A telescope is a vital astronomy instrument designed to form magnified images of distant objects by collecting and focusing radiation. By gathering more light than the human eye, telescopes allow astronomers to observe faint, distant celestial bodies.[1][2] They generally fall into two categories: refracting telescopes, which use lenses to focus light, and reflecting telescopes, which use curved mirrors. Modern advancements have expanded these capabilities across the electromagnetic spectrum , enabling deeper exploration of the universe through powerful ground-based and space-based observatories.[2][1]

How does a telescope help us see space?

Telescopes help us see space by collecting and focusing light from distant celestial objects, much like the human eye but with significantly greater power. Because our pupils are small, they capture limited light; telescopes use large apertures—either lenses in refractors or curved mirrors in reflectors—to gather far more light. This increased light-gathering capacity allows astronomers to detect faint, distant objects and resolve fine details. By focusing this light into an image, telescopes enable us to analyze radiation across the electromagnetic spectrum , revolutionizing our understanding of the universe .[1][2][3]

What are the main types of telescopes?

Telescopes are essential tools for collecting and analyzing radiation from celestial objects. They are primarily categorized by their light-gathering mechanisms:[1] Refracting telescopes (refractors) use lenses to bend light, bringing it to a focus.[2] Reflecting telescopes (reflectors) use mirrors to reflect light to a focus, avoiding the chromatic aberration common in refractors.[3] Catadioptric telescopes , such as the Schmidt telescope , combine both lenses and mirrors to achieve wide fields of view.[4] Radio telescopes use large antennas to detect radio emissions from space.[5] Modern astronomy also utilizes specialized instruments for other regions of the electromagnetic spectrum , including X-ray and gamma-ray telescopes.[1]

How do lenses and mirrors collect light?

Telescopes function by collecting light rays and focusing them to a point, much like the human eye but with significantly greater light-gathering power. Refracting telescopes use lenses to bend and focus light onto an eyepiece. Conversely, most modern reflecting telescopes utilize large, curved mirrors to collect and redirect light toward scientific instruments. By increasing the size of the light-collecting aperture, these devices allow astronomers to capture faint, distant objects that would otherwise remain invisible, enabling detailed observation and analysis of the universe.[1][2]

Why are some telescopes placed in space?

Telescopes are placed in space primarily to overcome the limitations imposed by Earth’s atmosphere . The atmosphere absorbs significant portions of the electromagnetic spectrum , including most ultraviolet, X-ray, and gamma-ray radiation, as well as certain infrared wavelengths, preventing them from reaching ground-based instruments.[1][2][3] Furthermore, space-based observatories avoid atmospheric turbulence and airglow, which distort images and limit the resolution of ground-based telescopes. By operating above the atmosphere, space telescopes like the Hubble Space Telescope can achieve much clearer, high-resolution observations, allowing astronomers to study the universe across all wavelengths and gain deeper insights into its origin and evolution.[1][2][4]

What discoveries have telescopes helped astronomers make?

Telescopes have revolutionized astronomy by revealing objects invisible to the naked eye. Early refracting telescopes allowed Galileo to discover lunar mountains, Jupiter’s satellites, and Venus’s phases. [1][2] Modern radio telescopes have detected pulsars , quasars , and the cosmic microwave background , providing evidence for the Big Bang. Furthermore, telescopes have enabled the discovery of extrasolar planets and dark energy , which accelerates the universe's expansion. By observing across the electromagnetic spectrum —including infrared —astronomers now map star formation and analyze the composition of distant planetary atmospheres.[3][4][5][2][6]

Telescope

Images & Videos

$refracting telescope$ $The Great Lick Refractor telescope, a large silver tube mounted inside the domed ceiling of Lick Observatory on Mount Hamilton, California.$

Diagram of a Cassegrain reflector telescope showing light entering the tube, reflecting off a concave primary mirror to a convex secondary mirror, and then focusing through an eyepiece at the back.

