For many years astronomers have lived with the reality of observing the stars and planets through the turbulence of the Earth's atmosphere. The launch of the Hubble space telescope attests to the value which the astronomical community places on obtaining sharp images, free of atmosphere-induced distortion. However, recent advances in adaptive optics and digital signal processing may provide a solution to the problem of atmospheric turbulence blurring groundbased telescope images. This paper will provide an example of the large computational effort required to provide ground-based telescopes with real-time adaptive optical elements.
The atmosphere is an unstable medium that exhibits variations in index of refraction as a function of position and time. These index variations introduce phase distortions on light waves, with the end result being beam spreading, blurring, and twinkling.
To achieve the sharpest image of a distant star or planet it is necessary to measure and correct the phase distortions. The corrections must be performed at a rapid rate compared to the rate of change of the atmosphere, approximately 1 ms. It is the role of an adaptive optical system to perform these measurements and corrections.
Ground-based laser installations may also benefit from adaptive optical systems. Light from ground-based lasers can be precorrected before transmission through the atmosphere to achieve minimum beam spreading and maximum energy delivery.
The scenario is relatively straightforward. A beacon, or known illumination signal, is sent through the atmosphere in the direction of the telescope. This beacon allows measurement of the atmosphere-induced distortion to be performed by measuring the slope of the incoming wavefront over small local regions. These local slopes (in the 3c and y directions)