A diagram of a large reflecting telescopeA diagram of a large reflecting telescope 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 Cassegrain-type reflecting telescopeA diagram of a Cassegrain-type 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 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.


Observatories on Mauna Kea, HawaiiObservatories on Mauna Kea, HawaiiA large telescope for viewing the night sky is housed in an observatory. The doors slide open to give a view of the stars. Modern observatories, where telescopes are housed, are built on mountain tops, above the densest, cloudiest parts of the atmosphere. Mauna Kea, Hawaii, one of the best observing sites in the world, has many observatories.

An image taken using long-exposure photographyAn image taken using long-exposure photography

Viewing the image

The larger the lens of a refracting telescope or the mirror of a reflecting telescope, the brighter or sharper the image of the object observed, and the fainter the objects that can be seen with the telescope. The light-gathering power, rather than magnifying power, is a telescope's most important feature. The more light a telescope can gather, the higher its resolution (the greater the detail it can provide). The image is viewed with an eyepiece lens, which works like a magnifying glass to make it appear much larger.

Most astronomical observation at observatories is now done by the use of photography. Cameras focus light on to special microchips, called charge-coupled devices (CCDs), that divide the image into thousands of pixelsLong-term exposure by digital cameras allows images to be made of objects, such as faint stars and galaxies, that would otherwise be invisible to the naked eye. Some telescopes can be controlled (aimed and focused) over the internet, from thousands of kilometres away. The images produced can then be viewed online. 

The VLT telescopesThe VLT telescopes


The Very Large Telescope (VLT) is an observatory located on Cerro Paranal in the Atacama Desert of northern Chile. Operated by the European Southern Observatory (ESO) since its completion in 1998, it has been described as the world's most advanced optical telescope. It consists of four individual telescopes, each with a primary mirror 8.2 metres (27 feet) across. The telescopes are usually used separately, but they can be used together, along with four smaller telescopes, to achieve very high resolution. This technique is known as astronomical interferometry.

Gran Canaria TelescopeGran Canaria Telescope

Gran Canaria Telescope

The Gran Canaria Telescope, sitting atop a 2400-metre (7900-foot) peak on the island of La Palma in the Canaries, is the world’s largest telescope. Its light-collecting mirror, measuring 10.4 metres 34 feet) across, consists of 36 hexagons fitted together in a honeycomb pattern. It is larger than the previous record holder, the Keck in Hawaii. Protected from the elements by a steel shell, the telescope is designed to seek out the most distant galaxies and quasars in the Universe. The very clear skies of the Canaries make it one of the best locations for astronomical observation.

Galileo's sketches of the Moon, made in 1609Galileo's sketches of the Moon, made in 1609


In 1609 Italian scientist Galileo Galilei became the first person to gaze at the night sky through a telescope, invented in 1608. To his amazement, he saw that the Moon had mountains and craters. Galileo also detected four moons circling Jupiter and witnessed the changing shape of Venus as it orbited the Sun. He also discovered many new stars, never before seen by humans.

 Chris Oxlade

See also in Technology

See also in Space

See also in Science

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.

The first photograph of an astronomical world, the Moon, was taken in 1840.

The four VLT optical telescopes are called Antu (Sun), Kueyen (Moon), Melipal (Southern Cross), and Yepun (Venus) in the native language of the local region of Chile. The competition to name them was won by 17-year-old Jorssy Albanez Castilla from Chuquicamata.

Using the VLT, astronomers have measured the age of the oldest star known in the Milky Way Galaxy: HE 1523-0910 is 13.2 billion years old, nearly as old as the Universe itself (13.82 billion years).

The first image of an exoplanet, a planet from another solar system beyond our own, was made by the VLT.

Each individual telescope in the VLT can detect objects 4 billion times fainter than visible with the naked eye.

When all the VLT telescopes are combined as the Very Large Telescope Interferometer, it would be possible to distinguish an astronaut standing on the Moon seen from Earth.

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