Abstract
Results from experiments measuring the contribution of the surface layer (2.3 to 15 m) to the optical seeing at the Observatorio Astronómico Nacional at San Pedro Mártir (OAN-SPM) are reported. Microthermal sensors placed at 7 heights on a 15-m-high instrumented mast were used to measure the structure constant of the refractive index C_{mathrm{n}}(2) . The integrated seeing parameter was measured with a Differential Image Motion Monitor (DIMM) during 23 nights. Log-normal statistics were found for the DIMM seeing with a median value of 0rlap .(arcsec) 84. The surface layer average seeing was found to be 0rlap .(arcsec) 16. The measured optical turbulence of this layer has a mean contribution of 5.2% to the total C_{mathrm{n}}(2) , which corresponds to a mean degradation of 3.2% of the total seeing. These values are similar to those found in other observatories around the world, suggesting that the presence of trees in the OAN-SPM does not have a significant effect on the surface layer seeing.
