Fizeau interferometric cophasing of segmented mirrors

Segmented mirrors have quickly become an integral part of large telescope design in optical astronomy. They mitigate many of the problems associated with monolithic mirrors, such as rigidity, fabrication and transport, and even allow for foldable primary mirrors such as for the James Webb Space Telescope (JWST). However, one significant disadvantage is the need to cophase the separate segments to ensure they conform to the optimum mirror shape. Many cophasing approaches have been proposed and employed in practice, but all suffer from significant problems. Most notably the introduction of non-common path error, a persistent problem plaguing both the fields of active and adaptive optics. One recent proposed cophasing algorithm eliminates non-common path error by removing the requirements for additional hardware and instead concentrating on measuring piston and tip/tilt aberrations by their effects on images. Fizeau Interferometric Cophasing of Segmented Mirrors (FICSM) yields a large capture range and allows phasing to interferometric precision; for these reasons it was recently selected as the backup phasing strategy for the JWST. Here we present an overview of numerical simulations and results of optical testbeds, including the first lab demonstration of FICSM which successfully phased a segmented mirror with more than 5 wavelengths of piston to an RMS of 25nm, a result consistent with the limit set by the accuracy of segment motion. These results suggest this approach is well suited to the task of segment cophasing for future space missions.