Phased Array Feed

Phased Array Feeds in the future design of the SKA

The Phased Array Feeds (PAF) element, part of the SKA Advanced Instrumentation Programme, includes the activities necessary to develop a wideband, multi-pixel, wide field-of-view receiver for the SKA.

Defining Phased Array Feeds

The Phase Array Feed (PAF) consortium is one of three Advanced Instrumentation Programmes (AIP) within the SKA, aimed at developing the next generation of technology for radio astronomy.

A PAF mounted in the focus of an ASKAP 12m antenna (Credit: CSIRO)

The PAF consortium is seeking to develop cost effective wideband, multi-pixel, wide field-of-view (FOV) receivers for the SKA. These receivers replace conventional single pixel feed receivers located at the focus of the offset parabolic dish antennas to provide a multi-pixel “camera” on the sky.

Each receiving element of the PAF, is in itself a small antenna which samples the energy on the focal plane of the dish antenna. This energy is then amplified using a Low Noise Amplifier (LNA) and filtered to remove out-of-band signals, to produce a signal that can be combined with signals from other elements to form a beam on the sky. By “weighting” the addition of all the signals, that is adjusting how much of each signal is used and how, multiple beams (pixels) can be created to look in slightly different directions, to create a montage of beams within a FOV.

Apertif was built by ASTRON for the Westerbork telescope (Credit: ASTRON)

Through careful design of the PAF receiving elements, LNAs and the remaining signal chain, wide bandwidths and high sensitivities are achievable; offering PAFs similar receiver sensitivities to conventional single pixel feeds.

Combined with their wide field-of-view, PAFs can provide a significant increase to the speed of radio sky surveys without a degradation in sensitivity and are ideal for use in a survey telescope such as the SKA. It is for this reason that the SKA1-mid antennas have been designed to incorporate PAF receivers in the future.

PAFs are already being used in several observatories for radio astronomy, including Apertif in the Netherlands & ASKAP in Australia, and early science with this new technology is just commencing.

The animation below explains the way PAF beamforming works, using the example of the Apertif. This is a technique which will also be used by the SKA’s low-frequency telescope.

The challenges

PAF receivers developed by the PAF Consortium will help address fundamental questions in radio astronomy, related to Dark Energy, the Epoch of Reionisation, Gravitation and Cosmic Magnetism; which all require observational statistics on very large scales that are supported by large sky surveys.

With their large field of view, PAFs provide the ability to rapidly survey large areas of sky to generate these statistics and image large scale structures, such as the Milky Way and nearby galaxies.

In addition, PAFs installed in the SKA interferometer can also be used as a transient detector to capture and locate Fast Radio Bursts (FRBs) in a newly emerging field of study. This can be done by pointing each separate SKA antenna in a different direction on the sky to generate a mosaic “fly’s-eye” view of a huge section of sky simultaneously.

The PAF consortium is led by Steve Barker at CSIRO.

Institutions involved in the PAF consortium include:

• Joint Laboratory for Radio Astronomy Technology (JLRAT), China
• National Institute for Astrophysics (INAF), Italy
• Max Planck Institute for Radio Astronomy (MPIfR), Germany
• Netherlands Institute for Radio Astronomy (ASTRON), The Netherlands
• Chalmers University/Onsala Space Observatory, Sweden
• University of Manchester, United Kingdom
• Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
• University of Malta, Malta

In the Netherlands, the APERTIF phased array feed gave the Westerbork telescope a much wider field of view, as shown in this image. (Credit: ASTRON)

SKA Global Consortia