Probing micro-arcsec astrometry with NACO

Relative astrometric measurements with a precision far better than 1 mas (milli-arcsec) are commonly regarded as the domain of interferometry. Pioneering work by Pravdo & Shaklan (1996), made in the optical, reached a precision of 150 micro-arcsec in direct imaging but is ultimately limited by atmospheric turbulence and differential chromatic refraction (DCR) effects. Neuhaeuser et al. (2006, 2007) demonstrated that AO assisted observations with NACO in a near-infrared narrow band filter allow measurements with a precision of ~50 muas (micro-arsec) on a 0.6 arcsec binary within one hour and are unaffected by DCR effects. This opens new possibilities for astrometric detections of extrasolar planets and the determination of their true masses. We discuss here how to improve the measurements and address the necessary calibrations.