| Abstract |  | We describe how to obtain accurate measurements of the Cn2 integral
in the free atmosphere and in the ground layer with a combined
MASS-DIMM instrument. The instrument is briefly presented. The
definition of "seeing" and its limits are re-stated. We evaluate
various systematic effects and biases in the MASS method
(semi-saturated scintillation, finite exposure, inner scale) and in
the DIMM method (noise, influence of aberrations, centering
algorithms, finite exposure) by combining analytical approach with
numeric simulation. Examples of the MASS-DIMM data are given.
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