The smallest angular separation between two point sources that a telescope can distinguish as separate objects. Measured in arcseconds, it determines how much fine detail an instrument can resolve and is fundamentally limited by the telescope's aperture size and the wavelength of light observed.
From Latin 'angulus' (corner, angle) and 'resolutio' (a loosening, solving). The concept emerged in the 19th century as telescope makers sought to quantify their instruments' ability to separate close double stars, leading to precise mathematical definitions of optical performance based on wave theory.
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