Testing an antenna layout for the SKA-Low telescope

Aerial photographs show the two SKA-Low prototype stations: AAVS2 with its pseudo-random antenna arrangement (left) and AAVS3 with the Vogel "sunflower" layout (right). The Vogel pattern maximizes spacing between antennas to reduce interference, but on-sky tests revealed a sensitivity loss at 125 MHz that led the Observatory to select a different layout for the final telescope. Image courtesy of SKA Observatory. Credit: Asayama et al., doi: 10.1117/1.JATIS.12.3.034004

The SKA-Low telescope, under construction in Western Australia, is designed to operate across a wide frequency range from 50 to 350 MHz. It will consist of 512 antennas spread across an area roughly 75 km wide. The Vogel layout, inspired by the spiral pattern in sunflower heads, was tested in AAVS3, which used 256 dual-polarized antennas and was tested both as a signal-combining beamformer and as an imaging array. The team combined computer simulations with observations to check the station's sensitivity and calibration accuracy. The researchers pointed it at the sun, bright cosmic radio sources, the galactic plane and several pulsars. They also developed two ways to calibrate the station—correcting the raw data so they accurately reflect the sky. The simpler method used the sun as a known reference point. The second, more thorough approach combined the sun with a sky-brightness model and detailed antenna response profiles.

The team found that the Vogel layout caused a sensitivity loss at 125 MHz due to repeated antenna spacings, about 2.4 meters apart, throughout the sunflower pattern. These spacings created interference patterns that reduced sensitivity. The older prototype, AAVS2, showed no such problem with its scattered layout. Observations confirmed the prediction. When the galactic plane passed directly overhead, images clearly showed the same dip in sensitivity at 125 MHz. This confirmed the problem was a built-in consequence of the antenna geometry, not a flaw in the equipment or method. The team separately checked the station's sensitivity at 230 MHz. At night, measurements closely matched predictions when the team used the more thorough calibration method. Daytime measurements showed larger gaps from predictions, most likely because unusually high solar activity during the observations made the sun's brightness harder to predict accurately.

The results of the AAVS3 tests gave SKA-Low's teams practical guidance for building and calibrating the stations during construction. Combined with lessons from AAVS2, this work has directly shaped the observatory's testing process and supported a key construction milestone, known as AA0.5, marking the first stations to come online. More information Shin'ichiro Asayama et al, On-sky verification of the Vogel layout with the SKA-low prototype station Aperture Array Verification System 3, Journal of Astronomical Telescopes, Instruments, and Systems (2026). DOI: 10.1117/1.jatis.12.3.034004.

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