Abstract
Based on data obtained at the Observatorio Astronómico Nacional, Tonantzintla, an evaluation of the quality of the local sky is presented. The evaluation is carried out through an absolute CCD calibration to a set of standard stars in the field of M67 in the B, V, R and I Johnson-Cousins system and from Boller & Chivens long-slit spectroscopic observations of the local sky at various elevations. The sky brightness is estimated from our CCD frames yielding a mean value of 18.5±0.6 mag arcsec^{-2}. The mean atmospheric extinction curve behaves midway between the normal extinction and that related to volcanic outbursts. From the long-slit spectra of the sky at OAN-Tonantzintla, HgI and NaI lines of high and low pressure lamps are identified. These results allow us to justify a major upgrading of the observatory, to convert the OAN-Tonantzintla into the Laboratory for Astronomy Education both for UNAM and for other educational institutions in the center-south part of the country.
