Summary

Based on the observational data obtained during 1996-1997 observing runs at SSO, we present the atmospheric extinction coefficients and transformation relations to the standard systems (SAAO and Landolt standard stars). The results are summarized as follows.

(1) The atmospheric extinction coefficients fluctuated by up to 0.05 mag around the mean value for a given passband. If the difference in airmass between standard stars and target objects is small, the mean extinction coefficients give accurate results. But if the difference is large, accurate values of the extinction coefficients, as well as of the zero points, are necessary to achieve reliable photometry.

(2) In the transformation to Landolt's standard U system using our initial U filter, a non-linear term related to the Balmer discontinuity was required. While most of this correction is due to the non-standard initial U passband of the CCD system, part is related to the differences in U-B between Landolt's system and the SAAO system.

(3) For the SAAO standard system with a revised U filter, a small but systematic non-linear correction is required for U, B, and I. The non-linear correction terms for U and B are related to the Balmer discontinuity, while that for I may be related to Paschen discontinuity at

. The maximum value of the non-linear correction is about 0.02 mag.

(4) After making the non-linear corrections, we achieved better than 0.004 mag difference relative to the SAAO system in zero point and obtained better than 0.015 mag in accuracy relative to the original SAAO standard value.

(5) From a direct comparison with Landolt's measurements of equatorial standard stars, we reconfirmed the mostly small but systematic differences between the two standard systems (SAAO and Landolt).

(6) Graham's observations of faint stars in E-regions show no systematic difference in colours, but the V magnitudes of Graham's faint stars (V > 12 ) are systematically brighter by about 0.05 mag.

© Copyright Astronomical Society of Australia 1997