Telescopes and Astronomy

About this topic

Telescopes collect and focus light. Refracting telescopes use lenses, and reflecting ones use mirrors, which can be larger. Larger apertures gather more light, revealing fainter objects. Beyond visible light, radio telescopes, infrared, UV, X-ray and gamma-ray observatories reveal different aspects of the universe, many placed in space to avoid atmospheric absorption.

Notable: Hubble, James Webb and the Event Horizon Telescope network. Spectroscopy of starlight reveals composition, motion and temperature. Gravitational-wave detectors add a new way to observe.

For a feel of how light gathering works, the 10-metre Keck telescopes collect about 2.5 million times more light than the unaided eye (a pupil of about 7 mm), and light-gathering power is proportional to the area of the mirror, so doubling the diameter gives four times the light. Resolution depends on aperture and wavelength, which is why radio telescopes need huge dishes or arrays, such as the Very Large Array and ALMA, working together as interferometers.

Observing in different wavelengths reveals different things: infrared sees through dust to newborn stars, X-rays reveal hot gas near black holes, and radio waves map hydrogen in galaxies. James Webb, with a 6.5-metre gold-coated mirror, sits about 1.5 million km from Earth and stays cold to detect faint infrared light. The Event Horizon Telescope linked dishes across the planet to image a black hole. A misconception is that telescopes mainly magnify; their key job is to collect light. Also, the Hubble images are often false-colour, with colours assigned to represent different wavelengths or elements.