On the Evolution of the Diurnal Atmospheric Opacity at EL Leoncito

MAZZA, N.; NUÑEZ, N.; BALMACEDA, L.; FRANCILE, C.; STENBORG, G

Córdoba, Argentina

Congreso; X Latin American Regional IAU Meeting; 2001

OAC

MICA (Mirror Coronograph for Argentina) is a solar telescope installed at the Estación de Altura Carlos U. Cesco", El Leoncito, San Juan, Argentina (depending from the Observatorio Astronómico Félix Aguilar (OAFA), UNSJ) since August 1997. The instrument is part of a bilateral science program between Argentina and Germany. The institutions involved in the cooperation agreement are, on the german side, the Max-Planck-Institute für Aeronomie (MPAe) and the Max-Planck-Institut für Extraterrestrische Physik (MPE). On the argentinean side are the Instituto de Astronomía y Física del Espacio (IAFE, Buenos Aires) on behalf of CONICET, and the University of San Juan through the OAFA Observatory. The automatic detection of the sky conditions at the site are of crucial importance for the determination of the operation mode of the telescope, the so called ?Sky Tester" and ?Sun Tester" devices being the auxiliary instruments needed for such a task. The first one records the sky brightness around the solar disk and the latter measures the integrated solar disc intensity, both in white light.Such data allow the daily determination of the variability of the diurnal atmospheric opacity. Moreover, statistical studies about the sky quality evolution at the site are also possible. We present a first approach to the analysis of the evolution of the mean diurnal atmospheric opacities (corresponding to the morning and the afternoon) in the period January 1988 - December 2000. Such study is of fundamental importance for the planification of specific observing campaigns with the solar instruments available at the Cesco Station (a solar coronograph: MICA and an Halpha telescope: HASTA). The daily mean values obtained are also useful parameters for the characterization of the sky quality at the site on the corresponding observing day, while their dispersion accounts for the variability of the sky brightness along the day. We conclude from the analysis of the period mentioned above, that the most appropriate season for observing campaigns would be autumn, as the lowest opacity value (in average) as computed in this work is obtained. Furthermore, this epoch exhibits the greatest number of days with favorable conditions either during mornings or afternoons. The database will be extended to year 2001 and future years in order to have an updated database.The sky variability will be analyzed in deeper detail to have a reliable estimator to account not only for the sky variability but also for the presence of clouds, features not considered in this work (as they were eliminated prior to the estimation of the opacity value).