Deep Astrometric Standards and Galactic Structure

The advent of next‐generation imaging telescopes, such as the Large Synoptic Survey Telescope (LSST) and the Panoramic Survey Telescope and Rapid Response System (Pan‐STARRS), has revitalized the need for deep and precise reference frames. The proposed weak‐lensing observations with these facilities put the highest demands on image quality over wide angles on the sky. It is particularly difficult to achieve a subarcsecond point‐spread function on stacked images, where precise astrometry plays a key role. Current astrometric standards are insufficient to achieve the science goals of these facilities. We thus propose the establishment of a few selected deep (V = 25) astrometric standards (DAS). These will enable a reliable geometric calibration of solid‐state mosaic detectors in the focal plane of large ground‐based telescopes, and will make a substantial contribution to our understanding of stellar populations in the Milky Way. In this paper we examine the need for such standards and discuss the strategy for selecting them and their acquisition and reduction techniques. The feasibility of DAS is demonstrated by a pilot study around the open cluster NGC 188, using the Kitt Peak National Observatory 4 m CCD Mosaic camera, and by Subaru Suprime‐Cam observations. The goal of reaching an accuracy of 5–10 mas in positions and obtaining absolute proper motions good to 2 mas yr−1 over a several square‐degree area is challenging, but reachable with the NOAO 4 m telescopes and CCD mosaic imagers, or a similar setup. Our proposed DAS aims to establish four fields near the Galactic plane, at widely separated coordinates. In addition to their utilitarian purpose for DAS, the data we will obtain in these fields will enable fundamental Galactic science in their own right. The positions, proper motions, and VI photometry of faint stars will address outstanding questions of Galactic disk formation and evolution, stellar buildup, and mass assembly via merger events.