Traditional Culture Encyclopedia - Photography and portraiture - Who can tell me the basics of astronomical telescopes?
Who can tell me the basics of astronomical telescopes?
An astronomical telescope costing 1,000-2,000 yuan is enough for home use. Generally available in photographic equipment markets. There are two types: refractive type and direct type
Basic knowledge
According to the different structures of the objective lenses, astronomical telescopes can be roughly divided into three categories: those using lenses as objective lenses are called refracting telescopes ; A telescope that uses a reflector as the objective lens is called a reflecting telescope; a telescope that includes both a lens and a reflector is called a catadioptric telescope. Some astronomy enthusiasts often buy a lens, thinking that this solves the problem of the telescope's objective lens. In fact, imaging with one lens will produce aberration. Nowadays, the objective lenses of regular refracting astronomical telescopes are mostly composed of 2 to 4 lenses. In contrast, refracting astronomical telescopes are more versatile and easy to use, making them more suitable for popularizing astronomy.
The optical paths of reflecting telescopes can be divided into Newtonian systems and Cassegrain systems. Generally speaking, for the popularization of astronomy, especially for amateurs with little observation experience, Newtonian reflecting telescopes are inconvenient to use, and their objective lenses need to be frequently coated, making maintenance troublesome. Catadioptric telescopes are composed of lenses and reflectors. Light from celestial bodies is subject to refraction and reflection. This type of telescope has the advantages of strong light power, large field of view and the ability to eliminate several major aberrations. This type of telescope is divided into Schmidt system, Maksutov system and Schmidt-Cassegrain system. Based on our many years of practical experience, the 120-refraction astronomical telescope produced by the Nanjing Astronomical Instrument Factory of the Chinese Academy of Sciences is a convenient and practical instrument for the popularization of astronomy and the majority of astronomy enthusiasts.
Optical performance of telescopes
Among the objects of astronomical observation, some celestial objects have viewing surfaces, and some have no discernible viewing surfaces; some celestial objects have extremely strong light, and some have They are extremely weak; some emit light on their own and some reflect light. Observers should choose different telescopes or use different methods to conduct observations according to the purpose of observation; generally speaking, popular astronomical observations are mostly comprehensive, and "one mirror has multiple uses" should be considered. When choosing an astronomical telescope, you must fully understand its basic optical properties.
Aperture refers to the effective diameter of the objective lens, often represented by D;
Relative aperture refers to the ratio of the effective diameter of the objective lens to its focal length, also known as focal ratio, often represented by A; That is A=D/F.
Generally speaking, the relative aperture of refracting telescopes is relatively small, usually between 1/15 and 1/20, while the relative aperture of reflecting telescopes is relatively large, usually between 1/3.5 and 1/5. When observing a celestial body with a certain viewing surface, the line size of its viewing surface is proportional to F, and its area is proportional to F2. The luminosity of the image is directly proportional to the amount of light collected, that is, directly proportional to D2, and inversely proportional to the area of ??the image, that is, inversely proportional to F2.
Magnification refers to the physical quantity of a visual telescope, that is, the magnification of the angle. It is equal to the ratio of the focal length of the objective lens to the focal length of the eyepiece.
When many people mention astronomical telescopes, the first thing they consider is the magnification. In fact, astronomical telescopes are different from microscopes. In addition to the quality of the instrument, the effectiveness of ground-based astronomical observations is also affected by the clarity and quietness of the earth's atmosphere, the environment of the observation site, and other factors. Moreover, an astronomical telescope has several eyepieces with different focal lengths, that is, several different magnifications are available. When observing, it is never the maximum magnification that is optimal, but the clearest observation target.
The resolution angle refers to the minimum angular distance that the telescope can resolve. During visual observation, the resolution angle of the telescope = 140 (arc seconds)/D (mm), and D is the effective diameter of the objective lens.
The field of view refers to the angular diameter of the starry sky seen by an astronomical telescope.
Penetrating power means that on a clear night, the faintest star magnitude can be seen by a telescope in the zenith direction. Penetrating power is mainly related to the effective aperture of the telescope.
For example, the optical performance of the 120 catadioptric astronomical telescope produced by Nanjing Astronomical Instruments is: the effective aperture of the main mirror is 120mm, the focal length is 1500mm, the relative aperture is 1/12.5, and the eyepiece magnification is: 37.5 times, 60 times, 100 times, 200 times, the theoretical resolution angle is 1"-2", the visual limit magnitude is 12th magnitude, and the field of view is less than 10. Its finder mirror objective lens has an effective aperture of 35mm, a focal length of 175mm, a magnification of 7 times, and a field of view of 500.
