Telescope

Introduction
Refracting telescopes
Light gathering and resolution
Reflecting telescopes
The Schmidt telescope
Multimirror telescopes
Special types of optical telescopes
- Solar telescopes
- Earth-orbiting space telescopes
- Astronomical transit instruments
- Astrolabes
The development of the telescope and auxiliary instrumentation
- Evolution of the optical telescope
- Advances in auxiliary instrumentation
  - Cameras
  - Spectrographs
  - Photomultiplier tubes
  - Charge-coupled devices
- Impact of technological developments
  - Computers
  - Rockets and spacecraft

References & Edit History Quick Facts & Related Topics

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

Quizzes

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

Astronomy and Space Quiz

Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.

All About Astronomy

Related Questions

How did Galileo use a telescope to study the Moon in 1609?
How did Isaac Newton’s reflecting telescope differ from Galileo’s refracting telescope?
How did Karl Jansky discover radio signals from the centre of the Milky Way?
How did Pyrex glass improve the construction of large telescope mirrors?
How did scientists correct the Hubble Space Telescope’s focusing problem?
How did the Keck telescopes contribute to the large group of observatories on Mauna Kea?
How did the Spitzer Space Telescope study extrasolar planets?
How did transit instruments help astronomers map the celestial sphere?
How do radio telescopes detect cosmic objects without using visible light?
How do reflecting telescopes avoid chromatic aberration?
How do right ascension and declination help astronomers point a telescope?
How do the 27 dishes of the Very Large Array work together?
How does Earth’s atmosphere blur images from ground-based telescopes?
How does FAST observe different parts of the sky even though its main dish does not move?
How does a Cassegrain telescope use its secondary mirror?
How does a Newtonian reflector direct light to an observer’s eyepiece?
How does a refracting optical telescope differ from a reflecting optical telescope?
How does adaptive optics reduce image distortion caused by atmospheric turbulence?
How does an equatorial telescope mount track a star across the sky?
How does an eyepiece change a telescope’s magnification?
How does an optical telescope collect and focus light?
How does an optical telescope collect and focus light from distant objects?
How does resolving power help astronomers distinguish nearby objects in the sky?
How does the James Webb Space Telescope investigate forming stars and galaxies?
How does the aperture size affect the performance of an optical telescope?
How does very long baseline interferometry improve radio telescope resolution?
How is a telescope’s light-gathering power related to its aperture diameter?
How much more light does a 25-centimetre telescope collect than a 12.5-centimetre telescope?
How was the Arecibo Observatory used before its central platform collapsed?
What are the main components of an optical telescope?
What did Galileo’s observations of Jupiter’s four largest moons show?
What discoveries did William Lassell make with his reflecting telescopes?
What discoveries did the Infrared Astronomical Satellite make about debris around stars?
Where is the Gran Telescopio Canarias located?
Which parts of the electromagnetic spectrum can ground-based telescopes observe most easily?
Who invented the first refracting optical telescope?
Why are X-ray and gamma-ray telescopes placed above Earth’s atmosphere?
Why are large refracting telescopes difficult to build?
Why are many large optical telescopes built on high mountains?
Why are many major observatories built on high mountains?
Why are modern large optical telescopes usually reflectors?
Why are radio telescopes usually shaped like large dishes?
Why are telescopes often built far from cities and their lights?
Why can mirrors in reflecting telescopes be supported more easily than large lenses?
Why do refracting telescopes use lenses?
Why do space-based optical telescopes capture clearer images than ground-based optical telescopes?
Why does a larger telescope aperture improve resolving power?
Why does a telescope with a larger aperture show fainter objects?
Why does chromatic aberration occur in refracting telescopes?
Why does magnification not reveal more detail when viewing distant stars?
Why is infrared astronomy useful for observing objects hidden by dust?
Why must many infrared telescopes operate at high altitude or in space?
Why was FAST built in a natural depression in Guizhou, China?
Why was the 1-metre Yerkes telescope significant in the history of refracting telescopes?
Why were Galileo’s telescope observations important to the history of astronomy?

Science Astronomy

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External Websites

University of Arizona - James C. Wyant College of Optical Sciences - Telescopes
Physics LibreTexts - The Telescope
National Center for Biotechnology Information - Telescopes
Boston University - Optics and Telescopes (PDF)
University College London - Optical Telescope

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

telescope - Children's Encyclopedia (Ages 8-11)
telescope - Student Encyclopedia (Ages 11 and up)

Telescope

astronomy

Also known as: telescope

Written by

B.L. Klock

Physical Scientist, Research and Engineering Directorate, Advanced Weapons Systems Division, Defense Mapping Agency, U.S. Department of Defense, Washington, D.C. Editor of Fundamental Star Catalogs.

B.L. KlockAll

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Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

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Last updated

Nov. 9, 2026 •History

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...

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