SURFACE LAYER SEEING AT SAN PEDRO M ARTIR REVISITED

Results from experiments measuring the contribution of the surface layer (2.3 to 15 m) to the optical seeing at the Observatorio Astron omico Nacional at San Pedro M artir (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 2 n . The integrated seeing parameter was measured with a Dieren tial Image Motion Monitor (DIMM) during 23 nights. Log-normal statistics were found for the DIMM seeing with a median value of 0: 84. The surface layer average seeing was found to be 0: 16. The measured optical turbulence of this layer has a mean contribution of 5.2% to the total C 2 n , 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 signican t eect on the surface layer seeing.