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The term adaptive optics (AO) indicates systems that correct distortions from the atmosphere using input from a sensor that measures the distortion. Typically, the sensor tracks the distortion (e.g., a slight mislocation) of a known fixed source, such as a star (guide star) or an artificial source of light reflected off the atmosphere itself from a laser (termed an LGS for laser guide star). This fixed source must be near the line of sight to the object under observation. The same distortion presumably affects objects in that area of the celestial sphere, including the observation target. Modern techniques use multiple guide stars and algorithms for better performance. Among the techniques are laser tomography AO (LTAO), using multiple LGSs, and multi-conjugate AO (MCAO), typically using multiple LGS plus a wavefront sensor, modeling sources of distortion at multiple distances from the telescope to better devise the optics adjustments to compensate. Single-conjagate AO (SCAO) indicates use of the wavefront sensor but without modeling more than one source of distortion. Extreme AO (XAO) uses advanced techniques and concentrates more effort on a single point within the image and is associated with the use of a coronagraph, e.g., for best possible AO for direct imaging of an extra-solar planet. Some current and planned telescopes using adaptive optics: