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Phase calibration transfer for International LOFAR

Submitter: Adam Deller (for the long baseline working group)
Description: Calibration of International LOFAR observations is challenging due to the large and rapid phase changes induced by changing ionospheric conditions. The use of a calibrator source should help extend the coherence time of International LOFAR observations, allowing fainter targets to be imaged. However, sufficiently bright and compact calibrator sources are relatively rare, meaning the nearest suitable source may be several degrees away. Because the ionospheric effects vary spatially as well as temporally, it was uncertain whether extrapolation of calibrator solutions over scales of order degrees improves coherence on the target source.

To investigate this question, the LOFAR long baseline working group made multi-beam commissioning observations of a group of calibrator sources at 140 MHz, and the result is shown in the image above. The image plots the calibrator phase solutions (in radians) against time (in days) for four international stations: from left to right, top to bottom, Effelsberg, Potsdam, Juelich, and Chilbolton. The time baseline is 2 hours.

The red and green lines show phase solutions on two of the calibrators, which are separated by 2.5 degrees. The blue line shows the residual phase difference between the two calibrator solutions, and the colour changes from light to dark every time the accumulated phase difference exceeds 1 radian, representing the coherence time.

It is clear that transferring solutions over 2.5 degrees does extend coherence time by a factor of several in most cases, up to a typical period of 5 minutes under these observing conditions.
Copyright: The LOFAR long baseline working group
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