Characterising the Apertif primary beam response

¹ ASTRON, the Netherlands Institute for Radio Astronomy, 7991 PD Dwingeloo, The Netherlands
e-mail: helgadenes@gmail.com
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
³ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, 18080 Granada, Spain
⁴ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
⁵ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
⁶ Sydney Institute for Astronomy, School of Physics, University of Sydney, Sydney, New South Wales 2006, Australia
⁷ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), Universitätsstrasse 150, 44780 Bochum, Germany
⁸ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁹ Department of Electrical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden
¹⁰ Department of Physics, Virginia Polytechnic Institute and State University, 50 West Campus Drive, Blacksburg, VA 24061, USA
¹¹ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune 411007, Maharashtra, India
¹² Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹³ Netherlands eScience Center, Science Park 140, 1098 XG Amsterdam, The Netherlands
¹⁴ West Virginia University, White Hall, Box 6315, Morgantown, WV 26506, USA
¹⁵ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
¹⁶ Tricas Industrial Design & Engineering, Popovstraat 48, 8013 RK Zwolle, The Netherlands
¹⁷ University of Oslo Center for Information Technology, PO Box 1059, 0316 Oslo, Norway

Received: 16 May 2022
Accepted: 2 August 2022

Abstract

Context. Phased array feeds (PAFs) are multi-element receivers in the focal plane of a telescope that make it possible to simultaneously form multiple beams on the sky by combining the complex gains of the individual antenna elements. Recently, the Westerbork Synthesis Radio Telescope (WSRT) was upgraded with PAF receivers to carry out several observing programs, including two imaging surveys and a time-domain survey. The Apertif imaging surveys use a configuration of 40 partially overlapping compound beams (CBs) simultaneously formed on the sky and arranged in an approximately rectangular shape.

Aims. This work is aimed at characterising the response of the 40 Apertif CBs to create frequency-resolved I, XX, and YY polarization empirical beam shapes. The measured CB maps can be used for the image deconvolution, primary beam correction, and mosaicking processes of Apertif imaging data.

Methods. We used drift scan measurements to measure the response of each of the 40 Apertif CBs. We derived beam maps for all individual beams in I, XX, and YY polarisation in 10 or 18 frequency bins over the same bandwidth as the Apertif imaging surveys. We sampled the main lobe of the beams and the side lobes up to a radius of 0.6 degrees from the beam centres. In addition, we derived beam maps for each individual WSRT dish.

Results. We present the frequency and time dependence of the beam shapes and sizes. We compared the compound beam shapes derived with the drift scan method to beam shapes derived with an independent method using a Gaussian Process Regression comparison between the Apertif continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good agreement between the beam shapes derived with the two independent methods.