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Measurements of the isopistonic angle using masked aperture interferometry

The isopistonic angle is the angle over which the atmospheric piston can be considered constant. It is an important parameter for ground based optical interferometry, particularly in the context of phasing using guide stars. This is analogous to the isoplanatic angle, which is the angle over which an adaptive optics correction may be considered valid for a single aperture. While the isopistonic angle is well understood theoretically, there have been relatively few measurements of it as compared to the isoplanatic angle. We have used masked aperture interferometry on binary stars of various angular separations at the USNO Flagstaff astrometry telescope to measure the isopistonic angle. We report on our measurement methodology, data processing, and experimental results.