Three basic forms of diffractive optics for X-ray telescopes. (a) A zone plate (ZP), with opaque and transparent regions, (b) A phase zone plate (PZP), in which the shaded zones transmit radiation with a phase shift of , (c) A phase Fresnel lens (PFL), in which the thickness is everywhere such that the phase is shifted by the optimum angle. The profile in (c) is drawn for a converging lens assuming the refractive index is less than unity.
