The Research and Development Laboratory at BDM Albuquerque, in cooperation with and for the Air Force Weapons Laboratory has designed and developed an optical phase measurement instrument capable of providing a phase error signal with a resolution of 3 nm at an update rate of 1 kHz. This phase error measurement of better than 1/160th of a wave detects the optical path difference (OPD) between two laser beams operating in the 0.5 micron region. The error measurement is used to translate mirrors which adjust the OPD to maintain phase coherence between the beams. By using several mirrors, each corrected to a reference, a large aperture can be synthesized. The 1 kHz update rate provides a high speed control signal for closing the mirror control loop. The design is part of a proof-of-concept program which will identify the feasibility of using feedback control techniques to simulate a large optical aperture with multiple small apertures.