Tied-array holography with LOFAR^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 Leiden, The Netherlands
e-mail: psalas@nrao.edu
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 Dwingeloo, The Netherlands
³ Department of Astronomy, University of Virginia, PO Box 400325, 530 McCormick Road, Charlottesville, VA 22904-4325, USA

Received: 12 April 2019
Accepted: 21 February 2020

Abstract

Context. A radio interferometer uses time delays to maximize its response to radiation coming from a particular direction. These time delays compensate for differences in the time of arrival of the wavefront at the different elements of the interferometer, and for delays in the instrument’s signal chain. If the radio interferometer is operated as a phased array (tied array), the time delays cannot be accounted for after an observation, so they must be determined in advance.

Aims. Our aim is to characterize the time delays between the stations in the core of the LOw Frequency ARray (LOFAR).

Methods. We used radio holography to determine the time delays for the core stations of LOFAR (innermost 3.5 km). Using the multibeaming capability of LOFAR we map the voltage beam faster than with a raster scan, while simultaneously calibrating the observed beam continuously.

Results. For short radio holographic observations (60 s and 600 s) of 3C196, 3C147, and 3C48 we are able to derive time delays with errors of less than one nanosecond. After applying the derived time delays to the beamformer, the beam shows residuals of less than 20% with respect to the theoretical beam shape.

Conclusions. Tied-array holography could be a way towards semi-real-time beam calibration for the Square Kilometer Array.