Precision astrometry with adaptive optics

We discuss the limits of ground-based astrometry with adaptive optics based on experiments using the core of the Galactic globular cluster M5. We have recently achieved less than or similar to 100 microarcsecond astrometric precision and accuracy at the Hale 200-inch telescope. Here we apply the same experimental design considerations and optimal estimation technique to explore the astrometric precision of the Keck II telescope. We find that high-precision astrometry at approximate to 50 microarcsecond level is possible at Keck in 20 seconds. We discuss the potential of differential astrometry for current and next generation large aperture telescopes based on these results.