ABSTRACT:
In order for a segmented-mirror telescope like Keck to achieve its full diffraction limit, the telescope must be phased; that is, the steps between segments must be reduced to a small fraction of the wavelength of light. Exactly how well things need to be phased depends upon on a number of factors; because Keck image quality is limited by segment figure errors and other effects (to be discussed), phasing the telescope to 30 nm has proved adequate. The problem is that the phasing requirement for the Thirty Meter Telescope (TMT) is 10 nm, and for many years we have not understood why we cannot do better at Keck. A recent investigation into the Keck phasing problems has revealed some subtle and unexpected optical effects, which have important implications for TMT and for large segmented-mirror telescopes in general.