A piston and tilt wavefront sensor dedicated to the cophasing of segmented optics in low light level conditions

We present a new interferometer to measure piston, tip and tilt of segmented mirrors, using the combination of an optical grating and a microlens array. Since this interferometer is able to collect all the incident light and to operate with a large spectra bandwidth, it is particularly suitable to operate in low light level conditions, for example for the co-phasing of segmented primary mirrors like James Webb Space Telescope. We tested our new device on a 37-segment deformable mirror and obtained an accuracy better than lambda/100. The measurement is self-referenced and errors can be estimated on the measurement itself thanks to closure relationships.