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J. Rhee, C. L. Slesnick, J. D. Crane, A. A. Polak, R. J. Patterson, S. R. Majewski (UVa), A. Kundu (Yale), W. E. Kunkel (LCO), D. Geisler, R. Munoz, J. Arenas, J. Seguel, W. Gieren (U. Concepcion), V. V. Smith (UTEP), G. F. Benedict (McDonald Obs.), K. V. Johnston (Wesleyan)
To perform wide-angle astrometry, SIM requires an astrometrically stable, precision reference frame of stars, the Astrometric Grid. The Grid must be composed of relatively bright stars (V \lesssim 13) to minimize impact on SIM's observing schedule. Subsolar metallicity K giants are good candidate stars for the SIM Astrometric Grid, because they are both relatively common at V \lesssim 13 and because they are luminous and therefore distant, so that any problems related to astrometric jitter (from large planets, starspotting, and flaring) is minimized. The goal of the GGSS is to discover the most distant metal-poor K giants in 1306 ``bricks'', each covering ~.5 deg{}2 at a mean separation of ~\circ, distributed over the entire sky. To date, photometric observations with the Washington M, T2 and the (gravity sensitive) DDO51 filters have been performed in 880 bricks at the Las Campanas 1-m Swope telescope and the 0.8-m telescope of McDonald Observatory. Semi-automated reduction pipelines generate a sample of giant candidates, along with photometric parallaxes and metallicities derived from the three-filter system. In each brick, up to four candidate Grid stars are selected as those K giants with M<13.5 having the largest distances. Preliminary results show that 93% of bricks have at least one Grid candidate, with median photometric distance and metallicity of d ~3.6 kpc and [Fe/H] ~-1.0, respectively. Follow-up, low resolution spectroscopy enables us to verify the luminosity class and determine spectroscopic abundances and radial velocities of the candidate giants. At present, we have obtained spectra of over 1600 Grid candidates in the wavelength range 4800 -- 6800 Å\ by using the Modular Spectrograph on the Swope telescope. Here we present preliminary results of the GGSS and discuss several checks on the derived photometric properties of the Grid candidates.