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An optical telescope is a telescope which is used to gather, and focus light, for directly viewing a magnified image, making a photograph, etc. The term is used especially for a monocular with static mounting for observing the sky. Handheld binoculars are common for other purposes. Light is made up of photons, and professional telescopes concentrate the light onto electronic detectors which collect the photons. There are three primary types of optical telescope: Refractors (Dioptrics) which use lenses, Reflectors (Catoptrics) which use mirrors, and Combined Lens-Mirror Systems (Catadioptrics) which use lenses and mirrors in combination (for example the Maksutov telescope and the Schmidt camera).
How it works
Angular resolution Ignoring blurring of the image by turbulence in the atmosphere (atmospheric seeing) and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the width of the objective, termed its "aperture" (the primary mirror, or lens.) The Rayleigh criterion for the resolution limit (in radians) is given by , where is the wavelength and is the aperture. For visible light (), this equation can be rewritten: . Here, denotes the resolution limit in arcseconds and is in millimeters. In the ideal case, the two components double stars can be split even if separated by slightly less than . This is taken into account by the Dawes limit . Essentially; the larger the aperture, the better the angular resolution It should be noted that the resolution is NOT given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images. For large ground-based telescopes, the resolution is limited by atmospheric seeing. This limit can be overcome by placing the telescopes above the atmosphere, e.g., space telescopes, balloon telescopes and telescopes on high-flying airplanes (Kuiper Airborne Observatory, SOFIA) or by adaptive optics or speckle imaging for ground-based telescopes. Recently, it has become practical to perform aperture synthesis with arrays optical telescopes. Very high resolution images can be obtained with groups of widely-spaced smaller telescopes, linked together by carefully-controlled optical paths, but these interferometers can only used for imaging bright objects such as stars or measuring the bright cores of active galaxies. Example images of starspots on Betelgeuse can be seen here. Focal length and f-ratio The focal length determines how wide an angle the telescope can view with a given eyepiece or size of a CCD detector or photographic plate. The f-ratio (or focal ratio, or f-number) of a telescope is the ratio between the focal length and the aperture (i.e., diameter) of the objective. Thus, for a given aperture (light-gathering power), low f-ratios indicate wide fields of view. Wide-field telescopes (such as astrographs) are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky. It is more difficult to reduce optical aberrations in telescopes with low f-ratio than in telescopes with larger f-ratio. Light-gathering power The light-gathering power of an optical telescope is directly related to the diameter (or aperture) of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light. Research telescopes Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are: The size of optical telescopes increased steadily in the 20th century, doubling between the 1910s and the 1940s, and doubling again between the late 1940s and the 1990s. The largest current telescopes are the 11m SALT and Hobby-Eberly telescopes and the 10.4m Gran Telescopio Canarias. In the 1980s a number of technological improvements were made which created a new generation of telescopes. These advances included the creation of multi-mirror telescopes and the invention of cheap personal computers which could control the mirrors. Another major advanced was the invention of rotating furnaces in which centrifugal force would shape a telescope mirror to close to its final shape. Names of types: See also | ||||||||||
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