Observation of standard stars is of crucial importance in stellar photometry.
We have studied the standard transformation relations of the UBVRI CCD
photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All
observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild
486 CCD, and standard Bessell UBVRI filters from 2003 August to 2007 September.
We observed many standard stars around the celestial equator observed by SAAO
astronomers. The atmospheric extinction coefficients, photometric zero points,
and time variation of photometric zero points of each night were determined.
Secondary extinction coefficients and photometric zero points were very stable,
while primary extinction coefficients showed a distinct seasonal variation. We
also determined the transformation coefficients for each filter. For B, V, R,
and I filters, the transformation to the SAAO standard system could be achieved
with a straight line or a combination of two straight lines. However, in the
case of the U filter and Fairchild 486 CCD combination, a significant
non-linear correction term - related to the size of Balmer jump or the strength
of the Balmer lines - of up to 0.08 mags was required. We found that our data
matched well the SAAO photometry in V, B-V, V-I, and R-I. But in U-B, the
difference in zero point was about 3.6 mmag and the scatter was about 0.02 mag.
We attribute the relatively large scatter in U-B to the larger error in U of
the SAAO photometry. We confirm the mostly small differences between the SAAO
standard UBVRI system and the Landolt standard system. We also attempted to
interpret the seasonal variation of the atmospheric extinction coefficients in
the context of scattering sources in the earth's atmosphere.Comment: 14 pages, 13 figure