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.