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Binoculars, also sometimes called “field glasses,” are two identical, or mirror-symmetrical telescopes, mounted side-by-side that allow the viewer to use both eyes to view distant objects. Most binoculars are sized to be held with both hands, although some are much larger. Hand-held binoculars range from small opera glasses used in theaters to large ones for outdoor use, such as birdwatching and amateur astronomy.

Different types of binoculars:

Galilean binoculars:
Soon after the invention of the telescope, the idea of mounting them side by side for binocular vision seems to have been explored. Most early binoculars used Galilean optics; that is they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification. This type of binocular is used for most opera glasses or theater glasses.

Porro prism binoculars:
The Porro prism binoculars were named after Italian optician Ignazio Porro who patented this image erecting system in 1854. Binoculars of this type use a Porro prism in a double prism Z-shaped configuration to erect the image. This feature results in binoculars that are wider, with objective lenses that are well separated but offset from the eyepieces. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective and wider spacing of the objectives gives better sensation of depth.

Roof prism binocular:
Most roof prism binoculars use either the Abbe-Koenig prism (named after Ernst Karl Abbe and Albert Koenig and patented by Carl Zeiss in 1905) or Schmidt-Pechan prism (invented in 1899) designs to erect the image and fold the optical path. They have objective lenses that are approximately in line with the eyepieces.

Porro prism binoculars are the more widely used models, although most birdwatchers and hunters prefer the lighter weight (but more expensive) roof-prism models. Binoculars are also used by amateur astronomers for comet and supernova seeking and general observation. Some of the night sky objects seen with binoculars include the Galilean moons of Jupiter, Ceres, Neptune, Pallas and Titan. Uranus and Vesta and the dwarf planets Pluto and Eris can also be seen with binoculars. Binoculars made specifically for astronomical uses have higher magnification and a larger aperture objective (in the 70mm or 80mm range) because the diameter of the objective lens determines the faintest star that can be observed. These binoculars usually require a mount.

The Large Binocular Telescope in Arizona, United States, comprises two 8-meter reflector telescopes. While obviously not intended to be held to the eyes of a viewer, and not what one would normally think of as “binoculars,“ the LBT does use two telescopes to view the same object, and is thus, technically, binoculars.


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