A collector optical system is provided in which radiation is collected from a radiation source and directed to an image focus. The collector optical system includes one or more mirrors, with each mirror being symmetric about an optical axis extending through the radiation source and each mirror having at least first and second reflective surfaces. The first and second reflective surfaces have a common focus, such that radiation from the source undergoes successive grazing incidence reflections at the first and second reflective surface and wherein the common focus is transversely offset by a predetermined distance Δr with respect to the optical axis.