Astrometry Survey Missions beyond the Magnitude Limit

Three planned astrometry survey satellites, the Full-Sky Astrometric Mapping Explorer ( FAME ), DIVA , and GAIA , aim at observing magnitude-limited samples. We argue that substantial additional scientific opportunities are within the reach of these missions if they devote a modest fraction of their catalogs to selected targets that are fainter than their magnitude limits. We show that the addition of (10 6 ) faint ( R > 15) targets to the 40 × 10 6 object FAME catalog can improve the precision of the reference frame by a factor of 2.5, to 7 μas yr -1 , increase the Galactocentric distance at which halo rotation can be precisely (2 km s -1 ) measured by a factor of 4, to 25 kpc, and increase the number of late M dwarfs, L dwarfs, and white dwarfs with good parallaxes by an order of magnitude. In most cases, the candidate quasars, horizontal-branch stars, and dim dwarfs that should be observed to achieve these aims are not yet known. We present various methods to identify candidates from these classes and assess the efficiencies of these methods. The analysis presented here can be applied to DIVA with modest modifications. Application to GAIA should be deferred until the characteristics of potential targets are better constrained.