2021 May Monthly Media is brought to you by our Monthly Media Megastar (for most prolific writer); Dr Jack Line, on some of his recent work. As discussed in Jack’s December 2018 Monthly Media, a radio telescope observes the Universe through a “window” of sorts, known as the primary beam. This primary beam is particular to every telescope, and a bright radio source seen through the window will always appear changed because of it.

The Murchison Widefield Array (MWA) is sensitive to certain types of light. Not just wavelength (the colour) but also polarisation (the way the light travels through space).

Different astronomical objects emit light of different polarisation types, or can change the polarisation of light passing through them (without necessarily emitting their own light). Observations of how the polarisation has been changed can be the only method for observing objects that don’t emit their own light, therefore making polarisation an important phenomenon to study.

The core the of MWA telescope, with kangaroos for scale. Photo credit: MWA Collaboration and Curtin University.

The dipoles of every MWA tile are made with two crossed antennas. One antenna is aligned E-W and the other aligned N-S, giving them their “spidery” appearance. These constant cardinal alignments act as a reference point so the MWA can measure the polarisation of the light that lands on it. However, the MWA, like all telescopes, does not see the sky perfectly, and the effect of the primary beam on the observed polarisation needs to be accounted for.

The GIF below shows the simulated effects of the primary beam on the observed polarisation of a bright polarised radio source, if the radio source is observed at different parts of the sky. As the source moves away from the centre of the primary beam (directly overhead) towards the edge (the horizon), polarisation errors caused by the primary beam swamps the polarisation signal of the radio source of interest.

Top and bottom left: Antenna gain for the N-S and E-W antenna alignments, respectively, in each dipole. Antenna gain is a measure of how well a telescope sees the sky; the higher the number the stronger the sky signal. Top right: Detected rotation measure over the whole sky. The rotation measure is a particular measurement of polarisation of interest, with the primary beam effects appearing at 0 as the radio source is no longer directly overhead. Bottom right: The primary beam pattern on the sky, and the location of the source of interest (orange star), starting at the overhead location and moving closer to the horizon for each observation.

The good news is, if we understand our instrument well enough, we can undo this effect. This is not trivial, and can (and has, and probably will again), caused many confused meetings for astronomers arguing whether zero points towards north or east and whether X means east or north.