1.1 How it works
Astronomical telescopes are amazing instruments that can make distant objects appear close. In order to better understand the working principle of astronomical telescopes, let us first consider such a question: why can't we see distant objects with the naked eye? For example, why can't you see a coin at 50 meters with the naked eye? The answer is simple: because distant objects don't take up enough space on the retina. If you have large eyes that focus more light from a distant object and make a bright image on your retina, then you can see that object. Two optics of a telescope can help you turn this assumption into reality:
The objective lens, which can focus the light emitted by the distant target into the focal point (image at the focal point);
Eyepieces, which magnify the image at the focal point of the objective so that it appears on your retina. This is the same principle as a magnifying glass, it magnifies small objects and displays them on your retina, so that small objects appear larger.
The main components of an astronomical telescope are: the main barrel, the objective lens, and the eyepiece. The function of the main lens barrel is: to fix the objective lens to keep it at a proper distance from the eyepiece; to prevent dust, moisture and stray light that interferes with image quality. The purpose of the objective lens is to gather light and present an image at the focal point. The purpose of the eyepiece is to magnify the image at the focal point of the objective and present it on your retina.
1.2 Types of Astronomical Telescopes
According to different optical structures, astronomical telescopes can be divided into many different types, but two are more commonly used: refracting astronomical telescopes (using optical lenses as objective lenses) and reflecting astronomical telescopes (using curved mirrors as objective lenses). Although the two can achieve the same effect, their optical structure is completely different.
Refracting astronomical telescopes: Refracting astronomical telescopes usually use an achromatic objective composed of two or more coated lenses. Generally speaking, it is very difficult to make a large-diameter (above 100mm) combined lens, so the diameter of common refracting astronomical telescopes does not exceed 100mm.
Reflective astronomical telescope: The objective of the reflective astronomical telescope is a curved mirror (primary mirror). A small flat mirror (secondary mirror) inclined at 45 degrees is placed in the optical path of the objective lens to divert the light reflected by the objective lens to the eyepiece on the side of the lens barrel. Reflective astronomical telescopes are relatively easy to achieve large clear apertures. This means that the reflective astronomical telescope can have a strong light-gathering ability, which can be used to observe dim deep space targets and to take astronomical photos.
1.3 Optical properties
Astronomical observers should choose different astronomical telescopes according to the purpose of observation. Generally speaking, popular astronomical observations are mostly comprehensive, and "one mirror can be used for multiple purposes" should be considered. When choosing an astronomical telescope, be sure to fully understand its basic optical properties. To evaluate the quality of a telescope, first look at its optical performance, and secondly look at its mechanical performance. The optical performance of an astronomical telescope is generally measured by the following indicators:
Effective aperture (D): refers to the effective diameter of the objective lens, usually represented by D; that is, the clear diameter of the telescope. The larger the diameter of the telescope, the stronger the light-gathering ability, and the more fainter celestial objects can be observed, which reflects the ability of the telescope to observe celestial objects. Therefore, if the economic conditions permit, enthusiasts should choose a larger-diameter telescope. . In the specification description of the astronomical telescope, it is usually marked.
Focal length (F): The focal length of a telescope mainly refers to the focal length of the objective lens. Similarly, in the specification description of the astronomical telescope, it should also be marked.
Relative aperture (NA): The relative aperture is also called optical power, which is the ratio of the effective aperture D of the telescope to the focal length F, and its reciprocal is called the focal ratio (F/D). The larger the effective aperture is for the observation of extended celestial bodies such as planets, comets, galaxies, nebulae, etc., because their imaging illuminance is proportional to the square of the aperture of the telescope; while the imaging illuminance of so-called linear celestial bodies such as meteors is related to the relative aperture A and the effective aperture. The product of D is proportional. Therefore, when doing astrophotography, you should pay attention to choosing the appropriate effective aperture A or focal ratio. Generally speaking, the relative aperture of the refracting telescope is relatively small, usually 1/8 to 1/20, while the relative aperture of the reflecting telescope is relatively large, usually 1/3.5 to 1/12.
Field of view (ω): The field of view of an astronomical telescope is approximately the ratio of the field of view of the eyepiece to the magnification of the astronomical telescope. The field of view of a telescope is inversely proportional to the magnification, the larger the magnification, the smaller the field of view. Different apertures, different focal lengths, and different optical systems determine the size of the field of view of the telescope.
Magnification (M): The magnification of an astronomical telescope is equal to the ratio of the focal length of the objective lens to the focal length of the eyepiece, as well as the ratio of the entrance pupil to the exit pupil of the objective lens. Therefore, the magnification of the telescope can be changed as long as different eyepieces are changed. However, due to the influence of factors such as the resolving power of the objective lens, the quietness of the atmosphere, and the diameter of the exit pupil cannot be too small, the magnification of the telescope cannot be increased indefinitely. ; In general, it should be controlled at 1-2 times the diameter of the objective lens (maximum should not exceed 300 times). When many people mention astronomical telescopes, the first thing they consider is the magnification.
In fact, astronomical telescopes are different from microscopes. The effect of ground astronomical observation is not only affected by the advantages and disadvantages of the instrument, but also affected by the clarity and tranquility of the earth's atmosphere, and is restricted by various factors such as the environment of the observation site. Also, an astronomical telescope has several eyepieces with different focal lengths, that is, several different magnifications are available. When observing, the maximum magnification is by no means the best, but the clearest observation target shall prevail.
Resolution: Refers to the minimum angular distance that a telescope can resolve. During visual observation, the resolution angle of the telescope = 140 (arcseconds)/D (millimeters), where D is the effective aperture of the objective lens. The resolving power of a telescope is measured by the inverse of the resolving angle of the telescope. The higher the resolution of the telescope, the darker and more celestial objects can be observed. Therefore, high resolution is one of the most important performance indicators of the telescope.
Penetration power: Refers to the faintest star magnitude that a telescope can see on a clear night. The penetration ability is mainly related to the effective aperture of the telescope. In a clear moonless night sky, our human eyes can generally see stars of about 6th magnitude; the number of stars that a telescope can see is mainly determined by the aperture of the telescope. The larger the aperture, the higher the magnitude of the stars seen ( For example, a 50mm telescope can see a 10th magnitude star, and a 500mm telescope can see a 15th magnitude star).
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