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Point-like sources separated by an angle smaller than the angular resolution can not be resolved. A single optical telescope has an angular resolution less than one arcsecond, but seeing and other atmospheric effects make attaining ing this very hard. The highest angular resolutions can be achieved by interferometry.
The angular resolution of a telecope can usually be approximated by R = L/D where L is the wavelength of the observed radiation and D is the diameter of the telescope. The resulting R is in radians. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.