The Micro-Arcsecond Metrology (MAM) testbed is a ground-based system of optical and electronic equipment for testing components, systems, and engineering concepts for the Space Interferometer Mission (SIM) and similar future missions, in which optical interferometers will be operated in outer space. In addition, the MAM testbed is of interest in its own right as a highly precise metrological system.

The designs of the SIM interferometer and the MAM testbed reflect a requirement to measure both the position of the starlight central fringe and the change in the internal optical path of the interferometer with sufficient spatial resolution to generate astrometric data with angular resolution at the microarcsecond level. The internal path is to be measured by use of a small metrological laser beam of 1,319-nm wavelength, whereas the position of the starlight fringe is to be estimated by use of a charge-coupled-device (CCD) image detector sampling a large concentric annular beam. For the SIM to succeed, the optical path length determined from the interferometer fringes must be tracked by the metrological subsystem to within tens of picometers, through all operational motions of an interferometer delay line and siderostats. The purpose of the experiments performed on the MAM testbed is to demonstrate this agreement in a large-scale simulation that includes a substantial portion of the system in the planned configuration for operation in outer space. A major challenge in this endeavor is to align the metrological beam with the starlight beam in order to maintain consistency between the metrological and starlight subsystems at the system level.

The MAM Testbed is designed to demonstrate concepts for a highly precise optical interferometer and especially to test for agreement between measurements of metrological and white-light interferometer path lengths. At the time of reporting the information for this article, agreement to within 150 pm had been demonstrated.

The MAM testbed includes an optical interferometer with a white light source, all major optical components of a stellar interferometer, and heterodyne metrological sensors. The aforementioned subsystems are installed in a large vacuum chamber in order to suppress atmospheric and thermal disturbances. The MAM is divided into two distinct subsystems: the test article (TA), which is the interferometer proper, and the inverse interferometer pseudo-star (IIPS), which synthesizes the light coming from a dis

tant target star by providing spatially coherent wavefronts out of two mirrors, separated by the MAM baseline, that feed directly into two siderostats that are parts of the TA. The two feed mirrors of the IIPS are articulated (in translation and tilt) in order to simulate stars located at different orientations in space, while still illuminating the TA siderostats. The spectrum of the simulated starlight of the IIPS corresponds to that of a blackbody at a temperature of about 3,100 K.

The figure schematically depicts the optical layout of the MAM testbed. A beam splitter is used as central main beam combiner that brings together light from the two arms of the interferometer to produce interference. A CCD camera records the white-light interference fringes. A delay line is used to adjust the steady component of the optical-path difference (OPD) between the two interferometer arms, while a voice-coil modulator superimposes an oscillating OPD component to scan the OPD for fringe fitting. In addition to the white light source, the IIPS contains a number of auxiliary light sources at different wavelengths that are used as beacons for aligning the optics. One of the auxiliary light sources makes it possible to perform an alternative metrological test in which a full-aperture beam (instead of a pencil beam) is used.

In the MAM testbed as in the SIM interferometer, the starlight beams (in this case, the simulated starlight beams) propagate in annuli that fill most of the apertures of the siderostats. The metrological laser beams propagate concentrically with these annuli within subapertures that are obscured to the starlight beams. The metrological beams are directed to small reference corner-cube reflectors at the centers of the siderostats. The differences between the optical footprints of the metrological and starlight beams put a premium on precise optical alignment.

MAM has recorded and processed data that show agreement between the metrological and starlight paths to better than 150 picometers, using the SIM narrow-angle (1 degree) astrometry observation scenario. This result is consistent with the basic requirement for astrometry on SIM at the 3-microarcsecond level for planet detection around nearby stars.

This work was done by Renaud Goullioud, Braden Hines, Charles Bell, Tsae-Pyng Shen, Eric Bloemhof, Feng Zhao, Martin Regehr, Howard Holmes, Robert Irigoyen, and Gregory Neat of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category.