Because of the physics of quasars, the spectral index may be variable with time as the source intensity. Almost simultaneous measurements at 2 frequencies are thus needed to estimate it accurately. In practice, it is impossible to follow all the 100 quasars used as phase reference at the IRAM interferometer. Even following the 20 ones typically used during a 4 month observing period is a time consuming task.
Accordingly, only a handful of strong quasars are monitored frequently, at 2 frequencies. These are the strongest quasars, typically used for bandpass calibration by all projects. A complementary flux check is done using the continuum source W3OH. As it is partially resolved by the interferometer, a model of the emission is however necessary. This additional procedure will be available soon.
To make the monitoring of strong quasars easy, for the observations and for the analysis, but also to minimize the calibration errors, a standard observing procedure has been devised. The two frequencies selected correspond to the SiOv1 line at 86 GHz and 13CO(2-1) line at 210 GHz. As the calibration accuracy is a crucial point, fluxes must be measured only by very good weather.
Of course, short ``holes'' (1 or 2 hours) in the observations by good weather can also be used to observe fluxes using the current frequency tuning but have to be considered as ``COMPLEMENTARY'' data. One need to obtain a few precise measurements at fixed frequencies rather than lots of data taken in incertain conditions.