 The latest proposed extremely large astronomical telescopes , such as the Thirty Meter Telescope and the European Extremely Large Telescope , will be of segmented design . Their primary mirrors will be built from hundreds of hexagonal mirror segments . Polishing and figuring these highly aspheric and non @-@ rotationally symmetric mirror segments presents a major challenge . Traditional means of optical testing compares a surface against a spherical reference with the aid of a null corrector . In recent years , computer @-@ generated holograms ( CGHs ) have begun to supplement null correctors in test setups for complex aspheric surfaces . Fig . 15 illustrates how this is done . Unlike the figure , actual CGHs have line spacing on the order of 1 to 10 µm . When laser light is passed through the CGH , the zero @-@ order diffracted beam experiences no wavefront modification . The wavefront of the first @-@ order diffracted beam , however , is modified to match the desired shape of the test surface . In the illustrated Fizeau interferometer test setup , the zero @-@ order diffracted beam is directed towards the spherical reference surface , and the first @-@ order diffracted beam is directed towards the test surface in such a way that the two reflected beams combine to form interference fringes . The same test setup can be used for the innermost mirrors as for the outermost , with only the CGH needing to be exchanged . 
