Keywords: flux

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# 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 to 30 between the RF bandpass calibrator and the phase calibrator introduces on the efficiencies estimates a variation of about 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: HO 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.

Subsections

- The Amplitude Calibrator is used for Pointings
- The Amplitude Calibrator is too weak for Pointings
- Using Pointings and Cross-correlations
- Use of W3OH as Flux Calibrator

Next: The Amplitude Calibrator is Up: Flux measurements with the Previous: Interferometry Contents Index