The radio-quiet quasars were selected from the catalogues of Véron-Cetty & Véron (1985); Hewitt & Burbidge (1987) and Irwin et al. (1991). All the sources are at high redshifts (z>1) with the sole exception of PG 2112+059 (z=0.466) and the majority of the sources also have high absolute magnitudes (-27<MV<-30). The photometric data presented here were obtained during a number of observing runs spanning a period of six years. The observations were carried out during September 1990, October/November 1991, August 1992, September 1992, February 1993, March 1994, December 1995 and May/June 1996. All the RQQSOs listed in Tables 1 (click here) and 2 (click here), were monitored during at least one of these observing periods between 1990 and 1996 and a few were observed over a number of years.
The observations were made with the one metre JK Telescope at the
Observatorio del Roque de los Muchachos. A GEC CCD with
pixels was used for the September 1990 and
October/November 1991 runs. An EEV CCD with 
pixels
and an image scale of 0.30 arcseconds pixel-1 was used for all
the subsequent observing runs. The latter CCD was preferred because
the larger field of view offered a greater choice of reference stars
for the differential photometry. The seeing typically varied between
1.0 and 1.8 arcseconds.
The CCD fields containing the sources were observed through B, V or R-band filters and integrations times varied between 3 and 14 minutes. The simultaneous observations of the source and several comparison stars allowed variations arising from fluctuations in atmospheric transmission and extinction to be removed. The radio-quiet quasars and the reference stars used in the analysis are identified in the CCD images (Fig. 1). Where possible reference stars of comparable brightness and colour to the source were selected for the differential photometry. In all cases two or more reference stars were used. The CCD frames were processed using the standard software package IRAF. The DAOPHOT routine was used to carry out aperture-photometry on each star in the CCD frame. The differential magnitudes were then calculated for any pair of objects in the frame.
Figure 1: The finding charts for the radio quiet quasars (RQQ)
and the reference stars 
used in the differential
photometry. Each frame is orientated so north is up and east to
the left. The length of the bar is one arc minute. B1950
coordinates have been used