SIM astrometric demonstration at 24 picometers on the MAM testbed

Future space-based optical interferometers, such as the Space Interferometer Mission (SIM), require fringe measurements to the level of picometers in order to produce astrometric data at the micro-arc-second level. To be more specific, it is necessary to measure both the position of the starlight central fringe and the change in the internal optical path of the interferometer to a couple of tens of picometers. The internal path is measured with a small metrology beam, whereas the starlight fringe position is estimated with a CCD sampling and a large concentric annular beam. One major challenge for SIM is to align the metrology beam with the starlight beam to keep the consistency between these two sensors at the system level while articulating the instrument optics. The Micro-Arcsecond Metrology testbed (MAM) developed at the Jet Propulsion Laboratory features an optical interferometer with a white light source, all major optical components of a stellar interferometer, and heterodyne metrology sensors. The setup is installed inside a large vacuum chamber in order to mitigate atmospheric and thermal disturbances. Recent data show agreement between the metrology and starlight paths to better than 24 pm in the one-degree narrow-angle field of regard of SIM. This paper describes how MAM processes its narrow-angle data, the testbed performance, and how it relates to SIM.