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One of the telescopes at the VLTOne of the telescopes at the VLT A telescope is an instrument that makes distant objects appear closer, allowing the viewer to see details that are not visible with the naked eye. Terrestrial telescopes are used for spotting wildlife (binoculars are made up of two telescopes, one for each eye) and on gunsights and in periscopes. Astronomical telescopes are used to study objects in space. Terrestrial telescopes and most astronomical telescopes are optical telescopes, which collect light coming from distant objects and use it to produce images of the objects. A powerful astronomical telescope will reveal details of planets, nebulae (clouds of dust or gas) and galaxies that are invisible to the naked eye.

A diagram of a large reflecting telescopeA diagram of a large reflecting telescope

Refractors and reflectors

There are two main types of optical telescope—refracting telescopes and reflecting telescopes. In a refracting telescope, a convex lens (bulging shape) collects light from the distant object and focuses it to form an image of the object. This image is very small, but is much larger than the image formed in the human eye.

A reflecting telescope uses a curved mirror to focus light and form an image. In both refracting and reflecting telescopes, the image is viewed with an eyepiece, or it falls on a light sensor like the sensor in a digital camera.

A diagram of a Cassegrain-type reflecting telescopeA diagram of a Cassegrain-type reflecting telescopeA Cassegrain telescope is a type of reflecting telescope. It uses a concave (dish-shaped) primary mirror to collect the light from the object and focus it on to a smaller secondary mirror. From there, the image is reflected on to a light sensor. Larger telescopes are nearly always reflecting telescopes because large mirrors are easier to manufacture than large lenses.

Segmented mirror

The word telescope comes from two Greek words: tele meaning "far" and skopein "to look or see".  The word was created in 1611 to describe one of Galileo's instruments, which Galileo himself called a perspicillum.

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