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The temporal properties of atmospheric seeing have become important parameters in the design of new-technology instruments such as adaptive wavefront correctors and optical interferometers. Surprisingly, very few long-term studies of these properties have been made at optical wavelengths. An example of how little is known about this topic is the fact that, although it is ``well known'' that the coherence time of the seeing at a good astronomical site is about 10 milliseconds, few people are clear as to which definition of the coherence time is being referred to, nor can they give an example of a site where such a measurement has been made. In fact, different definitions of the coherence time can give numerical values that differ by a factor of 10 for the same seeing, and measurements of coherence times at world-class astronomical sites have yielded values as small as 3 milliseconds for supposedly typical nights.
We present here measurements of the coherence time from the Mark~III optical interferometer on Mt Wilson, CA. The Mark~III incorporates a servo system which actively tracks the atmospherically-induced motion of the stellar fringes, and therefore produces temporal seeing information as a byproduct of every stellar observation. Furthermore, the Mark~III interferometer was in near-continuous operation from 1989 to the end of 1992, and therefore data are available from all times of year and under all astronomical observing conditions. We have written software to automatically analyze these data to produce measurements of the coherence time ($t_0$) of the atmospheric wavefront perturbations, as well as the spectral index of the high-frequency temporal spectrum of the fluctuations. At the time of writing of this abstract, one year's worth of data, comprising more than 180 nights have been reduced, and it is expected that several more years' worth will have been analyzed by the time of the meeting. These provide a database from which to analyze the statistics of the seeing in unprecedented detail. It will allow us to determine for at least one site what is meant by the term ``typical seeing coherence time''.
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