THE ATMOSPHERIC EXTINCTION AT THE ESO LA SILLA OBSERVATORY

The magnitude of the Earth atmospheric extinction has been determined during about 4'400 nights of astronomical photometric measurements, from the Swiss Telescope at the E.S.O. La Silla Observatory (Chile). This has been done at optical wavelengths corresponding to the seven passbands of the Geneva photometric system between November 1975 and August 1994. The atmospheric extinction during each of these nights can be characterized by a mean value and a standard deviation, since these nights are those of best astronomical quality, the so-called photometric nights. Two telescopes (40 cm and 70 cm), two photoelectric photometers and one CCD camera have been used on two different sites at La Silla. The effects these changes of sites and instrumentation produce on the long-term stability of the photometric system and, thus, on the determination of the atmospheric extinction coefficients, are analysed. The extinction law at minimum extinction is determined and the relative contributions, at each wavelength, of the Rayleigh-Cabannes molecular diffusion, of the selective absorption by molecular bands (essentially ozone) and of the extinction due to aerosols (dust and various condensations, including water) are given. The long-term extinction variation, due to the volcanoes El Chichon (1982, in Mexico) and The Pinatubo (1991, in The Philippines), is described. The maximum mean effect was more than twice larger for The Pinatubo (respectively 0.037 and 0.087 mag in the V band) and the extinction law from The Pinatubo aerosols, at the period of the observed maximum, was hater, or gray. The decantation of the volcanic aerosols in the atmosphere was very slow, lasting 1000-1300 days. The extinction law evolved with time, probably because of the increase of the mean aerosol size. Apart from these long-term effects due to the volcanoes,the minimum value of the atmospheric extinction remains relatively stable during the year, while the maximum values are reached during the southern Summer. The extinction law of the meteorological aerosols is very precisely defined, k(p)(lambda) = b(p) lambda(-1.39), indicating that the distribution of the radii of the particles, forming the atmospheric hare (mainly water droplets) during the photometric nights, is quite stable and has its mode near the value 0.3 mu. There is a tendency to have series of a few (less than about five) consecutive nights with similar extinction values. This is to be related to the typical duration of periods with similar meteorological conditions at La Silla.