Thus, the larger the diameter, the smaller the minimum angular separation and the higher the resolving power. The smallest angular separation that can be unambiguously distinguished is called the resolving power of the telescope and is proportional to the ratio of the wavelength of light being observed to the diameter of the telescope. If two stars are very close, a given telescope may not be able to separate them into two distinct points. The laws of diffraction make a certain amount of blurring unavoidable, because of the wave nature of light. The resolution of the telescope is a measure of how sharply defined the details of the image can be. The brightness of the image depends on the total light gathered and hence is proportional to the area of the objective or the square of the diameter of the telescope. (152 cm) would produce an image of the moon 0.6 in. For example, the angular size of the moon's diameter is about 1-2°, or roughly 1-100 radian a telescope with a focal length of 60 in. The size of the object's image is the product of this and the focal length of the image-forming lens or mirror.
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The angle in radians determined by the object is given by the ratio of the object's diameter to its distance from the telescope.
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The angular size of an object as seen from the position of the telescope may be expressed in degrees or in radians (1 radian equals about 57°). In a terrestrial refracting telescope used to view objects on the earth, an additional lens is used to invert the image a second time, so that objects appear as they do when viewed with the unaided eye in an astronomical telescope, image inversion is unimportant and no lens is added to invert the image a second time. The real image produced is inverted i.e., top and bottom are reversed, as are left and right. The properties of the image produced by a telescope are similar, whether formed by lenses or mirrors. The third type of telescope, the catadioptric system, focuses light by a combination of lenses and mirrors. Typically, a glass disk is ground to this shape and then coated with a thin layer of silver or aluminum to make it highly reflecting. If parallel light rays are to be reflected so that they converge to a single point, the mirror must be paraboloid in shape. In a reflecting telescope, or reflector, light is reflected by a concave mirror and brought to a focus in front of the mirror. The distance from the lens to the focus is called the focal length. Parallel light passing through the lens is refracted so that it converges to a point behind the lens, called the focus. The objective lens is convex, i.e., thicker at the middle than the edges. In the refracting telescope, or refractor, light is bent, or refracted, as it passes through an objective lens. There are three major types of optical telescopes, classified according to the element that gathers and focuses the incoming light. Click the link for more information.), or infrared or ultraviolet radiation. X-ray astronomy dates to 1949 with the discovery that the sun emits X rays. Study of celestial objects by means of the X rays they emit, in the wavelength range from 0.01 to 10 nanometers. Click the link for more information.), X rays (see X-ray astronomy X-ray astronomy, Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes, Study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy other types of astronomical telescopes gather radio waves (see radio astronomy radio astronomy,
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Telescope, traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it.