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Telescopes

A telescope collects light (or other radiation) from the sky over a much larger area than the eye, letting us see fainter objects and finer detail. Its power depends mainly on the size of its main mirror or lens. Because Earth's atmosphere blurs images and blocks many wavelengths, astronomers combine giant ground-based telescopes with observatories in space such as Hubble and the James Webb Space Telescope.

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Beyond Single Mirrors

Resolution depends on wavelength as well as aperture: the longer the wavelength, the coarser the detail. At the same diameter, a radio telescope's resolution is about 1,000 times worse than an optical one's. Interferometers solve this by combining dishes so that they act like one telescope as wide as their separation (the baseline). The Very Large Array, up to 22.62 miles across, reaches 0.2 to 0.04 arcseconds, and ALMA, with antennas up to 16 km apart, beats Hubble's visible-light resolution by up to ten times.

Infrared astronomy faces two problems: the atmosphere blocks the longer infrared wavelengths, and everything warm — the air, the telescope, even the detectors — glows in the infrared. Ground-based infrared telescopes are therefore built high up in dry climates, and Webb shields itself from the Sun, Earth and Moon with a five-layer sunshield.

Info: Hubble's 2.4 m mirror was polished very smoothly but to slightly the wrong curvature — off by about 1/50th of the width of a human hair — which blurred its first images until astronauts installed corrective optics in 1993.
Full explanation — the complete reference version every reading depth is based on

What it is

Your eye catches only the light that enters its small pupil. A telescope is a light bucket: a large lens or mirror gathers light over a much bigger area and brings it to a focus, where it forms an image for an eye, camera or spectrograph. The earliest record of a telescope is a Dutch patent application of 1608, and Galileo was the first to turn one on the sky, finding Jupiter's four largest moons in 1610.

Key ideas

  • Light-gathering power is proportional to collecting area, which grows with the square of the diameter: doubling the diameter collects four times the light.
  • Resolution — sharpness — also improves with a bigger mirror, but gets worse at longer wavelengths.
  • Refractors use lenses; reflectors use curved mirrors. The first telescopes were refractors, but large mirrors can be made thinner and lighter than lenses, so big telescopes, especially in space, are reflectors.
  • Telescopes exist for many kinds of light: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma ray.
light1light2=A1A2=(D1D2)2\dfrac{\text{light}_1}{\text{light}_2} = \dfrac{A_1}{A_2} = \left(\dfrac{D_1}{D_2}\right)^2

Ratio of light collected by two telescopes: the ratio of their collecting areas, which for full circular mirrors is the square of the ratio of their diameters.

Why go to space?

Turbulent air blurs starlight — the same effect that makes stars twinkle — and the atmosphere absorbs ultraviolet light and blocks X-rays and gamma rays. Ground telescopes fight the blurring with adaptive optics, computer-controlled mirrors that change shape in real time. Space telescopes avoid the atmosphere entirely: Hubble (launched April 1990, 2.4 m mirror) orbits about 483 km up, and the infrared James Webb Space Telescope (launched 25 December 2021, 18-segment mirror about 6.5 m across) orbits the Sun 1.5 million km from Earth.

Worked example

A quick estimate treats both mirrors as full circles: (6.5 ÷ 2.4)² ≈ 7.3. But Webb's mirror is a honeycomb of hexagons, not a full circle, so NASA's measured collecting areas are better: 25.37 m² for Webb and 4.55 m² for Hubble, and 25.37 ÷ 4.55 ≈ 5.6. Webb collects about 5.6 times as much light — a reminder that a formula is only as good as its assumptions.

Common misconception: A common misconception is that a telescope's main job is to magnify. Its most important job is to collect light: a bigger mirror reveals fainter objects and finer detail, while magnification on its own just enlarges a blurry image.
Info: Radio astronomers link dishes into interferometers that act like one telescope as wide as the distance between the dishes. The Event Horizon Telescope used this idea on an Earth-wide scale to image black holes.

Where it connects

Telescopes depend on how mirrors and lenses handle light (Light). They are how we discovered other galaxies (Galaxies), find planets round other stars (Exoplanets), image black holes (Black holes) and measure the expansion of the universe (Cosmic expansion); putting them in space depends on rockets and orbits (Spaceflight).

Assumptions and limits

  • Comparing light with (D₁/D₂)² assumes full circular mirrors; gaps, segment shapes and obstructions reduce the real collecting area.
  • Adaptive optics corrects blurring but cannot restore wavelengths that the atmosphere absorbs.
  • An interferometer reaches the resolution of its full width but collects only the light falling on its separate dishes, so it works best on bright sources.

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Sources and methodology

  • The size of a telescope's main mirror or lens determines how well it collects light: the larger the mirror or lens, the more light it collects and the fainter the objects it can detect. (awaiting scientific review)
  • A telescope's light-gathering power is proportional to its total collecting area. (awaiting scientific review)
  • Refracting telescopes use a lens as the main optical element and reflecting telescopes a mirror; the first telescopes, in the 1600s, were refractors, while large mirrors can be made thinner and lighter than lenses of the same size, which makes reflectors ideal for space. (awaiting scientific review)
  • The earliest record of a telescope is a patent application made in Holland on 2 October 1608 by the spectacle maker Hans Lippershey, and Galileo was the first to use the telescope for astronomy. (awaiting scientific review)
  • Galileo Galilei first observed Jupiter's four largest moons in 1610 using an early telescope. (awaiting scientific review)
  • The larger a telescope's mirror, the finer the detail it can resolve; at the same diameter a radio telescope's resolution is about 1,000 times worse than a visible-light telescope's, because radio waves are much longer. (awaiting scientific review)
  • An interferometer combines two or more telescopes so that they act like a single telescope as wide as the separation between them, although it collects less light than one telescope that size would. (awaiting scientific review)
  • ALMA can combine up to 66 antennas spread over distances of up to 16 kilometres, giving resolution up to ten times better than the Hubble Space Telescope achieves at visible wavelengths. (awaiting scientific review)
  • Turbulence in Earth's atmosphere blurs telescope images and makes stars twinkle; adaptive optics uses computer-controlled deformable mirrors to correct this distortion in real time, giving images almost as sharp as those taken from space. (awaiting scientific review)
  • Earth's atmosphere absorbs ultraviolet light and blocks X-rays and gamma rays, so astronomy at those wavelengths must be done from space, whereas radio waves pass through the atmosphere easily. (awaiting scientific review)
  • The Hubble Space Telescope, launched on 24 April 1990 aboard the space shuttle Discovery, has a 2.4-metre primary mirror and orbits roughly 300 miles (483 km) above Earth's surface. (awaiting scientific review)
  • Soon after launch, Hubble's primary mirror was found to have spherical aberration — its curvature was off by about 1/50th of the width of a human hair — and astronauts installed corrective optics in 1993. (awaiting scientific review)
  • With its optics corrected, Hubble can distinguish objects with an angular diameter of just 0.05 arcseconds in visible light. (awaiting scientific review)
  • The James Webb Space Telescope's primary mirror is about 6.5 metres in diameter and made of 18 segments. (awaiting scientific review)
  • Webb's primary mirror has a collecting area of 25.37 square metres, 5.6 times the 4.55 square metres of Hubble's primary mirror. (awaiting scientific review)
  • The James Webb Space Telescope launched on 25 December 2021 and, unlike Hubble, does not orbit Earth: it orbits the Sun 1.5 million kilometres from Earth at the second Lagrange point (L2). (awaiting scientific review)
  • The Very Large Array's 27 active radio dishes reach resolutions from 0.2 to 0.04 arcseconds, and its widest arrangement is 22.62 miles across. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.