High-contrast imager for complex aperture telescopes (HiCAT): 11. System-level demonstration of the Apodized Pupil Lyot Coronagraph with a segmented aperture in air

We present the final results of the Apodized Pupil Lyot Coronagraph (APLC) on the High-contrast imager for Complex Aperture Telescopes (HiCAT) testbed, under NASA's Strategic Astrophysics Technology program. The HiCAT testbed was developed over the past decade to enable a system-level demonstration of coronagraphy for exoplanet direct imaging with the future Habitable Wolds Observatory. HiCAT includes an active, segmented telescope simulator, a coronagraph, and metrology systems (Low-order and Mid-Order Zernike Wavefront Sensors, and Phase Retrieval camera). These results correspond to an off-axis (un-obscured) configuration, as was envisioned in the 2020 Decadal Survey Recommendations. Narrowband and broadband dark holes are generated using two continuous deformable mirrors (DM) to control high order wavefront aberrations, and low-order drifts can be further stabilized using the LOWFS loop. The APLC apodizers, manufactured using carbon nanotubes, were optimized for broadband performance and include the calibrated geometric aperture. The objectives of this SAT program were organized in three milestones to reach a system-like level demonstration of segmented-aperture coronagraphy, from static component demonstration to system-level demonstration under both natural and artificial disturbances. HiCAT is, to this date, the only testbed facility able to demonstrate high-contrast coronagraphy with a truly segmented aperture, as is required for the Habitable World Observatory, albeit limited to ambient conditions, corresponding to NASA's Technology Readiness Level (TRL) 4. Results presented here include 6 x 10(-8) (90% CI) contrast in 9% bandpass in a 360 deg dark hole with inner and outer working angles of 4.4./D-pupil and 11 lambda/D-pupil. Narrowband contrast (3% bandpass) reaches 2.4 x 10(-8) (90% confidence interval). We first explore the open-loop stability of the entire system quantify the baseline testbed performance. Then we present dark hole stabilization using both high-order and low-order loops under both low-order and segment level drifts in narrow and broadband. We compare experimental data with that obtained by the end-to-end HiCAT simulator. We establish that current performance limitations are due to a combination of ambient conditions, detector and deformable mirrors noises (including quantization), and model mismatch.