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Time for a Tune-Up

Submitter: Jason Hessels, Olaf Wucknitz, and Ben Stappers on behalf of the LOFAR PWG
Description: Coherently combining the LOFAR Superterp into a `tied-array' beam
provides a sensitive means to observe pulsars and other rapidly
varying objects. Using the Superterp as a single station also has
interesting applications for long-baseline imaging. Although we've
known for a while that the phasing of the Superterp is good, but not
perfect, we have never quantified the performance by mapping the beam.
Until now...

The top two plots show a zoomed-in (left) and zoomed-out (right) view
of the Superterp beam respectively. These were made by simultaneously
recording 217 coherent and 1 incoherent beam for all 12 Superterp
High-Band Antenna (HBA) sub-stations. In each beam map, the bright
pulsar B0329+54 was observed for 5 minutes and the color maps reflect
the signal-to-noise ratio of the dedispersed and folded signal in
various directions. The background color (aqua) reflects the
signal-to-noise ratio of the simultaneously acquired incoherent beam.

The two lower plots show the expected beam shape based on a model that
takes into account the positions of the HBA sub-stations and their
size. Alhough the observed beam pattern generally agrees well with
what one would expect, there is an obvious asymmetry in the brightness
of the sidelobes. Re-running the beam simulations with the known
phase offsets between the stations we are able to qualitatively
reproduce this asymmetry.

Obviously the next step is to calibrate out these remaining delays.
That work is currently underway as part of the broader effort to
extend the single clock system to all Superterp stations.
Copyright: Hessels/Wucknitz
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