The geometry for estimating the virtual-source size Sv seen by the KB mirrors. The lens was placed close to the KB mirrors indicated by the gray box. D is the distance from the source point being imaged by the KB mirror to the lens and F is the focal length of the lens. α is the acceptance angle of the lens, typically defined by the entrance slits to the KB mirrors. In this work, D = 56.5 m, F varies from 4.4 m to 9.4 m (see Table 1), and the KB mirrors are placed close to the lens at 57 m from the virtual source.