Analysis of Integration Time and Photometric Error of the CCD Photometry for the BL Lac Object ON231

In the CCD photometry the integration time significantly affects the photometric error. We used the 1m telescope of the Yunnan Observatory to observe the BL Lac object ON231 at the I band in different integration times, from 20s to 470s in an incrementing sequence with a 10s interval. All the images were reduced by using the CCDRED and APPHOT tasks in the IRAF software package, resulting in a total of 46 datapoints. We also analyzed the light curve of ON231 and found it in a low state with no variation during our observation time. We obtained the relations between integration time and photometric error of ON231 and two standard stars at the I band. We apply the polynomial fit to the relation of ON231, and calculate the optimal integration time (corresponding to the minimum photometric error). The optimal integration time for ON231 at the low state at the I band is about 294s.