At a Glance

telescope summary

Telescope

astronomy

Also known as: telescope

Written by

B.L. KlockAll

Fact-checked by

Britannica Editors

Last updated

Nov. 9, 2026 •History

Top Questions

What is a telescope?
A telescope is a vital astronomy instrument designed to form magnified images of distant objects by collecting and focusing radiation. By gathering more light than the human eye, telescopes allow astronomers to observe faint, distant celestial bodies.[1][2]

They generally fall into two categories: refracting telescopes, which use lenses to focus light, and reflecting telescopes, which use curved mirrors. Modern advancements have expanded these capabilities across the electromagnetic spectrum, enabling deeper exploration of the universe through powerful ground-based and space-based observatories.[2][1]
2 Britannica Sources
1. telescope - Light gathering and resolution
2. How Does a Telescope Work?
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does a telescope help us see space?
Telescopes help us see space by collecting and focusing light from distant celestial objects, much like the human eye but with significantly greater power. Because our pupils are small, they capture limited light; telescopes use large apertures—either lenses in refractors or curved mirrors in reflectors—to gather far more light. This increased light-gathering capacity allows astronomers to detect faint, distant objects and resolve fine details. By focusing this light into an image, telescopes enable us to analyze radiation across the electromagnetic spectrum, revolutionizing our understanding of the universe.[1][2][3]
2 Britannica Sources
1. telescope
2. How Does a Telescope Work?
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the main types of telescopes?
Telescopes are essential tools for collecting and analyzing radiation from celestial objects. They are primarily categorized by their light-gathering mechanisms:[1]
- Refracting telescopes (refractors) use lenses to bend light, bringing it to a focus.[2]
- Reflecting telescopes (reflectors) use mirrors to reflect light to a focus, avoiding the chromatic aberration common in refractors.[3]
- Catadioptric telescopes, such as the Schmidt telescope, combine both lenses and mirrors to achieve wide fields of view.[4]
- Radio telescopes use large antennas to detect radio emissions from space.[5]
Modern astronomy also utilizes specialized instruments for other regions of the electromagnetic spectrum, including X-ray and gamma-ray telescopes.[1]
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do lenses and mirrors collect light?
Telescopes function by collecting light rays and focusing them to a point, much like the human eye but with significantly greater light-gathering power. Refracting telescopes use lenses to bend and focus light onto an eyepiece. Conversely, most modern reflecting telescopes utilize large, curved mirrors to collect and redirect light toward scientific instruments. By increasing the size of the light-collecting aperture, these devices allow astronomers to capture faint, distant objects that would otherwise remain invisible, enabling detailed observation and analysis of the universe.[1][2]
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why are some telescopes placed in space?
Telescopes are placed in space primarily to overcome the limitations imposed by Earth’s atmosphere. The atmosphere absorbs significant portions of the electromagnetic spectrum, including most ultraviolet, X-ray, and gamma-ray radiation, as well as certain infrared wavelengths, preventing them from reaching ground-based instruments.[1][2][3]

Furthermore, space-based observatories avoid atmospheric turbulence and airglow, which distort images and limit the resolution of ground-based telescopes. By operating above the atmosphere, space telescopes like the Hubble Space Telescope can achieve much clearer, high-resolution observations, allowing astronomers to study the universe across all wavelengths and gain deeper insights into its origin and evolution.[1][2][4]
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What discoveries have telescopes helped astronomers make?
Telescopes have revolutionized astronomy by revealing objects invisible to the naked eye. Early refracting telescopes allowed Galileo to discover lunar mountains, Jupiter’s satellites, and Venus’s phases. [1][2]

Modern radio telescopes have detected pulsars, quasars, and the cosmic microwave background, providing evidence for the Big Bang. Furthermore, telescopes have enabled the discovery of extrasolar planets and dark energy, which accelerates the universe's expansion. By observing across the electromagnetic spectrum—including infrared—astronomers now map star formation and analyze the composition of distant planetary atmospheres.[3][4][5][2][6]
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Telescope, device used to form magnified images of distant objects. The telescope is undoubtedly the most important investigative tool in astronomy. It provides a means of collecting and analyzing radiation from celestial objects, even those in the far reaches of the universe.

