  Temporal fluctuations of the atmospheric piston are critical for
interferometers as they determine their sensitivity. We characterize an
instrumental set-up, termed the piston scope, that aims at measuring the
atmospheric time constant, tau0, through the image motion in the focal plane of
a Fizeau interferometer. High-resolution piston scope measurements have been
obtained at the observatory of Paranal, Chile, in April 2006. The derived
atmospheric parameters are shown to be consistent with data from the
astronomical site monitor, provided that the atmospheric turbulence is
displaced along a single direction. Piston scope measurements, of lower
temporal and spatial resolution, were for the first time recorded in February
2005 at the Antarctic site of DomeC. Their re-analysis in terms of the new data
calibration sharpens the conclusions of a first qualitative examination.
