10.3.3 Determining the absolute flux scale on a project

10.3.3.0.1 The method

Fig.10.3 is a printout of the ``standard calibration procedure'' used in CLICThis procedure uses the CLIC command SOLVE FLUX which works on cross-correlation only as follows:

1.

The flux of the reference source is fixed to F(Ref)

2.

F(Ref) is used to measure the antenna efficiency by dividing by antenna temperature of the reference ( T_A^*(Ref)): $\ensuremath{\mathcal{J}} _I=T_A^*(\mathrm{Ref})/F(\mathrm{Ref})$

3.

$\ensuremath{\mathcal{J}} _I$ is used to compute the flux of all other sources in the index: $F(source) = \ensuremath{\mathcal{J}} _I \times T_A^*(source)$

The flux density of the amplitude calibrators will be used in the final step of the amplitude calibration to fix the flux of the source of astronomical interest.

10.3.3.0.2 The practice

In the automatic procedure, the reference sources are the calibrators where Fixed flux is set to YES and the reference values are in the variable Input Flux. Flux in file corresponds to the value stored with the data (by using the observational command FLUX, see lecture 6 by R.Neri for details). The calculation is performed by clicking on SOLVE and the results are displayed inside the variable Solved Flux.

If you want to iterate using one of these values as reference, you need to write it in the variable Input Flux and set Fixed flux to YES. Like in the CLIC command SOLVE FLUX, the individual antenna efficiencies ( $\ensuremath{\mathcal{J}} _I$) are computed; these values are only averaged values on the time interval using all sources. They are then affected by many small biases like pointing or focus errors and atmospheric decorrelation. They are then usually worse than the canonical values given in table above (for biases, see end of this section).

When you are satisfied by the flux calibration, you need to click on the following sequence of buttons: 1) Get Results in order to update the internal variables of the CLIC procedure, 2) Store to save the flux values inside the header file (hpb file) and 3) Plot to display the result of your calibration. The plot shows the inverse of the antenna efficiencies ( $1/\ensuremath{\mathcal{J}} _I$) versus time for all selected sources. If the flux calibration is correct, all sources must have the same value e.g. $1/\ensuremath{\mathcal{J}} _I$. This plot is systematically done in mode amplitude scaled (written on the top left corner). In this mode, the antenna temperature of each source T_A^*(source) in K is divided by its assumed (variable Input flux) flux density F(source) in Jy (the value you have just stored), the result is then $T_A(source)^*/F(source)= 1/\ensuremath{\mathcal{J}} _I(source)$which must be the same for all sources and equal to $1/\ensuremath{\mathcal{J}} _I$. If it is not the case, for example if one source appears systematically lower or higher than the others, this means that its flux is wrong and you need to iterate.

Note that the scan range, applied on all calibrators, which is by default the scan range of the ``standard calibration procedure'' can be changed. This option is useful when there is some shadowing on one calibrator because the shadowing can strongly affect the result of a SOLVE FLUX. If you change the scan range, do not forget to click on UPDATE.

  

Figure 10.3: User interface of the ``standard calibration procedure'' of CLIC corresponding to the flux calibration.