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The approximate nature of the adaptive-optics time constant measurements withMASS is examined. The calibration coefficient C derived from numericalsimulations of polychromatic scintillation shows dependence on the height ofthe turbulence layer, wind speed, and seeing. The previously recommended valueof C=1.27 is a good match to typical conditions, while C can vary from 0.6 to1.6 in other circumstances. For two nights, MASS was compared with the timeconstant measured with adaptive optics, and the expected agreement was found.We show that the single-layer approximation used in some AO systems to derivethe AO time constant can give wrong results. A better approach is to estimateit from the speed of focus variation (the FADE method). The analysis of thespeed of scintillation developed recently by V.~Kornilov will lead to moreaccurate measurements of the AO time constant with MASS.

Atmospheric time constant with MASS and FADE