An assembly for collimating broadband radiation, the assembly comprising: a convex refractive singlet lens having a first spherical surface for coupling the broadband radiation into the lens and a second spherical surface for coupling the broadband radiation out of the lens, wherein the first and second spherical surfaces have a common centre; and a mount for holding the convex refractive singlet lens at a plurality of contact points having a centroid coinciding with the common centre.