Optical testbed demonstration of Fizeau Interferometric Cophasing of Segmented Mirrors

The recent trend for segmented primary mirrors in designs for large optical telescopes has introduced a new problem: that of precisely aligning the separate segments to conform to the desired figure. Existing solutions for this "cophasing" procedure are far from ideal, often requiring dedicated hardware and/or motion of various optical elements, introducing non-common path errors. Our recently developed approach, termed Fizeau Interferometric Cophasing of Segmented Mirrors (FICSM), overcomes many of these disadvantages by allowing this process to be accomplished with an ordinary science camera with no specialist hardware. FICSM has been adopted as the primary backup strategy for cophasing the James Webb Space Telescope (JWST). Numerical simulations have shown it has the potential to phase the JWST primary mirror from an initial configuration with piston steps as large as 300 microns, to nanometre precision. Here we also report first results from a dedicated laboratory demonstration of FICSM utilizing a MicroElectroMechanical System segmented mirror as a proxy for the JWST primary mirror. This test illustrates the principles behind the method and demonstrates that it is robust against noise and imperfections encountered in real world settings. This is promising for its application to JWST and other segmented mirrors.