= Solution
<Adaptive-optics sky coverage> is limited by the need for a <guide star> bright enough to measure the <wavefront error> on the atmospheric evolution time. A faint guide gives noisy measurements; a guide too far from the target samples a different turbulent column. That angular mismatch is <anisoplanatism>, with useful separation characterized by the <isoplanatic angle>. Thus a conventional single-guide system cannot provide equally good correction at every target position.
A <laser guide star> supplies an artificial bright reference and improves coverage. However, its finite-distance beam does not sample the full stellar turbulence column, and conventional laser systems need a natural reference for absolute image motion. The required reference brightness, angular separation, and desired correction quality therefore still constrain observations.
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