All the millimetre/submillimetre (mm/submm) observations of 3C 273 were grouped together into seven light curves: 3.3mm (90GHz), 2.0mm (150GHz), 1.3mm (230GHz), 1.1mm, 0.8mm, 0.45mm and 0.35mm (see Table 1). The 3.3mm light curve is shown in Fig. 2.
All the submillimetre observations, as well as most observations at 1.1mm, 1.3mm and 2.0mm were performed on Mauna Kea, Hawaii. The QMC/Oregon photometer (Ade et al. 1984) was used on the 3.8m United Kingdom Infrared Telescope (UKIRT) until 1985. Since January 1986, a new common user photometer UKT14 (Duncan et al. 1990) was installed on the UKIRT, before being moved in March 1988 to the 15m James Clerk Maxwell Telescope (JCMT). Finally, in July 1996, UKT14 was replaced on the JCMT by the Submillimetre Common-User Bolometer Array (SCUBA), described by Robson et al. (1998). Details on the observations performed with the UKT14 photometer both on the JCMT and the UKIRT, as well as calibration techniques are given by Robson et al. (1993). The earlier observation techniques with the QMC/Oregon photometer are described in Robson et al. (1983).
A second important source of millimetre observations at 90 and 230GHz is the 15m Swedish-ESO Submillimetre Telescope (SEST) on the European Southern Observatory (ESO) site at La Silla, Chile. Until June 1995, the 90GHz observations were obtained using a dual polarization Schottky receiver, and since then measurements were made with a superconductor-insulator-superconductor (SIS) receiver. As a back-end, wide band (1GHz) acousto-optic spectrometers (AOS) were used. For the 230GHz observations a Schottky receiver and a wide band AOS were initially used, but since 1991 measurements are mainly obtained with a single channel bolometer. The flux density measurements were made in a dual beam-switching mode, and calibrated against planets. All the SEST observations until June 1994 were published by Tornikoski et al. (1996), together with more details on the observation techniques.
Interferometer observations of 3C 273 at around 90GHz (75-115 GHz) were obtained since 1986 with the Berkeley-Illinois-Maryland Association (BIMA) millimetre array located at Hat Creek, California (Welch et al. 1996). The 3C 273 flux densities were measured relative to planets assuming the planetary brightness temperatures given by Ulich (1981). The larger error bars include the uncertainties in the correction for atmospheric decorrelation on the longer baselines. All BIMA observations are included in the 3.3mm light curve. This light curve also contains several unpublished 87.3GHz observations performed since 1985 at the Metsähovi Radio Observatory, Finland (Teräsranta et al. 1992).
An other major source of 3C 273 data in the millimetre domain are the published observations at 90, 150 and 230GHz obtained using the heterodyne detectors of the "Institut de Radio-Astronomie Millimétrique'' (IRAM) 30m telescope at Pico Veleta, Spain. Details on the observations and the calibrations are given by Steppe (1992), whereas the measurements were published by Steppe et al. (1988, 1992, 1993) and by Reuter et al. (1997).
Other mm/submm observations from the literature were added to the database. In the 3.3mm light curve, we included the huge set of observations made at 90GHz with the Aerospace 4.6m telescope from 1965 to 1975 (Epstein et al. 1982) and the less frequent measurements made at 85.2 or 90GHz with the 11m NRAO telescope located on Kitt Peak (Hobbs & Dent 1977). The 3.3mm light curve also contains the very well sampled observations performed in 1982-83 with the 25m telescope at Hat Creek, California (Backer 1984), the 87GHz observations obtained in 1981-82 at the 14m telescope of the Five College Radio Astronomy Observatory (FCRAO) (Barvainis & Predmore 1984), and the 77GHz observations obtained at Metsähovi in 1984 (Teräsranta et al. 1987). Early monitoring of 3C 273 was also performed at 1mm with the 5m Hale telescope from 1973 to 1980 (Elias et al. 1978; Ennis et al. 1982). Early UKIRT mm/submm observations together with NRAO measurements at various wavelengths are reported by Robson et al. (1983, 1986) and Gear et al. (1984). Other isolated millimetre observations are reported by several authors (Chini et al. 1984; Clegg et al. 1983; Courvoisier et al. 1987; Geldzahler et al. 1981; Jones et al. 1981; Landau et al. 1980; Owen & Puschell 1982; Owen et al. 1978; Roellig et al. 1986; Sherwood et al. 1983). All these data were included in the respective light curves.
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