Sung, HwankyungBessell, Michael2015-12-102015-12-101323-3580http://hdl.handle.net/1885/38606We discuss variations of the atmospheric extinction coefficients and transformation equations to the standard UBVRI system based on observations of standard stars during 1996-1997 at Siding Spring Observatory using a thinned SITe CCD and coloured glass filters. In the transformation from the initial natural system to the Landolt version of the standard system, a large nonlinear term related to the Balmer discontinuity was required for the U transformation. We then modified the U filter, and the subsequent transformation to the SAAO version of the standard UBVRI system had only small nonlinear correction terms for U, B, and I. The correction terms relating to U and B are evidently due to the Balmer discontinuity, while that relating to I seems to be due to the Paschen discontinuity at λ ≈ 8200 Å. We also compared the results with Landolt's observations, and confirmed the difference between the two sets of standard stars (SAAO and Landolt).Keywords: Standards; Stars: imaging; Techniques: photometric20002015-12-09