for telescopes with diameters larger than "r", the image resolution is determined primarily by the atmosphere and is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to "r". "r" also corresponds to the length-scale over which the turbulence becomes significant (10–20 cm at visible wavelengths at good observatories), and "t" corresponds to the time-scale over which the changes in the turbulence become significant. "r" determines the spacing of the actuators needed in an adaptive optics system, and "t" determines the correction speed required to compensate for the effects of the atmosphere.