National Radio
Astronomy Observatory

5.9 Self-Calibration

Once you have a model image or set of model components reconstructed from your data using one of the deconvolution techniques described above, you can use it to refine your calibration. This is called self-calibration as it uses the data to determine its own calibration (rather than observations of special calibration sources).

In principle, self-calibration is no different than the calibration process we described earlier (§ 4). In effect, you alternate between calibration and imaging cycles, refining the calibration and the model as you go. The trick is you have to be careful, as defects in early stages of the calibration can get into the model, and thus prevent the calibration from improving. In practice, it is best to not clean very deeply early on, so that the CLEAN model contains correct components only.

One important thing to keep in mind is that the self-calibration relies upon having the most recent source model inside the MS. This is indeed the case if you follow the imaging (using clean) directly by the self-calibration. If you have done something strange in between and have lost or overwritten source model (for example done some extra cleaning that you do not want to keep), then use the ft task (see § 5.7 above), which adds a source model image or clean component lists to an MS.

Likewise, during self-calibration (once you have a new calibration solution) the imaging part relies upon having the CORRECTED_DATA column contain the self-calibrated data. This is done with the applycal task (§ 4.6.1).

The clearcal command can be used during the self-calibration if you need to clear the CORRECTED_DATA column and revert to the original DATA. If you need to restore the CORRECTED_DATA to any previous stage in the self-calibration, use applycal again with the appropriate calibration tables.

ALERT: In later patches we will change the tasks so that users need not worry what is contained in the MS scratch columns and how to fill them. CASA will handle that underneath for you!

For now, we refer the user back to the calibration chapter for a reminder on how to run the calibration tasks.

See the example of cleaning and self-calibrating the Jupiter 6cm continuum data given below in Appendix F.2.

More information about CASA may be found at the CASA web page

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