Adaptive optics
Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry itself. The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts of the wavefront in two dimensions (equal to correction of the position offsets for the image). This is performed using a rapidly moving tip-tilt mirror which makes small rotations around two of its axes.