Jeffrey Meisner

Optical Engineering v35, #7, pp. 1927-1935 (1996)

Technical note:

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they have been scanned at 600 dpi. I recommend
you right click

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**ABSTRACT**

The performance of a long-baseline optical stellar interferometer is greatly enhanced if the instantaneous atmospheric delay tau(t), can be tracked to within a fraction of a wavelength, permitting coherent integration of the optical correlation (fringe visibility). Real-time fringe- tracking involves a control system that servos a rapidly responding path-length compensator in real-time.

However precise delay-tracking can be achieved at somewhat lower signal levels by employing an off-line delay-tracking system, in which the raw data measured by the interferometer is stored for subsequent analysis. Then the estimate of tau at time t, is based on data collected both before and after time t.

An optimum delay-tracking algorithm embraces the a priori statistics
of the atmospheric delay process. Rather than simply estimating tau
at a point in time, a superior estimate of tau will be obtained by comparing
all possible functions, tau(t), over a time period. Using Bayes'
theorem, the a posteriori probability density of any tau(t) function
can be determined. An algorithm has been developed which determines
one or more functions which maximize that probability. Even the ambiguous
estimates which result at lower signal levels, can be employed for the
coherent integration of optical correlation.