2 Observation and data reduction

The observation presented here were carried out with the one-meter telescope of Yunnan Observatory during November 1996 to January 1997. The telescope is equipped with a direct CCD (1024$\times$1024 pixels) camera at the Cassergrain focus. The CCD was bought from Princeton Instruments Company, U.S.A. The field is about 6.5$^\prime$$\times$6.5$^\prime$. The filters are standard Johnson broadband filters as:
B - GG385(2 mm) + BG12(1 mm) + BG18(2 mm)
V - GG495(2 mm) + BG18(2 mm)
R - RG610(3 mm) + 66.2500(1 mm)
I - RG715(3 mm) + 60.5050(1 mm).
The exposure time is 300 - 400 seconds for B and V filters, 100 - 200 seconds for R and I filters. The sources we observed are listed in Table 1. They are selected arbitrarily. The standard stars in the field are taken from Smith et al. (1985, 1991) (see Table 1).

  

Table 1: The list of the sources

The results are given in Table 2. The first column is the observing date, the second the Julian date, the third the magnitude, the fourth the observational 1$\sigma$ uncertainty level and the last the filter used. All observing data were processed using the photometric tool, APPHOT, in IRAF software package. The flat field images were taken at dusk and dawn when possible. The bias (taken at least 15 images each night) were taken at the beginning of the observation and the end. The source magnitude is given as the average of those derived with respect to the two brightest standard stars in the image frame containing the source which present the smallest variations in their differential magnitudes. The errors quoted in Table 2 are calculated from the two standard stars, star 1 and star 2, in the usual way,

$$\sigma = \sqrt{\frac{\Sigma\delta_{i}}{N-1}} \quad i=1,2,...N,$$ (1)

where $\delta_{i}$=$(m_{2}-m_{1})_{i}-\overline{m_{2}-m_{1}}$, m₂-m₁ is the differential magnitude between star 2 and star 1, $\overline{m_{2}-m_{1}}$ is the mean differential magnitude, and N is the number of the observations. Star 1, which is the brightest standard star, is also used for calibration, and star 2 is standard star whose brightness is comparable with the source or a little fainter than the source. When all the standard stars are brighter than the source, the actual observational error for the source is greater than the $\sigma$ calculated as above, and the errors have been given according to the typical uncertainties for sources of brightness comparable with the source.