4 YEARS OF ASTROMETRIC MEASUREMENTS WITH THE MARK-III OPTICAL INTERFEROMETER

Repeated measurements of the eleven FK5 stars selected by Shao et al. [AJ, 100, 1701 (1990)] were performed with the north-south and east-so uth astrometric baselines of the Mark III optical interferometer in or der to estimate the accuracy of wide-angle astrometry. Even though the declination range of these stars is insufficient to determine absolut e declinations, we were able to determine corrections to the FK5 posit ions at four epochs with an accuracy of about 13 milliarcseconds (mas) in declination and 23 mas in right ascension. Measurements at two dif ferent wavelengths were used to correct for refractive index fluctuati ons in the turbulent atmosphere. The pathlength difference between the two arms of the interferometer was monitored during the night with an internal white-light interferometer. The accuracy of the positions is limited by systematic errors due to unmonitored changes in the baseli ne coordinates and due to low-frequency water vapor fluctuations. Howe ver, these results demonstrate the potential of future optical interfe rometers for the measurement of stellar positions with mas accuracy.