Full Characterisation of the Polarisation Primary Beam of the GRAO 32-m Telescope

The universe is filled with invisible forces that shape stars, galaxies, and the very space between them. One of the most powerful ways to study these forces is by looking at how light from distant objects vibrates. This vibration, known as polarisation, acts like a fingerprint for magnetic fields, revealing their strength and direction across the cosmos. To read these fingerprints, astronomers use giant radio dishes that collect faint signals from space. However, the telescope itself is not a perfect mirror; its metal surfaces and internal structures can subtly twist or mix these vibrations as the signal passes through. If astronomers do not understand exactly how their own instrument alters the light, they might mistake the telescope's quirks for real cosmic phenomena, leading to incorrect maps of the magnetic universe.

A team of researchers has now created a precise, three-dimensional map of how the Ghana Radio Astronomy Observatory's 32-metre telescope handles these vibrations. Located in Kutunse, Ghana, this large dish is a vital tool for African astronomy, capable of tracking everything from pulsars to the birth of stars. The scientists did not simply point the telescope at the sky and guess how it performed. Instead, they built a detailed computer model of the entire dish, including its curved mirrors, support struts, and the complex path the radio waves take to reach the receiver. Using this model, they simulated how the telescope responds to signals coming from every possible direction within its field of view, calculating exactly how the instrument mixes different types of polarisation at two specific frequencies used for observation.

The study reveals that the telescope behaves differently depending on where a source of light is located relative to the centre of the dish's view. While the telescope is excellent at the very centre, its performance changes as you look toward the edges of its vision. At a frequency of 5.0 gigahertz, the researchers found that the beam of circularly polarised light is slightly shifted, with the two opposing spins of the signal peaking at points separated by nearly 10 arcseconds. This tiny shift, while small compared to the size of the beam, is significant enough to create systematic errors if not accounted for. At a higher frequency of 6.7 gigahertz, this shift is so small it cannot be resolved by their model, suggesting the telescope is even more stable at that frequency.

Perhaps most importantly, the team discovered that the telescope's ability to keep polarisation signals pure degrades as you move away from the centre. In the middle of the beam, the instrument is incredibly precise, with a measure of purity reaching 80 decibels, meaning it almost perfectly separates the different types of vibration. However, as the signal moves toward the edge of the main beam, this purity drops sharply, and the telescope begins to leak a small amount of total brightness into the polarisation measurements. By the time the signal reaches the first ring of fainter light surrounding the main beam, the instrument's ability to distinguish true polarisation from noise becomes much weaker. The researchers also found that the telescope is slightly more efficient at 5.0 gigahertz, capturing about 56.4 per cent of the available signal in its main beam, compared to 55.6 per cent at 6.7 gigahertz.

These findings provide a crucial foundation for future observations. The researchers emphasise that the telescope cannot be described by a single, simple number that applies to the whole sky. Instead, its behaviour is a complex, direction-dependent property that must be mapped out in detail. The computer model they created serves as a baseline, a reference point that future astronomers can use to correct their data. When the telescope is eventually used to observe real cosmic sources, scientists will be able to compare the actual measurements against this simulated map to remove the instrument's own influence. This ensures that when they finally see the magnetic fields of distant galaxies or the swirling gas around newborn stars, they are seeing the universe as it truly is, not as the telescope has altered it.