The impact of station far sidelobes on EoR/CD power spectra

Abstract: Stations of dipole antennas for SKA1-Low will comprise 256 elements spread over an area with a diameter of 38~m. We consider the effect of residual unsubtracted sources well outside of the main beam for differing numbers of unique station configurations, in the Epoch of Reionisation (EoR) and the Cosmic Dawn (CD), both in simulation and with theoretical considerations. We find that beam sidelobes imprint power that renders the cosmological signal unobservable over a range of scales unless compact sources are subtracted beyond \theta_Z=30 degrees from zenith, and that station apodization will likely be required to control far sidelobes. An array with N_b=4 unique station configurations is sufficient to reduce the contamination, with an increase to N_b=8 showing little improvement. Comparison with an achromatic Airy disk beam model shows that beam sidelobe level is the main contributor to excess power in the EoR window, and beam chromaticity is less relevant. In the EoR, z=8.5, subtracting sources above 200~mJy out to \theta_Z=45 degrees, will be required to access relevant modes of the power spectrum.