John Trauger/Jet Propulsion Laboratory

Among the leading architectures for the imaging and spectroscopy of nearby exoplanetary systems is the space coronagraph, which provides in principle very high (10 billion to one) suppression of diffracted and scattered starlight at very small separations (a few tenths of arcseconds) from the star. The concept of a band-limited Lyot coronagraph provides the theoretical basis for mathematically perfect starlight suppression. In practice, the optical characteristics of available materials and practical aspects of the fabrication processes impose limitations on contrast levels and spectral bandwidths that are achievable in the real world.

We take guidance from the ACCESS ASMCS study, which assessed the performance of four major coronagraph architectures (Lyot, vector vortex, pupil mapping, and shaped pupil) in the context of a conceptual space observatory platform of high technology maturity. The hybrid Lyot coronagraph has produced the best laboratory-validated performance among the major coronagraph types for contrast, spectral bandwidth, inner working angles, and overall throughput, and alone it has demonstrated high-contrast imaging performance at levels required for exoplanet exploration. Our objective is to advance the hybrid Lyot technology, thereby advancing a key enabling technology for a scientifically compelling medium-class coronagraph mission.

Our objectives are to design and fabricate hybrid Lyot masks for high contrast in each of three spectral bands corresponding to the V, R, and I photometric bands, each designed for a spectral bandwidth of 20%. Masks will provide inner working angles of 2-3 λ/D, where λ/D is the central wavelength of the respective spectral bands, and where the precise inner working angle of each physical mask will be determined by selecting the f/number for the coronagraph system. Our milestones are demonstrations of coronagraph performance and validations of model predictions in the HCIT coronagraph, including contrast performance of 10-9 or better over spectral bandwidths of 20% or more in all three spectral bands.

The proposed technology development addresses three of the five areas of interest called out in the TDEM NRA. (1) The occulting masks are expected to demonstrate 10-9 contrast ratios in three 20% spectral bands in the visible. (2) Wavefront control is an integral component of the proposed mask design, which then informs the wavefront sensing and control procedures in the proposed HCIT contrast demonstrations. And (3) the proposed work will seek the optimal hybrid Lyot coronagraphic mask component for 20% bandwidth. Further, the proposed key technology demonstrations enable a reliable
assessment of science performance and relevance of the space coronagraph to the NASA ExEP program objectives.

Technology Development for Exoplanet Missions
Solicitation NNH09ZDA001N-TDEM