Interferometric seeing measurements at the La Palma Observatory

We describe an interferometric seeing monitor is described which measures the wavefront perturbations due to the atmosphere directly, rather than depending on an assumed model for these perturbations. It is able to measure simultaneously both the spatial and temporal parameters of the seeing ($$r_0\enspace\text {and}\enspace t_0$$) and to test theoretical models. Results are reported from the commissioning run on the William Herschel Telescope. It is found that the seeing at optical wavelengths is well described by a model where the perturbations are due to turbulence with Kolmogorov statistics, and evidence is shown that the outer scale of this turbulence is about 2 m at the La Palma Observatory.