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When stellar light passes through a turbulent layer and propagates down, its intensity fluctuates. Spatial scale of these variations depends on the distance to the layer. This dependence is used to separate the contributions from different layers by means of four concentric-ring apertures that work as a matched spatial filter. Turbulence profile is derived from the statistical processing of the series of photon counts with 1 ms sampling.

MASS is an instrument to measure the vertical distribution of turbulence in terrestrial atmosphere by analysing the scintillation (twinkling) of bright stars.