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Astronomical seeing is quantified by a single parameter, the turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor (DIMM). A new instrument, the multi‐aperture scintillation sensor (MASS), permits one to measure the seeing in the free atmosphere above ∼0.5 km and, together with a DIMM, to estimate the ground‐layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ∼10 per cent when their biases are carefully controlled. Pushing this limit to 1 per cent appears unrealistic because the seeing itself is just a model‐dependent parameter of a non‐stationary random process.

Accurate seeing measurements with MASS and DIMM