Abstract

The Mark III interferometer is a two-element stellar interferometer with four fixed base lines that uses active optical systems to keep optical path differences well below a wavelength of light for phase coherent operation. The instrument is designed to be used on a regular basis for astronomical observations as well as a test bed for technology development of instruments for phase coherent interferometric imaging and astrometry. Open loop pointing of the two siderostats is accurate to ~10 sec of arc, within the field of view of the star tracker. Closed loop operation of the star trackers should coalign the beams to ~0.2 sec of arc. As the interferometer base line rotates with the earth, the time varying optical path difference between the two siderostats and the star is compensated by an optical delay line. The delay line is controlled by a laser interferometer and can be set with an accuracy of ~10 nm over a range of 20 m. The optical path difference from the beam combiner to the star via the two arms is determined by measuring the phase of the stellar fringe pattern. The stellar fringe tracker is designed to compensate for optical path fluctuations due to turbulence to ~λ/8. Initially, the interferometer will be used for wide angle astrometry. The addition of an imaging capability is planned.

© 1986 Optical Society of America