3 Photometry

The photometry of the blended quasar images was performed by applying the MCS deconvolution algorithm (Magain et al. 1998). This algorithm has already been successfully applied to several monitoring data of lensed quasars (e.g., Burud et al. 2000; Burud et al. 2002). Its main advantage is its ability to deconvolve all the frames from different epochs simultaneously, hence constraining the astrometry of the two quasar images and the shape of the lensing galaxy using the total S/N of the whole data-set. The intensity of the point sources are allowed to vary from image to image, which produces the light curves.

The two quasar components are clearly distinguished on our deconvolved image of SBS 1520+530 (see Fig. 1), and the lensing galaxy is resolved. The lensing galaxy can easily be taken into account in order to avoid contamination of the photometry of the quasar images.

The light curves of SBS 1520+530 consist of 57 data points in the R-band as presented in Fig. 3 and Table 2.

Figure 3: R-band lightcurves of SBS 1520+530 and of the reference star S4 (see Fig. 2), calculated relative to the reference star S3 (Fig. 2). The error bars include photon noise and PSF errors estimated from the deconvolution of a reference star. The B component is shifted vertically by -0.4 mag and star S4 is shifted by +0.5 mag on the plot.

In addition to the two stars just to the South-East (S1) and North-West (S2) of the quasar (Fig. 1), three other reference stars in the field were deconvolved, in order to check the relative photometry and to check for systematic errors (stars 3 to 5 in Fig. 2). The magnitudes of the quasar are relative to the magnitudes of Star 3 in Fig. 3. The error-bars include both photon noise and additional systematic errors, e.g., PSF errors. The latter is estimated by using the reference stars, as explained in Burud et al. (2000).