Daily Image
31-01-2014
StefStefCal edging into the Mainstream
| Submitter: | Jan Noordam |
| Description: | During the 70's, Hans van Someren Greve used a simple but effective technique to calibrate the WSRT antenna phases on a bright point source, prior to a 12-hour observation. The invention of selfcal around 1980 saw the rise of the least-squares fit through matrix inversion. The latter proved general enough to solve for the extra parameters of ever more sophisticated Measurement Equations over the next few decades. Inevitably, all this required more and more processing, which we could not really afford with the ever increasing data volumes. A few years ago, Stef Salvini from Oxford independently hit upon the old WSRT technique in his search for a more efficient solver(*). But he improved on it by introducing a little averaging trick that considerably speeded up its convergence. This "StefCal trick" is illustrated in the picture, which features Stef on the left, and Stefan Wijnholds on the right. The importance of StefCal was first recognised by Oleg Smirnov, who also coined the name(**). It has been available in the MeqTrees package for some time. In the meantime, Stefan has implemented it for LOFAR station calibration, and he and Oleg have teamed up with Stef to make the technique robust and even faster, and to put it on a firm mathematical basis. It is not yet clear whether StefCal alone will be sufficient for the most demanding calibration. But it certainly comes close, especially since it works for full polarization, and for direction-dependent effects (DDE). The next step is to take account of the known continuity of instrumental parameters, in frequency and time. This might yet require a little matrix inversion. StefCal is definitely moving into the mainstream(***). Because of its simplicity and speed, it promises to become the workhorse of radio astronomical calibration. It will soon be made routinely available in LOFAR data reduction. In addition, the emergence of StefCal solves important problems for the LOFAR AARTFAAC project, and should do the same for the giant SKA. (*) At the behest of the late Steve Rawlings, and closely associated with the MeqTrees project and the European SKADS program. (**) The official name is a clumsy acronym, StEFCal, which is generally ignored. (***) The picture was taken at a recent 2-day workshop in Dwingeloo that was devoted entirely to the practicalities of StefCal. |
| Copyright: | Madroon Community Consultants (MCC) |
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