? Magellan? Telescope (GMT) and 100 meter aperture telescope (overwhelming? Large size? Telescope (owl for short) Scientists point out that these new telescopes can not only take better photos than Hubble space photos, but also collect more light. More clear and reliable space image data can make people know more about the initial stars and cosmic gases when galaxies formed 65.438+0 billion years ago, and observe the planets around distant stars more clearly.
The spherical mirror in the catadioptric reflecting telescope is used for imaging, while the refractive mirror can be used for correcting aberration. At the same time, it can avoid difficult large aspheric surface processing and obtain good image quality. Schmidt telescope is widely used. Schmidt correction plate is placed in the center of the spherical mirror. One side is flat and the other side is slightly deformed aspheric surface, which makes the central part of the beam converge slightly and the peripheral part diverge slightly, just to correct spherical aberration and coma.
Another is Maksutov telescope, in which a meniscus lens is added in front of the spherical reflector. By choosing appropriate parameters and positions of the meniscus lens, spherical aberration and coma can be corrected at the same time. And the derivatives of these two kinds of telescopes, such as super Schmidt telescope, Baker-Noon camera and so on. Reflective telescope is characterized by large aperture, even greater than 1, strong light, large field of view and excellent imaging quality. Suitable for sky survey photography and observation of nebulae, comets, meteors and other celestial bodies. The mirror of the reflective telescope is protected by the secondary mirror, which is not easy to be attacked by pollutants such as dust.
history
The first folding reflecting telescope in the world appeared in 18 14.
193 1 year, the German optician Schmidt used a thin aspheric lens similar to a parallel plate as a correcting mirror, and cooperated with a spherical reflector to make a Schmidt-type folded reflecting telescope that can eliminate spherical aberration and off-axis aberration. The telescope has strong optical power, large field of view and small aberration, which is suitable for taking large-area photos of the sky area, and the effect of shooting dim nebulae is very outstanding. Today, the Schmidt telescope is an important tool for astronomical observation.
1940, Maksutov made a new type of folding reflecting telescope. Maksutov uses a meniscus lens as a correction lens, making its two surfaces into two spherical surfaces with different curvatures, with little difference, but great curvature and thickness. All its surfaces are spherical, which is easier to polish than the correction plate of Schmidt telescope, and its lens barrel is shorter, but its field of view is smaller than that of Schmidt telescope, and its clarity and brightness are smaller, but its magnification is larger, which requires higher glass.
The catadioptric telescope absorbs the advantages of refractive telescope and reflective telescope respectively, which is very suitable for amateur astronomical observation and the best choice for astronomers.