The capability to perform Phased-Reference Imaging and Narrow-Angle Astrometry with the VLTI will be given by the PRIMA instrument, which is based on the simultaneous observation of two celestial objects. A highly accurate metrology system is required to monitor the optical path followed by the two objects inside the VLTI. In the framework of PRIMA, this paper reviews the requirements as well as the constraints for such a metrology system and discusses its implementation.
Samuel A. Leveque,
"Metrology for phase-referenced imaging and narrow-angle astrometry with the VLTI", Proc. SPIE 4006, Interferometry in Optical Astronomy, (5 July 2000); doi: 10.1117/12.390231; https://doi.org/10.1117/12.390231