Sky-projected illustration of the rough zones in which equal-brightness optical binary stars (two red crosses) at angular separation ρ and position angle θ can be resolved by Gaia. In the partially resolved region, the stars are resolved into two components depending on the scan angle ψ_i of the observation i, because of the asymmetric PSF, which has the highest spatial resolution in the along-scan direction. The bottom right inset shows a typical PSF profile (Fabricius et al. 2016) that is rotated and scaled in the background image to represent the expected PSF of the upper right component of the binary star for the given scan angle. East direction (increasing RA) is towards the left.