2 Observations and data reduction

Weekly observations of SBS 1520+530 were carried out at the NOT from February 1999 to May 2001. The detector ALFOSC (AndaLucía Faint Object Spectrograph and Camera) with a pixel size of 0 $.\!\!^{\prime\prime}$188 was used for 95% of the frames obtained. The remaining 5% were obtained with HiRAC (High Resolution Adaptive Camera) or with the stand-by camera StanCam which have pixel sizes of 0 $.\!\!^{\prime\prime}$107 and 0 $.\!\!^{\prime\prime}$176 respectively.

The target is relatively bright ($I \sim 18$) and an exposure time of 300 s in the R-band was sufficient to obtain a $S/N \sim 100$ for each quasar image. The seeing varied from 0 $.\!\!^{\prime\prime}$5 to 2 $.\!\!^{\prime\prime}$2 during the two years of observations, 1 $.\!\!^{\prime\prime}$0 being the most frequent value.

Our automated pipeline employing routines in the IRAF/NOAO/CCDRED package was used in order to pre-process the CCD frames in an efficient and homogeneous way.

Figure 1: Deconvolved R-band image ( ${\it FWHM} = 0$ $.\!\!^{\prime\prime}$28) of $12\hbox {$^{\prime \prime }$ }\times 12\hbox {$^{\prime \prime }$ }$ around SBS 1520+530 obtained from the simultaneous deconvolution of 59 frames. We see the two stars just to the North-West (S2) and South-East (S1), the quasar components A and B, the lensing galaxy L close to B and an additional galaxy M, NE of the lens. North is up and East is to the left.

Figure 2: The field ($\sim$5 $\hbox {$^\prime $ }\times 5$$^\prime$) around SBS 1520+530 observed with ALFOSC at the NOT. North is up and East to the left. See Faure et al. 2002 for a detailed investigation of the spatial distribution of secondary lenses in this field.