Overview of the SIM PlanetQuest Light (SIM-Lite) mission concept
  The Space Interferometry Mission PlanetQuest Light (or SIM-Lite) is a new concept for a space borne astrometric instrument, to be located in a solar Earth-trailing orbit. SIM-Lite utilizes technology developed over the past ten years for the SIM mission. The instrument consists of two Michelson stellar interferometers and a precision telescope. The first interferometer chops between the target star and a set of Reference stars. The second interferometer monitors the attitude of the instrument in the direction of the target star. The telescope monitors the attitude of the instrument in the other two directions.   SIM-Lite will be capable of one micro-arc-second narrow angle astrometry on magnitude 6 or brighter stars, relative to magnitude 9 Reference stars in a two degree field. During the 5 year mission, SIM-Lite would search 65 nearby stars for planets of masses down to one Earth mass, in the Habitable Zone, which have orbit periods of less than 3 years. SIM-Lite will also perform global astrometry on a variety of astrophysics objects, reaching 4.5 micro-arc-seconds absolute position and parallax measurements. As a pointed instrument, SIM-Lite will be capable of achieving 8 micro-arc-second astrometric accuracy on 19th visual magnitude objects and 15 micro-arc-second astrometric accuracy on 20th visual magnitude objects after 100 hours of integration.   This paper will describe the instrument, how it will do its astrometric measurements and the expected performance based on the current technology. 
R. Goullioud, J. H. Catanzarite, F. G. Dekens, M. Shao, J. C. Marr IV