Basic Astrometric Methods

William van Altena

Yale UniversityBasic Astrometric Methods

Yale University

July 18-22, 2005

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

Expectation for space-based limits

• SIM (103 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 (109 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

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

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.