An inertial adaptive optics system must have a way to sense the wavefront with enough spatial resolution and speed to apply a real-time correction . The direct approach employs a step where there is an explicit determination of the phase or optical path difference (OPD) of the wave. After the wavefront is reconstructed, the information is used as feedback to correct the unwanted components of the phase. The indirect approach never reduces the information to an explicit indication of the wavefront; rather, it translates information related to the phase into signals that are used to compensate for the wavefront. For example, the multidither system mentioned in Section 4.2 is an inertial indirect system.