10.3.4 Possible biases and remedies

10.3.4.0.1 Flux densities are more important than efficiencies

In the final amplitude calibration performed on the source (see next section), the flux of the source is determined by reference to the flux of the amplitude calibrator which is usually also the phase calibrator. This means that the averaged efficiencies $\ensuremath{\mathcal{J}} _I$ computed by SOLVE FLUX and the automatic procedure are not directly used and in many case variations of $\ensuremath{\mathcal{J}} _I$ does not affect the accuracy of the final amplitude calibration because they are corrected. It is then fundamental to have a good estimate of the flux of the amplitude calibrator but not necessarily to know precisely the averaged $\ensuremath{\mathcal{J}} _I$.

10.3.4.0.2 Possible biases

Using the automatic procedure, the possible biases are the following:

1.

There is some shadowing on the reference source, the estimate of the $\ensuremath{\mathcal{J}} _I$ can be wrong. Use another reference.

2.

One or several antennas are off focus: $\ensuremath{\mathcal{J}} _I$ is larger than $\ensuremath{\mathcal{J}} _S$but flux densities can still be correct if there is no significant drift during the time interval used to measure the fluxes. If the data are affected by a significant focus drift, it also affects the accuracy of the flux measurements. Depending of the observation time of the reference and of the sources, the estimated flux densities can be either too low (reference taken at the beginning when the focus is correct, sources at the end when the focus is off) or too high (opposite situation). In both cases, it is necessary to check the focus (data called FOCUS or have a look to the show.ps file). In the first case, the measured fluxes are correct. In the second case, the flux calibration must be done on a smaller interval of time where the focus drifts remain negligible.

3.

The pointing on the reference is bad, $\ensuremath{\mathcal{J}} _I$ is overestimated implying that the flux of all other sources (with good pointing) is also overestimated. Check the pointing on the possible reference sources (data called POINT or have a look to the show.ps file) and select a better reference.

4.

There is a strong atmospheric decorrelation. Flux measurements are performed on cross-correlations of about 4 minutes and the atmospheric phase fluctuations are high (check them on an individual cross-correlation taken on a strong quasar e.g. the RF calibrator), there are two possibilities: i) the atmospheric correction works well (clear sky), this is usually the case and it is necessary to apply it to measure the fluxes or 2) the atmospheric correction does not work (cloudy sky), the data can be used at 3mm and the flux scale can be computed by selecting the best scans on a small interval of time but at 1.3mm data are useless.

5.

The interferometric efficiencies $\ensuremath{\mathcal{J}} _I$ are really very different to $\ensuremath{\mathcal{J}} _S$ because there is a wonderful mixing of the points mentioned above... Ask to an expert (your local contact first, myself later if needed...).

Note that the biases 3) and 4) do not affect flux estimates when they are performed on pointing data (like in the case of the flux sessions).