| Description: | Our main goal in the 4th Pulsar Busy Week (Sept. 21-25, 2009) was to combine the beams of multiple LOFAR stations online. With 5 Dutch stations at our disposal, we took more than a dozen datasets to diagnose how the online pulsar pipeline is performing when it comes to adding station signals together. In the simplest case, the intensities of the various station beams are summed, with an appropriate time delay, to form an incoherent array beam. This combination of stations retains the original single station field of view and should gain in sensitivity over that of a single station as the square root of the number of stations that are combined. We summed different numbers of stations to show that indeed the measured peak signal-to-noise ratio of the pulsar we observed, B1508+55, increases in the expected way. The corresponding cumulative pulse profiles of the five observations are also shown, where the standard deviations of the off-pulse noises have been scaled to 1 in each case. Note that these observations were not taken simultaneously, so scintillation and RFI are partly responsible for the remaining scatter in the measured signal-to-noise ratios.
The next challenge will be to make a coherent summation of the station beams to form what are called "tied-array" beams. This involves summing the station signals with the appropriate phase delay before converting to intensity. The complication is that both the separate clocks of the stations and the different ionospheric patches through which they are looking create dynamic phase delays between stations. To create proper tied-array beams will require calibration of these phase delays in near real time. We have started tackling this problem and will focus on it in the next Pulsar Busy Week. |
