Polarization leakage in beam formed data

The report below concerns beam-formed data recorded with COBALT between 2014-07-12 and 2015-01-21.

The new clock correction for coherent beam-forming of the core stations, introduced at 2014-07-12, influences the data in two negative ways:
  • Polarization studies: The different stokes parameters are mixed, most notably stokes Q and V, but this is direction dependent.
  • Flux measurements may also be affected because the addition of the signal is not fully coherent.
At 2015-01-21 the previous situation has been restored. Please note that at this point LOFAR does not have any polarization calibration and a smaller incoherency effect may still be present in the data. The effect is described in more detail below.

The single clock that is used for the core stations delivers the same clock signal to these stations. This allows for coherent beam-forming.  However, there is a different delay between the clock and each state.  This delay gives a frequency dependent phase shift, a phase gradient, but there is also a phase zero offset. The values can be different for the X and Y polarization. If the phase zero offset is not applied correctly, there is leakage between the stokes parameters. With the previous correlator, BG/P, only the phase gradient could be applied, but no phase zero offset. With COBALT, it is possible to apply the phase zero offset. This was introduced at 2014-07-12. However, the values that were applied did not decrease spectral leakage, but instead increased the values, creating a stronger Stokes V intensity, probably from the Stokes Q parameter, but this can be direction dependent. This was reported by a user at 2015-01-07. Following this report, tests were performed to see what the current optimum values are by trying three different settings. The best result was obtained by setting the phases back to 0 at 2015-01-21. Please note that there may still be polarisation leakage, but less than before this date. There is currently no polarisation calibration method available to correct this. We are still attempting to find optimum calibration values, also to add the core coherently and will also investigate the polarization aspect. We are also investigating a conversion method for data taking during this period. A similar issue is known at international stations, but no fix will be available on the short term.

Imaging data will have had different delays during this period as well, but this is corrected in the standard calibration procedures.
Design: Kuenst.    Development: Dripl.    © 2021 ASTRON