Editing Optical telescope (section)

===Angular resolution===
Ignoring blurring of the image by turbulence in the atmosphere ([[astronomical seeing|atmospheric seeing]]) and optical imperfections of the telescope, the [[angular resolution]] of an optical telescope is determined by the diameter of the [[primary mirror]] or lens gathering the light (also termed its "aperture").

The [[Angular resolution#The_Rayleigh_criterion|Rayleigh criterion]] for the resolution limit <math>\alpha_R</math> (in [[radian]]s) is given by
:<math>\sin(\alpha_R) = 1.22 \frac{\lambda}{D}</math>
where <math>\lambda</math> is the [[wavelength]] and <math>D</math> is the aperture. For [[visible light]] (<math>\lambda</math> = 550&nbsp;nm) in the [[small-angle approximation]], this equation can be rewritten:
:<math>\alpha_R = \frac{138}{D}</math>
Here, <math>\alpha_R</math> denotes the resolution limit in [[arcsecond]]s and <math>D</math> is in millimeters.
In the ideal case, the two components of a [[double star]] system can be discerned even if separated by slightly less than <math>\alpha_R</math>. This is taken into account by the [[Dawes limit]]
:<math>\alpha_D = \frac{116}{D}</math>
The equation shows that, all else being equal, the larger the aperture, the better the angular resolution. The resolution is not given by the maximum [[magnification]] (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

For large ground-based telescopes, the resolution is limited by [[astronomical seeing|atmospheric seeing]]. This limit can be overcome by placing the telescopes above the atmosphere, e.g., on the summits of high mountains, on balloons and high-flying airplanes, or [[space telescope|in space]]. Resolution limits can also be overcome by [[adaptive optics]], [[speckle imaging]] or [[lucky imaging]] for ground-based telescopes.

Recently, it has become practical to perform [[aperture synthesis]] with arrays of optical telescopes. Very high resolution images can be obtained with groups of widely spaced smaller telescopes, linked together by carefully controlled optical paths, but [[List of astronomical interferometers at visible and infrared wavelengths|these interferometers]] can only be used for imaging bright objects such as stars or measuring the bright cores of [[active galaxies]].