Laboratory Demonstration of High Contrast Using PAPLC with High-Order Wavefront Sensor

The search for life in the cosmos is one of the next great challenges in astronomy. Based on Astro 2020's recommendation, NASA is starting a Great Observatories Maturation Program for the Habitable Worlds Observatory (HWO), the future flagship to search for Earth Twins in the habitable zone of ~100 nearby stars. Achieving this goal will require advanced starlight suppression technologies operating behind a large segmented aperture telescope. After the successful launch and commissioning of JWST, segmented primaries have now been proven as a practical, high-heritage architecture for a large-aperture, high-contrast system. However, high-performance coronagraphs on segmented telescopes present technological challenges that must be solved to enable a statistically robust search for life on other worlds. One coronagraph that has provided excellent performance in simulations is the Phase-Apodized-Pupil Lyot Coronagraph (henceforth PAPLC). This coronagraph uses a one-sided dark zone with wide spectral bandwidth (up to 20% relative bandwidth), high throughput (up to 40% core throughput) and low inner-working-angle (up to 2 lambda/D). Furthermore, optimizations demonstrate robustness to stellar diameter up to 0.1 lambda/D. The PAPLC is compatible with off- and on-axis monolithic or segmented telescope pupils considered for HWO. This performance makes the PAPLC highly competitive for characterizing exo-Earths. A first demonstration on a segmented telescope pupil was performed at the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed, reaching a contrast of ~4e-8 in 9% broadband light on a one-sided dark zone from 2.2 -- 13 lambda/D_circumscribed using a 37 hexagonal segmented off-axis pupil. By imprinting a Zernike dimple on the knife-edge focal-plane mask of the PAPLC, implemented on HiCAT as two optically-conjugated separate substrates, we demonstrated its ability to measure both low- and high-order aberrations on the incoming pupil using only light rejected by the coronagraph. The sensing performance was shown to be limited at about half that of a dedicated Zernike sensor which would use all the light from the telescope. High-order wavefront sensing telemetry enables in-band sensing and control techniques, and allows for post processing of science images. The static performance of the PAPLC on HiCAT is shown to be limited by ambient wavefront drifts and by the current-generation coronagraph design. We propose a three-year effort aimed at in-vacuum laboratory validation of the PAPLC concept for HWO. Our high-contrast demonstration will demonstrate <1e-9 raw contrast at 2 lambda/D in 10% broadband light at the High Contrast Imaging Testbed (HCIT) facility at JPL with supporting testing and component validation performed on the HiCAT testbed at STScI. Simultaneously, we will demonstrate simultaneous high-order sensing capabilities using an improved knife-edge focal plane mask with etched Zernike dimple, fabricated at JPL's MicroDevices Laboratory (MDL). Finally, we will advance optimization techniques to extend as-measured tolerances to low-order aberrations to match model predictions. Our broadband <1e-9 contrast demonstration, at very small inner working angles, of the PAPLC concept on a monolithic and segmented pupil, in combination with its integrated simultaneous high-order wavefront sensor, will mature it from technology readiness level 4 to 5.

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