1. Basic Astrometric Methods William van Altena Yale University Basic Astrometric Methods Yale University July 18-22, 2005

3. Current limits to imaging precision Space-based limits –HST ~ ±1 mas/position FGS, WFPC2 & ACS (Benedict, et al., Anderson & King, Drukier, et al.) Ground-based limits –OPTIC ~ ±2 mas/position In R-band at WIYN with Tip-Tilt correction (Vieira & van Altena) Projection of ±1 mas in Z- band

4. Expectation for space-based limits SIM (10 3 stars to V=20) –± 4 µas parallaxes 10 s.d. π @ 25 kpc 25 s.d. π at Gal. Cen. 5 s.d. π at Mag. Clouds –± 2.5 mas/yr pm’s ± 5 km/s at 400 kpc Gaia (10 9 stars to V=20) –± 10 parallaxes 10 s.d. π at 10 kpc 12 s.d. π at Gal. Cen. 2 s.d. π at Mag. Clouds –± 4 mas/yr pm’s ± 5 km/s at 250 kpc

5. Science potential for Astrometry Definitive –Distances to: Anything in the Galaxy (Globular clusters & Cepheids) The Galactic Center The Magellanic Clouds –Velocities of: Magellanic Clouds Remnant merger streams

6. The Challenges HST increase was projected at 10x –Very difficult to achieve. Increase in projected precision by 100x –We will need outstanding astrometrists now and in the near future to achieve/approach those goals. –US Naval Observatory Flagstaff is looking for astrometrists now.