Astronomical interferometer

An instrument combines signals from multiple telescopes to observe astronomical objects with resolution finer than a single telescope’s can provide. Its effective resolving power depends chiefly on the separation between the telescopes.

Interferometry: A measurement method that uses interference between waves to infer properties of their sources or paths. The instrument applies this broader method to electromagnetic waves arriving from astronomical sources.
Electromagnetic radiation: Energy that propagates through space as coupled electric and magnetic fields. Astronomical interferometers combine electromagnetic signals gathered from celestial sources.
Very Long Baseline Array: A continent-spanning network of ten radio telescopes operated by the United States National Radio Astronomy Observatory. Its long baselines deliver exceptionally fine radio angular resolution.
Single-dish radio telescope: A radio telescope that collects radiation with one primary reflecting surface. Its resolution is set mainly by dish diameter rather than separation between collectors.
Baseline (interferometry): The separation vector between two observing elements in an interferometer. Baseline length sets the angular scale an astronomical array can resolve.
Coherence (physics): A measure of the stable phase relationship between waves at different positions or times. Correlated signals require enough coherence across the telescope array to preserve interference.
Event Horizon Telescope: A global millimeter-wavelength array that produces images of horizon-scale structures around supermassive black holes. It combined widely separated observatories to resolve the shadows of M87* and Sagittarius A*.
Optical telescope: A telescope that collects and focuses visible light using lenses, mirrors, or both. A single optical telescope avoids the signal-combination demands of an array but has a smaller aperture.
Visibility (interferometry): A complex quantity describing the amplitude and phase of an interferometer's measured spatial coherence. Each telescope pair samples a visibility that encodes structure across the sky.
Angular resolution: The smallest angular separation between details an observing system can distinguish. Interferometer baselines can improve this measure beyond a single telescope's aperture.

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