Angular resolutionAngular resolution
tells the resolving power of a telescope
. Point-like sources separated by an angle
smaller than the angular resolution can not be resolved. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.
Highest angular resolutions can be achieved by interferometry.
A single optical telescope has an angular resolution less than one arcsecond, but seeing and other atmostpheric effects make reaching this very hard.
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.