Galileo revolutionized astronomy when he applied the telescope to the study of extraterrestrial bodies in the early 17th century. Until then, magnification instruments had never been used for this purpose. Since Galileo’s pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring radiation in every region of the electromagnetic spectrum. Observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems. These developments have contributed dramatically to advances in scientific knowledge about the solar system, the Milky Way Galaxy, and the universe as a whole.

This article describes the operating principles and historical development of optical telescopes. For explanation of instruments that operate in other portions of the electromagnetic spectrum, see radio telescope; X-ray telescope; and gamma-ray telescope.

Refracting telescopes

The Great Lick Refractor telescope, a large silver tube mounted inside the domed ceiling of Lick Observatory on Mount Hamilton, California. — refractor at Lick ObservatoryThe historical 91-cm (36-inch) refractor at the Lick Observatory on Mount Hamilton, near San Jose, California, U.S.

Commonly known as refractors, telescopes of this kind are typically used to examine the Moon, other objects of the solar system such as Jupiter and Mars, and binary stars. The name refractor is derived from the term refraction, which is the bending of light when it passes from one medium to another of different density—e.g., from air to glass. The glass is referred to as a lens and may have one or more components. The physical shape of the components may be convex, concave, or plane-parallel. This diagram illustrates the principle of refraction and the term focal length. The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and traveling a distance of one focal length. In a refractor the first lens through which light from a celestial object passes is called the objective lens. It should be noted that the light will be inverted at the focal plane. A second lens, referred to as the eyepiece lens, is placed behind the focal plane and enables the observer to view the enlarged, or magnified, image. Thus, the simplest form of refractor consists of an objective and an eyepiece, as illustrated in the diagram.

The diameter of the objective is referred to as the aperture; it typically ranges from a few centimetres for small spotting telescopes up to one metre for the largest refractor in existence. The objective, as well as the eyepiece, may have several components. Small spotting telescopes may contain an extra lens behind the eyepiece to erect the image so that it does not appear upside-down. When an object is viewed with a refractor, the image may not appear sharply defined, or it may even have a predominant colour in it. Such distortions, or aberrations, are sometimes introduced when the lens is polished into its design shape. The major kind of distortion in a refractor is chromatic aberration, which is the failure of the differently coloured light rays to come to a common focus. Chromatic aberration can be minimized by adding components to the objective. In lens-design technology, the coefficients of expansion of different kinds of glass are carefully matched to minimize the aberrations that result from temperature changes of the telescope at night.

View of the Andromeda Galaxy (Messier 31, M31).

Britannica Quiz

Astronomy and Space Quiz

Eyepieces, which are used with both refractors and reflectors (see below Reflecting telescopes), have a wide variety of applications and provide observers with the ability to select the magnification of their instruments. The magnification, sometimes referred to as magnifying power, is determined by dividing the focal length of the objective by the focal length of the eyepiece. For example, if the objective has a focal length of 254 cm (100 inches) and the eyepiece has a focal length of 2.54 cm (1 inch), then the magnification will be 100. Large magnifications are very useful for observing the Moon and the planets. However, since stars appear as point sources owing to their great distances, magnification provides no additional advantage when viewing them. Another important factor that one must take into consideration when attempting to view at high magnification is the stability of the telescope mounting. Any vibration in the mounting will also be magnified and may severely reduce the quality of the observed image. Thus, great care is usually taken to provide a stable platform for the telescope. This problem should not be associated with that of atmospheric seeing, which may introduce a disturbance to the image because of fluctuating air currents in the path of the light from a celestial or terrestrial object. Generally, most of the seeing disturbance arises in the first 30 metres (100 feet) of air above the telescope. Large telescopes are frequently installed on mountain peaks in order to get above the seeing disturbances.

Telescope

Thank you for signing up!

Refracting telescopes