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The Mark III Astrometric Interferometer

The Mark III stellar interferometer is a phase coherent long baseline telescope Michelson stellar interferometer that has been under construction since 1984. The instrument is designed primarily for accurate wide angle astrometry at the milliarcsecond level. It will use two color measurements to reduce the error due to atmospheric turbulence and a 20-meter baseline, 2.5 milliarcsecond resolution, that is laser monitored to achieve high precision. Initial operation of the Mark III is scheduled for late 1986. The Mark III interferometer uses a number of active subsystems to improve the ease of operation of the stellar interferometer. The siderostat pointing system uses laser interferometers for open loop control and an image correlation star tracker for closed loop control. The laser system, in addition to pointing the instrument, will be used to measure the errors in the gears as well as thermal drift. An optical delay line is used to equalize the path lengths in the interferometer. The delay line is programable in 50Å steps with 100A accuracy over a peak to peak range of 20 meters (1 part in 109) with maximum slew rate of 60 cm/sec. A Kalman filter phase estimator will be developed for the stellar fringe tracker. In addition to astrometry, we are planning to build additions to the instrument for optical aperture synthesis. Operation in the phased imaging mode for bright objects as well as the amplitude mode for photon starved objects are eventually planned with a multispectral, multi-r° aperture fringe detector and high speed, real time signal processor.