Preliminary measurements of star positions with the Mark III stellar interferometer

Preliminary astrometric measurements with the Mark III optical stellar interferometer using a 12.005 m north-south baseline were made during seven nights in November 1986. A total of 22 stars were observed over large angles in a 3000-A wide band centered at 7000 A. As expected, the estimated precision of the relative positions increases with hour-angle coverage. For the ten stars observed over at least 90 deg of hour angle, the right ascensions are consistent at the 5 ms of time (1sigma) level, while the uncertainties in declination are at the 22 mas (1sigma) level. With this small data set, the question of systematic errors cannot be fully examined. A discussion of observational techniques, errors, limitations of this system, and future plans is presented.