How to Determine the Absolute Flux Scale on a Project

Contrarily to a common idea, determining the absolute flux scale in an interferometric project is the hardest and finest task of the calibration. Furthermore the CLIC command SOLVE FLUX is very dangerous, particularly when used on bad calibrated data.

This command permits the determination of the absolute flux scale which is fixed by the CLIC command STORE.

During this step, all parameters which can vary in time have a critical influence on the quality of the final results: POINTING, FOCUS, weather (amplitude and phase noise) and can introduce somes biases. POINTING errors are the most important but can be easily cancelled by pointing before doing the cross-correlations on the RF AND the PHASE CALIBRATOR. The evolution in time of the phase noise is most difficult to cancel. For example, a phase noise fluctuation from 15$^o$ to 30$^o$ between the RF bandpass calibrator and the phase calibrator introduces on the efficiencies estimates a variation of about $\sim$ 5.5 %.

In order to avoid them, the best solution is to apply the SOLVE FLUX command only on a short time interval where the weather conditions: H$_2$O vapor contents and phase noise are similar (compare the CALIBRATIONS).

In standard interferometric projects, we find the following data:

        - POINTING on RF Bandpass Calibrator
        - CROSS-CORELATION on RF Bandpass Calibrator (typically 1/4h)
        - POINTINGS on Phase Calibrator (each 2h)
        - CROSS-CORELATION (by 4min) on Phase Calibrator

We us them to determine the absolute flux scale. Depending of the flux of the amplitude (phase) calibrator, there are several possibilities.