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OAST: a testbed for cophasing instrumentation

The Optical Aperture Synthesis Technologies project is aimed at the development of technologies for space interferometry. One of the key technologies is the path length stabilization (or cophasing) of light beams from a guide star. A setup has been developed for testing a system for controlling the cophasing (a delay line) in combination with a system for measuring the cophasing (a cophasing interferometer). The light from a simulated guide star passes a delay line system and it is imaged by the cophasing interferometer on a detector. From the captured (white light) fringe patterns the optical path difference (OPD) is determined in real time. The OPD data are used to drive the delay lines in a control loop system for fast stabilization of the optical paths to a sub-wavelength accuracy. The major design drivers for the testbed development were the optimal mechanical and thermal stability (nanometer level), the control loop bandwidth, the OPD stabilization accuracy and the wavefront quality. The instrumentation (optomechanical breadboard and control system) that has been developed for this project is described, and a first set of test results.