Full Aperture Metrology for High Precision Astrometry

POINTS (Precision Optical INTerferometry in Space) is a proposed dual astrometric interferometer with a nominal measurement accuracy of 5 microarcseconds for pairs of stars approximately 90 degrees apart. Each interferometer measures the arc-second-scale offset of its optical axis from a target star, and a metrology system, described by Phillips and Reasenberg (in this Digest), measures the approximately 90 degree angle between the two interferometers. To achieve the nominal accuracy will require monitoring the positions of the critical optical elements to a few picometer (pm). Even with the most stable materials and active temperature control, the larger optical elements, especially the 25 cm primaries, will experience thermal distortions much larger than this limit. In one subsystem, required in both interferometers, a novel technique called full aperture metrology is used to measure the appropriate average optical path length and correct for any changes. Laser light is injected into the interferometer at the primary beamsplitter and follows the starlight paths, but in the reverse direction, illuminating the active area of each optical element back to the primary mirrors. Most of the metrology light is sent out toward the target stars, but shallow phase-contrast zone plates on the primaries focus a few percent of the metrology light to axial points near the specular foci of the mirrors. The two metrology beams are collimated by athermal lenses and interfere at an auxiliary beamsplitter. The phase of the interference measures the path length difference between the two arms of the interferometer and is used to drive a null servo.