Telescope

For other uses, see Telescope (disambiguation).

A telescope is a device used to see faraway objects. It works by picking up light or other kinds of energy that these objects give off, absorb, or reflect. The first telescopes were optical instruments that used lenses, curved mirrors, or both to make distant objects look closer. These are called optical telescopes. Today, the word "telescope" covers many kinds of instruments. They can detect different parts of the electromagnetic spectrum, and some can even use other kinds of detectors.

The first practical telescopes were refracting telescopes. These used glass lenses. They were invented in the Netherlands at the start of the 1600s. People used them for looking at faraway places on land and for studying astronomy.

The reflecting telescope uses mirrors to gather and focus light. It was invented only a few decades after the first refracting telescope.

In the 1900s, many new kinds of telescopes were invented. Radio telescopes came in the 1930s. Infrared telescopes came in the 1960s.

Etymology

The word telescope was made up in 1611 by the Greek mathematician Giovanni Demisiani. He used it to name one of Galileo Galilei's instruments. The instrument was shown at a banquet at the Accademia dei Lincei. Earlier, in the Starry Messenger, Galileo had used the Latin word perspicillum.

The word comes from two Ancient Greek words: τῆλε, tele, meaning 'far', and σκοπεῖν, skopein, meaning 'to look or see'. Put together, they form τηλεσκόπος, teleskopos, meaning 'far-seeing'.

History

Main article: History of the telescope

The earliest known record of a telescope is an 1608 patent sent to the government in the Netherlands by spectacle maker Hans Lipperhey. It was for a refracting telescope. No one knows for sure who invented it, but news of it spread across Europe. Galileo heard about it. In 1609, he built his own version and used it to study objects in the sky. Soon after, people began testing whether the objective, the part that gathers light, could be a mirror instead of a lens. Parabolic mirrors could reduce spherical aberration and avoid chromatic aberration, so many designs were suggested and several reflecting telescopes were tried. In 1668, Isaac Newton built the first practical reflecting telescope. This design is now called the Newtonian reflector. John Dobson invented the Dobsonian telescope in 1956.

The achromatic lens was invented in 1733. It partly fixed the colour problems of simple lenses. This allowed shorter and more useful refracting telescopes. Reflecting telescopes did not have those colour problems, but early mirrors made of speculum metal tarnished quickly. Silver-coated glass mirrors in 1857 and aluminized mirrors in 1932 helped solve this. Refracting telescopes can only be about 1 meter (39 inches) across. Because of this, most large research telescopes built since the start of the 20th century have been reflectors. The largest reflecting telescopes now have objectives larger than 10 meters (33 feet). Work is underway on several designs of 30–40 m. During the 20th century, telescopes were also developed to work with a wide range of wavelengths, from radio to gamma-rays. The first telescope made just for radio waves began working in 1937. Many complex instruments have been developed since then.

Main article: Space telescope

Earth's atmosphere blocks most kinds of light, so only a few bands can be seen from the ground. These are visible light, near-infrared, and part of the radio waves. That is why X-ray and far-infrared telescopes must be placed in orbit. Even when a wavelength can be seen from the ground, a telescope in space may still work better. This is because of problems such as clouds, astronomical seeing, and light pollution. Space telescopes have some disadvantages, including cost, size, maintenance, and upgrades.

Some well-known space telescopes from NASA include the Hubble Space Telescope, which detects visible light, ultraviolet, and near-infrared. The Spitzer Space Telescope detects infrared. The Kepler Space Telescope discovered thousands of exoplanets. The most recent telescope launched is the James Webb Space Telescope. It was launched on 25 December 2021 from Kourou, French Guiana, on an Ariane 5 rocket. Webb detects infrared light and orbits the L₂ Lagrange Point of the Earth-Sun system.

By electromagnetic spectrum

Telescopes detect different kinds of electromagnetic radiation. Astronomers collect each kind in a different way. Longer wavelengths are easier to handle with antenna technology. Near-infrared light can be collected in a way similar to visible light. Far-infrared and submillimetre telescopes work more like radio telescopes. The James Clerk Maxwell Telescope uses a curved aluminum antenna, while the Spitzer Space Telescope uses a mirror. The Hubble Space Telescope and its Wide Field Camera 3 can observe from ultraviolet to infrared light.

Shorter wavelengths need special mirrors. TRACE and SOHO are examples, and they reflect extreme ultraviolet. A larger opening collects more light and lets astronomers see more detail. Telescopes can be grouped by where they are, such as a space telescope or a flying telescope. They can also be grouped by who uses them: professional astronomers or amateur astronomers. A place with one or more telescopes is called an observatory.

Radio telescopes are directional radio antennas, often with a large dish that collects radio waves. Several dishes can work together as astronomical interferometers to make detailed images. Radio telescopes also collect microwave radiation, which can pass through the atmosphere and clouds of gas and dust. Ultraviolet light is mostly absorbed by the Earth's atmosphere, so it must be seen from high up or from space. X-rays are much harder to focus, so X-ray telescopes use special mirrors. Very high energy gamma rays are detected with Imaging Atmospheric Cherenkov Telescopes. The Fermi Gamma-ray Space Telescope was launched in June 2008.

Main articles: Radio telescope, Radio astronomy, and Submillimetre astronomy

Main articles: Infrared telescope and Infrared astronomy

Main articles: Optical telescope and Visible-light astronomy

Main article: Ultraviolet astronomy

Main articles: X-ray telescope and X-ray astronomy

Main article: Gamma-ray astronomy

Lists of telescopes

• List of optical telescopes
• List of largest optical reflecting telescopes
• List of largest optical refracting telescopes
• List of largest optical telescopes historically
• List of radio telescopes
• List of solar telescopes
• List of space observatories
• List of telescope parts and construction
• List of telescope types