An introduction to the Guide-2 telescope testbed for the SIM Planet Quest Light mission

The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror (FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet) system.