Laboratory Demonstrations of High Contrast with Black Silicon Coronagraph Masks

A major recommendation of the Astro2020 Decadal Survey is to directly image and spectrally characterize about 25 Earth-like exoplanets with a future 6-meter flagship observatory. One of the most pressing needs to achieve this, as indicated by being in Tier 1 of NASA APD's technology gap list, is having efficient, robust coronagraphs that can reach the needed 1e-10 planet-to-star contrast ratio. Thus far, the only coronagraph architecture to have achieved better than 1e-9 contrast in broadband light is the classical Lyot coronagraph, but compared to other coronagraph types it is neither efficient in throughput nor robust to low-order optical aberrations. In this proposal, we aim to demonstrate that black silicon apodizer masks are a safe, predictable technology for characterizing Earth-like exoplanets with the Astro2020-recommended flagship. Apodizers are a robust way of overcoming diffraction created by segment gaps and obscurations in a telescope aperture, which is why they were baselined in the coronagraph designs for both the LUVOIR-A and LUVOIR-B mission concepts. The Roman Space Telescope Coronagraph Instrument project pioneered black silicon apodizer masks and demonstrated their performance down to 4e-9 broadband contrast, but those were inherently limited in contrast and throughput by the large pupil obscurations of the Roman Space Telescope. To be a viable technology for the Astro2020 flagship, black silicon apodizers must be demonstrated and understood at the 1e-10 contrast level and provide higher throughput. The primary goal of this proposal is to advance black silicon apodizer masks from TRL 4 to TRL 5 for the future exoplanet imaging mission prioritized by Astro2020. We will fabricate shaped pupil Lyot coronagraph masks in JPL's Microdevices Laboratory (MDL) and test them in vacuum both at the University of Arizona's (UArizona's) Space Coronagraph Optical Bench (SCoOB) and at JPL's High Contrast Imaging Testbed (HCIT) facility on the Decadal Survey Testbed 2 (DST2). On the DST2, we will use black silicon coronagraph masks to demonstrate <=4e-10 broadband contrast with high throughput for monolithic and segmented telescope apertures. The secondary goal of this proposal is to create and validate accurate models of the incoherent light created by the apodizer mask itself. It is well known that if mask features are close in size to the wavelength of light, then vector diffraction becomes non-negligible. However, it is unknown at what feature size vector diffraction can be ignored at the 1e-10 contrast level. The UArizona team will create a model of the polarization aberrations created by small mask features, and we will then validate that model with tests on SCoOB and DST2. Similarly, we will create and validate a model of the incoherent light from specular reflection off the black silicon parts of the mask. These validated models will enable accurate contrast error budget predictions for any apodizer designs considered for the Astro2020-recommended flagship.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk.

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