The ESO Key-Program observations of the Cloverleaf were made using several different telescopes, and with a number of different CCD detectors. All telescopes and cameras used are listed in Table 1, together with the filters, observing dates, average seeing and the name of the principal investigator(s). The ESO telescopes that were used are the 3.5 m New Technology Telescope (NTT), the 2.2 m ESO-Max Planck Institut (MPI) telescope and the Danish 1.54 m telescope (DAN) at La Silla, Chile (the ESO 3.6 m telescope has also been used occasionally).
Observations from the NOT Monitoring program were made with the 2.56 m Nordic Optical Telescope, at Roque de los Muchachos, La Palma, Canary Islands (Spain) from April 1990 to August 1994. During the first four years, a Tektronix CCD camera with 0.197'' pixels was the only detector available. In 1994, a more sensitive CCD camera with 0.177'' pixels was installed. Reasonable signal-to-noise ratios were obtained for exposure times of 4 minutes or more when the seeing was better than 1''. The best NOT images show point source profiles with FWHM = 0.5'', which clearly resolve the four optical components of the Cloverleaf. Note that a non-linearity problem with the NOT stand-by CCD camera in observations made before 1993 has been numerically corrected for (see Østensen et al. 1996).