An active control system is described for the automatic alignment of the mirrors in a two-beam interferometer. From an initial unaligned position the active control system determines the degree of misalignment and adjusts the relative mirror positions accordingly. An embodiment of the system is described for a Michelson interferometer in which one of the mirrors is mounted upon three piezoelectric transducers PZT arranged so that they can both tilt and retard the mirror. Laser sources and corresponding photodetectors are also incorporated such that a control system can use the PZT actuators to produce a series of mirror movements relative to the fixed mirror and give a set of two-dimensional diffraction patterns one for each of the laser photodetectors. Amplitude and phase information is extracted from these patterns which enables the control system to align the mirrors such that the diffraction pattern maxima is at the center of the instrument central photodetector. In a further stage of the control algorithm, the alignment accuracy is refined using information from the laser photodetectors during retardation of the mirror. In this manner, the initial mirror alignment, maintaining that alignment during retardation and diagnosis of possible misalignment, become part of a single active control technology for instrument alignment.