Eyepieces of astronomical telescopes
When people understand the basic optical properties of astronomical telescopes, some people often only pay attention to the objective lens and ignore the eyepiece, which is one of the terminal equipment of the telescope. As a result, no matter how good the telescope is, it often cannot give full play to its capabilities and can only look at the sky and sigh.
The eyepiece of an astronomical telescope has two main functions: first, to magnify the image formed by the objective lens, which is very important for observing celestial bodies and close double stars; second, to make the outgoing The beam is parallel light, making the observation comfortable and labor-saving for the observer. There are many types of eyepieces, the more commonly used ones are: Huygens eyepieces, represented by the letter H, MH or HM represent improved types of Huygens eyepieces. These eyepieces are suitable for low or medium magnification observations. Ramsden eyepieces, represented by the letter R, are suitable for use as eyepieces equipped with crosshairs or rulers for measurement observation at low or medium magnifications.
The Kellner eyepiece, represented by the letter K, is an improved version of the Ransten eyepiece, which eliminates the chromatic aberration of the Ransten eyepiece. This eyepiece has a large field of view and is often used for low-magnification observations, such as comets or large-area celestial bodies. . Steinheier's monocentric eyepieces, Chase's distortion-free eyepieces, Abbe's distortion-free eyepieces, and Hick's distortion-free eyepieces are all used for high-magnification observations, such as the observation of details on the planet or lunar surface.
An astronomical telescope should be equipped with a variety of eyepieces, so as to facilitate different observations and maximize the emission. play its due role. I once saw a situation where a department ordered a better astronomical telescope from abroad, but it only had two eyepieces. But the manual states that it has a variety of eyepieces. Why are there only two? The seller said that the buyer specified this when ordering. This is a lesson. Therefore, when ordering an astronomical telescope, you must do thorough research in advance, have complete and reliable information, have an expert check it, and carefully review the ordering procedures.
Star finder mirror and guide mirror
The main mirror of the astronomical telescope plays the leading role in observation. However, many astronomical observations cannot be completed successfully with the primary mirror alone. It also needs an assistant, which is a finderscope or guidescope.
In order to quickly search for the celestial objects to be observed, a small astronomical telescope is often attached next to the main mirror, which is a finderscope. Most finderscopes use refracting astronomical telescopes. Its optical axis is parallel to the optical axis of the primary mirror, so that it can maintain the same target as the primary mirror. The diameter of the finder scope objective lens is generally about 5 to 10 centimeters, the field of view is about 30 to 50, the magnification is about 7 to 20 times, and the focal plane is equipped with a reticle for calibration. When observing, first use the finderscope to find the celestial object to be observed, and adjust the celestial object to the center of the field of view. At this time, the celestial body is naturally in the center of the primary mirror's field of view.
When the primary mirror performs long-term observations, in order to promptly correct tracking errors, a monitoring station is set up next to the primary mirror. A telescope is called a guidescope. Telescopes used for popular astronomy also use guide mirrors instead of guide mirrors. Telescope installation and tracking. An ideal astronomical telescope should not only have an excellent optical system, but also must solve a series of mechanical structural problems. For example, how to set up the lens tube? In order to observe any celestial object on the horizon, according to the choice of the axis direction, astronomical telescope devices are usually divided into two categories: horizon devices and equatorial devices. In the horizon device, the mirror is the horizon longitude of the celestial body. When it changes along the horizontal axis, it represents the horizon latitude of the celestial body. Due to the diurnal apparent motion of the celestial sphere, both quantities of the celestial body in the horizon coordinates change with time and represent only the instantaneous position. Therefore, generally speaking, horizontal devices are not convenient for long-term continuous observation.
The equatorial device solves this problem. One of its axes is parallel to the celestial axis and is called the polar axis. The other axis is perpendicular to the polar axis and is called the declination axis. This is the change in diagonal angle when the tube rotates around the polar axis, and the change in declination when rotating around the declination axis. The declination of celestial bodies does not change with the daily movement and is a constant. Therefore, as long as the lens tube follows the movement of the celestial body around the pole, the purpose of keeping the celestial body within the field of view can be achieved. This is the basic principle of tracking celestial objects. Apparently this is all about overcoming the relative position changes caused by the Earth's rotation. The earth rotates from west to east at a rate of 10 per 4 minutes, and the tracking celestial bodies should also move around the polar axis from east to west at a constant speed of 10 per 4 minutes. How to make the lens barrel rotate like this? The mechanical system that drives the tracking device is called a rotary clock. Before this century, the power of the rotary clock was provided by a chain-type weight or spring, and the speed of the rotary clock was controlled by a centrifugal governor. Nowadays, the power of the rotary clock is driven by a motor, and the speed is controlled by an astronomical clock or a radio oscillator. The guide star is to compensate for the error problem in tracking.
It can be seen that for the popularization of astronomy, it is best to use an astronomical telescope with an equatorial device that can track celestial bodies.
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