Design and demonstration of hybrid Lyot coronagraph masks for improved spectral bandwidth and throughput

Coronagraph focal-plane occulting masks have generally been described in terms of attenuation profiles free of any phase shift. However, phase shifts are expected and observed in physical occulting masks, with significant effect at billion-to-one coronagraph contrast levels in spectrally broad light, as required for the direct imaging and spectroscopy of nearby exoplanet systems. We report progress in the design and fabrication of hybrid focal-plane masks for Lyot coronagraphy. These masks, composed of thickness-profiled metallic and dielectric thin films superimposed on a glass substrate, are in principle band-limited in both the real and imaginary parts of the occulter characteristics. Together with a deformable mirror for control of wavefront phase, these masks offer Lyot coronagraph contrast performance better than 10-9 over spectral bandwidths of 30% or more with throughput efficiencies up to 67%. We report recent laboratory coronagraph demonstrations with vacuum-deposited nickel masks on fused silica, and preparations for the fabrication of masks with superimposed metal and dielectric layers.