According to one aspect, a part has two reflective surfaces, one being a conic surface portion having an axis with a focus thereon, and the other being part of a spherical surface with a centerpoint at the focus. According to a different aspect, a method includes fabricating a part with first and second reflective surfaces, the first being a conic surface portion with an axis and a focus on the axis, and the second being a spherical surface portion with a centerpoint at the focus. The second surface is used to position the part so that the focus coincides with the centerpoint of a spherical wave from an interferometer. Then, a reflective further spherical surface portion on a member is used with the interferometer to position a centerpoint of the further surface at the focus. The interferometer then evaluates the first surface for accuracy.