Analysis of the Aperture Photometry for Blazar

In this paper, we studied the influence of the parameters in IRAF software package for differential photometry of blazar by using different values of the photometric aperture, background, and width of sky annulus. The results indicate that the selected values of aperture parameters affect the differential photometry greatly, but the values of the width of sky annulus affect little, and the background parameter has hardly any effect on the differential photometry. We also discussed a correction factor Γ, to account for the effect of incorrect choice of the standard stars used in the differential photometry on studying microvariability of blazer. We applied the correction to our analysis of the data of PKS1510-089 and OJ 248 observed in March 2007.