ATMOSPHERIC DELAY TRACKING IN A LONG-BASE-LINE OPTICAL STELLAR INTERFEROMETER

The performance of a long-baseline optical stellar interferometer is g reatly enhanced if the instantaneous atmospheric delay tau(t) can be t racked to within a fraction of a wavelength, permitting coherent integ ration of the optical correlation (fringe visibility). Heal-time fring e tracking involves a control system that servos a rapidly responding pathlength compensator in real time. However, precise delay tracking c an be achieved at somewhat lower signal levels by employing an off-lin e delay-tracking system, in which the raw data measured by the interfe rometer are stored for subsequent analysis. Then the estimate of tau a t time t is based on data collected both before and after time t. An o ptimum delay-tracking algorithm embraces the a priori statistics of th e atmospheric delay process. Rather than simply estimating tau at a po int in time, a superior estimate of tau will be obtained by comparing all possible functions tau(t) over a time period. Using Bayes's theore m, the a posteriori probability density of any tau(t) function can be determined. An algorithm is developed that determines one or more func tions that maximize that probability. Even the ambiguous estimates tha t result at lower signal levels can be employed for the coherent integ ration of optical correlation. (C) 1996 Society of Photo-Optical Instr umentation Engineers.