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| Binoculars From Wikipedia, the free encyclopedia Binocular telescopes, or binoculars, (also known as field glasses) are two identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most are sized to be held using both hands, although there are much larger types. Unlike a monocular telescope, binoculars give users a three-dimensional image: the two views, presented from slightly different viewpoints to each of the viewer's eyes, merge to produce a single perceived view with a sensation of depth, allowing distances to be estimated. There is no need to close or obstruct one eye to avoid confusion, as is usual with monocular telescopes. When using hand-held binoculars the two hands and the head form a steady three-point platform, with less tendency to shake than a single-eye instrument. General use Hand-held binoculars range from small 3x10 Galilean opera glasses, used in theaters, to glasses with 7 to 12 diameters magnification and 30 to 50 mm objectives for typical outdoor use. Porro prism models predominate although bird watchers and hunters tend to prefer, and are prepared to pay for, the lighter but more expensive roof-prism models. Many tourist attractions have installed pedestal-mounted, coin-operated binoculars to allow visitors to obtain a closer view of the attraction. In the United Kingdom, 20 pence often gives a couple of minutes of operation, and in the United States, one or two quarters gives between one-and-a-half to two-and-a-half minutes. Military Binoculars have a long history of military use. Galilean designs were widely used up to the end of the 19th century when they gave way to porro prism types. Binoculars constructed for general military use tend to be more heavily ruggedized than their civilian counterparts. They generally avoid more fragile center focus arrangements in favor of independent focus. Prism sets in military binoculars may have redundant aluminized coatings on their prism sets to guarantee they don’t lose their reflective qualities if they get wet. Military binoculars of the cold war era were sometimes fitted with passive sensors that detected active IR emissions, while modern ones usually are fitted with filters blocking laser beams. Further, binoculars designed for military usage may include a stadiametric reticle in one ocular in order to facilitate range estimation. There are binoculars designed specifically for civilian and military use at sea. Hand held models will be 5x to 7x but with very large prism sets combined with eyepieces designed to give generous eye relief. This optical combination prevents the image vignetting or going dark when the binoculars are pitching and vibrating relative to the viewer's eye. Large, high-magnification, models with large objectives are also used in fixed mountings. Very large binocular naval rangefinders (up to 15 meters separation of the two objective lenses, weight 10 tons, for ranging World War II naval gun targets 25 km away) have been used, although late-20th-century technology made this application redundant. Astronomical Binoculars are widely used by amateur astronomers; their wide field of view making them useful for comet and supernova seeking (giant binoculars) and general observation (portable binoculars). The Galilean moons of Jupiter, Ceres, Neptune, Pallas and Titan are invisible to the naked eye but can readily be seen with binoculars. Although technically visible unaided in pollution-free skies, Uranus and Vesta require binoculars for practical observation. 10x50 binoculars are limited to a magnitude of around +9.5, which means asteroids like Interamnia, Davida, Europa and, except under exceptional conditions Hygiea, are too faint to be seen with binoculars. Likewise too faint to be seen with binoculars are all moons except the Galileans and Titan, and the dwarf planets Pluto and Eris. Of particular relevance for low-light and astronomical viewing is the ratio between magnifying power and objective lens diameter. A lower magnification facilitates a larger field of view which is useful in viewing large deep sky objects such as the Milky Way, nebula, and galaxies, though the large exit pupil means some of the gathered light is wasted. The large exit pupil will also image the night sky background, effectively decreasing contrast, making the detection of faint objects more difficult except perhaps in remote locations with negligible light pollution. Binoculars specifically for most astronomical uses have higher magnification and a larger aperture objective because the diameter of the objective lens determines the faintest star that can be observed. Much larger binoculars have been made by amateur telescope makers, essentially using two refracting or reflecting astronomical telescopes, with mixed results. A very large professional instrument, although not one that would normally be called binoculars, is the Large Binocular Telescope in Arizona, USA, which produced its "First Light" image on October 26, 2005. The LBT comprises two 8-meter reflector telescopes. While obviously not intended to be held to the eyes of a viewer, it uses two telescopes to view the same object, giving higher resolving power than a single instrument of the same light-gathering power, and allowing interferometric use. |
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