Optical seeing at La Palma Observatory. I - General guidelines and preliminary results at the Nordic Optical Telescope

The results of one night of observations corresponding to an intensive site assessment campaign, held at the Observatorio del Roque de los Muchachos (La Palma) on July 22 1990, are presented and discussed. The goal of the campaign was to evaluate the contribution of free atmosphere, boundary layer, surface layer and dome + mirror seeing to image degradation at the Nordic Optical Telescope (NOT). To fulfill this objective the Scidar instrument was adapted to the NOT focus, while launched balloons and an instrumented mast were used to evaluate the extra-dome seeing. Although the seeing measurements reported here result from a single night of observations only, and thus have no statistical bearing on the seeing quality at La Palma, they nevertheless revealed a potentially excellent site; seeing contributions of respectively 0.08, 0.50, and 0.40 arcsec were measured for the surface layer, boundary layer and free atmosphere, with a resulting net effect of 0.69 arcsec. At the beginning of the night, extremely good seeing conditions (about 0.5 arcsec) where encountered at the NOT focus, demonstrating that, compared to the atmospheric conditions, there was virtually no significant dome and mirror seeing contributing to the image degradation.