Editing Angular resolution (section)

===Single telescope===

Point-like sources separated by an [[angle]] smaller than the angular resolution cannot be resolved. A single optical telescope may have an angular resolution less than one [[arcsecond]], but [[astronomical seeing]] and other atmospheric effects make attaining this very hard.

The angular resolution ''R'' of a telescope can usually be approximated by
:<math>R = \frac {\lambda}{D} </math>
where ''λ'' is the [[wavelength]] of the observed radiation, and ''D'' is the diameter of the telescope's [[Objective (optics)|objective]]. The resulting ''R'' is in [[radian]]s. For example, in the case of yellow light with a wavelength of 580&nbsp;[[nanometer|nm]], for a resolution of 0.1 arc second, we need D=1.2&nbsp;m. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.

This formula, for light with a wavelength of about 562&nbsp;nm, is also called the [[Dawes' limit]].