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What Are Binocular Telescopes?

Binocular telescopes contain prisms.
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  • Written By: Synthia L. Rose
  • Edited By: Lauren Fritsky
  • Last Modified Date: 20 October 2014
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Binocular telescopes are two telescopes that are jointly mounted, enabling a person to look through two lenses simultaneously. This binary vision is beneficial because it mimics the double-eyed vision of most living creatures. Commonly called binoculars, binocular telescopes can come in both small, personal sizes and gigantic sizes.

This instrument can be used in both astronomy and nature-watching, depending on the strength of magnification. Binoculars have become a mainstay of amateur sky and nature watchers due to their inexpensive price and ease of use. Small binocular telescopes are easily portable and can be held in the hands or worn around the neck. Large-sized binocular telescopes require the use of a tripod stand. Most selections can be bought in camera stores, electronic departments or sports shops.

Telescopes vary widely in power and visual effect, with different models offering different size lenses, different light-gathering capabilities, and different magnification power. Binocular telescopes, however, are made with twin telescopes of the same exact specifications, ensuring that the view and magnification on the right is the same as the one on the left. A pair of binocular telescopes consists of several parts: a tubular casing, two sets of lenses on each side, a prism, and focus controls.

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Users look through the casing’s tubes through the first lens, which is known as the eyepiece. At the end of the tube is the second lens, known as the objective lens. The objective lens is responsible for gathering light, magnifying images, and sharpening their focus. In between the two lenses is a prism, which is a mirror that reverses the orientation of the image and shortens the distance observed. A knob or dial for adjusting focus might be at the top of the casing.

The optical design of binocular telescopes is often the most important characteristic distinguishing one model from another. Binocular telescopes come in three distinct optical designs: roof prism, porra prism, and reverse porra prism. The roof prism design relies on linear characteristics and even spacing; the narrow barrels, the eyepieces and the lenses are all mounted in a straight line an equal distance apart. Focus is internal, not external, for roof prism binoculars, allowing users to shift the lens inside its casing in order to sharpen focus. A big advantage of this streamlined design is that roof prism binoculars are often waterproof, allowing for use in any type of weather or environment.

Porra prism binoculars tend to be heavier and larger than roof prism models. Not evenly spaced, this design arranges the eyepieces closer together than the objective lenses. Focus controls are on the outside of the binocular casing, allowing for external focus only. This design is the most common and most traditional design for binocular telescopes. Despite some aforementioned awkward traits, this design allows for a high amount of depth perception and light gathering. It is also typically the least expensive design.

Reverse porra prism binoculars set the objective lenses closer together than the eyepieces — this is opposite of the porra prism design. Among the smallest of binocular telescopes, reverse porra prism models do not perform well when great distance, magnification and depth are needed. They have external focus controls and are usually not waterproof.

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