3.58 m New Technology Telescope (NTT) in Chile, which commenced operation in March 1988 (1). The instrument employs 75 adjustable pressure pads on the back of the primary to modify automatically the shape of the mirror when it is in different positions (1). ADAPTIVE OPTICS Images of astronomical objects are blurred and degraded by atmospheric turbulence . "Adaptive optics" is a technology for sharpening turbulence-degraded images, by using fast-moving, flexible mirrors to "unscramble" the optical distortion and thereby improve the angular resolution . The key elements of an adaptive optics system are a wave-front sensor, an adaptive mirror, and a control computer, as shown in the diagram below. Let us talk through this diagram to explain how the system works. An optical wavefront passing through air is distorted by turbulence. The light is collected by the telescope, and fed to the adaptive optics system . The wave-front sensor measures the distortion, the control computer calculates the mirror shape needed to remove the distortion, and this correcting shape is applied to the adaptive mirror by a series of fast-acting actuators , to reconstruct the undistorted image. This procedure is repeated about 1000 times per second, to track the rapidly varying turbulence. It is this speed that is the major difference between adaptive and active optical systems. In correcting the distorted image, the system uses as a reference either a real guide star in the field of view or an artificial guide star created by laser light backscattered off air molecules in