Knowledge of the aberrated wavefront is the critical input for an adaptive optics system. Measuring the wavefront requires sufficient signal to keep the estimation error low. An astronomical adaptive optics system wavefront sensor requires a sufficiently bright source in the direction of the science object. If a natural star or the science object itself is bright enough then we use that. If it is not, then up until the mid-1980s we would all have just gone home. However, because of the development of artificial laser guide stars, we don't get to go home anymore; we get to shine lasers up into space. Saying the guide star is "bright enough" has some definite meaning: It must be bright enough (have enough photons that reach our sensor) so that the signal to noise ratio is high enough.
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