Galactic structure from wide-field surveys

The possible contributions of a wide-field space imager, such as might be designed for a JDEM mission (such as SNAP or DESTINY), to Galactic structure are explored. The Sloan Digital Sky Survey serves as an example of an extragalactic survey that has made important contributions to our understanding of the distribution of Galactic stars, showing in particular how imaging alone can make major contributions. Since we expect the imaged sources in JDEM to be faint (20 < V < 30), the images will sample Galactic stars fainter than G dwarfs, including white dwarfs and brown dwarfs. Many of these stars have evolved little, and are good tracers of stellar formation histories. A faint imager will discover large numbers of faint main sequence stars and brown dwarfs. Brighter stars will be sampled in other galaxies. There is also great promise for the astrometric capability of a wide-field imager in space.