An instrument designed to achieve high angular resolution by connecting two or more telescopes or antennas (elements) to operate as a single instrument. The signals from each element are combined to form an interference pattern. The maximum resolution of the array is then determined not by the size of the individual elements, but by their maximum separation. Interferometers are widely used in radio astronomy, and also in infrared and optical astronomy. At optical and infrared wavelengths, the incoming beam is split and then recombined with itself to form an interference pattern. This can give high resolution of spectral lines, as in a Fabry–Perot interferometer. See also Optical Interferometer; Radio Interferometer.