Metrology and pointing will be enabling technologies for a new generation of astronomical missions having large and distributed apertures and delivering unprecedented performance. The UV interferometer Stellar Imager would study stellar dynamos by imaging magnetic activity on the disks of stars in our Galaxy. The X-ray interferometer Black Hole Imager would study strong gravity physics and the formation of jets by imaging the event horizons of supermassive black holes. These missions require pointing to microarcseconds or better, and metrology to nm accuracy of optical elements separated by km, for control of optical path difference. This paper describes a metrology and pointing system that meets these requirements for the Stellar Imager. A reference platform uses interferometers to sense alignment with a guide star. Laser gauges determine mirror positions in the frame of the reference platform, and detector position is monitored by laser gauges or observations of an artificial star. Applications to other astronomical instruments are discussed.
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