Telescope

A telescope collects and focuses electromagnetic radiation to reveal distant objects or signals. Different designs use lenses, mirrors, antennas, or detectors suited to particular wavelengths.

Objective (optics): The primary lens or mirror that gathers incoming radiation and forms an image. Its collecting area sets how much light an optical telescope can capture.
Electromagnetic spectrum: The full range of electromagnetic radiation, ordered by frequency or wavelength. Telescopes are designed around particular portions of this spectrum.
Optical telescope: A telescope designed to collect and focus visible light. It is the familiar form used to observe stars, planets, and galaxies in visible wavelengths.
Refracting telescope: An optical telescope that uses lenses to gather and focus light. It is the lens-based design, contrasted with telescopes that use primary mirrors.
Galileo's telescope: The refracting telescope Galileo used for astronomical observations beginning in 1609. Its observations of Jupiter’s moons and Venus challenged Earth-centered cosmology.
Focal length: The distance over which an optical system brings parallel rays to a focus. It governs image scale and helps determine a telescope’s magnification.
Electromagnetic radiation: Energy that propagates through space as oscillating electric and magnetic fields. It is the signal telescopes collect, whether visible light, radio waves, or other wavelengths.
Infrared astronomy: The study of celestial objects through their infrared radiation. Infrared telescopes can reveal cool objects and sources obscured by dust.
Reflecting telescope: An optical telescope that uses mirrors to gather and focus light. It avoids the chromatic aberration that affects simple lens-based designs.
Hubble Space Telescope: A space observatory launched in 1990 that observes primarily ultraviolet, visible, and near-infrared light. Its sharp images helped establish the scale and evolution of the universe.

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