Sub-parameter cyclefactor

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5.3.4.1 Sub-parameter cyclefactor

Inside the Toolkit:

The im.setmfcontrol method sets the parameters that control the cycles and primary beam used in mosaicing.

This sub-parameter is activated for imagermode=’csclean’ and ’mosaic’.

The cyclefactor parameter allows the user to change the threshold at which the deconvolution cycle will stop and then degrid and subtract the model from the visibilities to form the residual. This is with respect to the breaks between minor and major cycles that the clean part would normally force. Larger values force a major cycle more often.

This parameter in effect controls the threshold used by CLEAN to test whether a major cycle break and reconciliation occurs:

cycle threshold = cyclefactor * max sidelobe * max residual

If mosaic or csclean diverges on your data, try a larger cyclefactor. A larger value typically increases the robustness of your deconvolution. The price, however, will be a slower algorithm. On the other hand, if you find that the cleaning is slow due to taking too many major cycle breaks, then reduce cyclefactor.

Note that currently the cycle_threshold will saturate at a maximum value of 0.80 even when you set cyclefactor to a very high value or you have very high PSF sidelobes. This means that with a gain = 0.1 you will get 3 minor cycles per major cycle when hitting the limit.

Some rules of thumb:

If you have data taken with a small number of antennas, for example from ALMA in the commissioning and early-science phase, then you will have high sidelobes in the PSF. In this case, you will have to reduce cyclefactor considerably, likely into the range 0.25 to 0.5, if you want efficient cleaning of simple source structures (e.g. point sources). You can use the viewer to look at your PSF image and see what the maximum sidelobe level is and judge accordingly.

However, if your uv-coverage results in a poor PSF and you have complex source structure, then you should reconcile often (a cyclefactor of 4 or 5). For reasonable PSFs, use cyclefactor in the range 1.5 to 2.0. For good PSFs, or for faster cleaning at the expense of some fidelity, we recommend trying a lower value, e.g. cyclefactor = 0.25, which at least in some of our mosaicing tests led to a speedup of around a factor of two with little change in residuals.

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