Telescope

A telescope is an instrument designed to observe distant objects by collecting electromagnetic radiation and producing a magnified image. It increases the brightness of faint objects and reveals details that are not visible to the unaided eye. The concept applies across wavelengths from visible light to radio, infrared, and beyond.

Most optical telescopes use an objective element to gather light and form an image at a focal plane, where an eyepiece or detector magnifies or records it. The main specifications are aperture, the diameter of the light-collecting element, which determines light-gathering ability and angular resolution, and focal length, which influences magnification. Practical telescopes also rely on precise mounts and tracking systems to follow celestial objects as the Earth rotates.

There are several major types. Refracting telescopes use lenses to bend and focus light; reflecting telescopes use mirrors to collect and direct light; and catadioptric designs combine lenses and mirrors in a compact form. Other classes include radio telescopes, which observe radio waves, and space-based telescopes, which operate above Earth's atmosphere to avoid atmospheric distortion. Telescopes range from small amateur instruments to large professional facilities, often mounted on alt-azimuth or equatorial mounts and equipped with detectors, spectrographs, and adaptive optics to improve image quality.

Historically, telescopes were developed in the early 17th century, with Galileo popularizing their astronomical use. Key advances include Newton’s reflecting telescope and modern large-aperture facilities. Telescopes underpin much of astronomy, enabling planetary studies, deep-sky surveys, spectroscopy, and astrometric measurements, and continuing to expand our understanding of the universe.