Programmable Phase Nulling Interferometer for Giant Mirrors

We propose the development of a programmable phase nulling interferometer that extends the capability of a computer generated hologram (CGH) to null test a range of meter class telescope mirrors, instead of a single one for which it was designed. The active element is a crystal on silicon (LCoS) spatial light modulator (SLM). The SLM corrects for the residual wavefront remaining when a CGH is used to test a mirror for which it was not designed. An aperture F-number trade space exists in which a range of mirrors can be null tested with one CGH. The prototype can be modularly added-on to a railed interferometric test setup with a CGH. In Phase I a system will be constructed with a 512x512 SLM in a brassboard Twyman-Green interferometer to null test a 10 inch F3.5 or F4 parabolic mirror. It will resemble the proposed prototype, but without a CGH, as in this case the SLM alone can null the phase error from the mirror. Tests of null quality and repeatability will be performed and measures to eliminate modulo 2p phase artifacts will be implemented. The Phase 1 brassboard will be controlled with a labVIEW executable and delivered to NASA.