High-contrast spectroscopy testbed (HCST): tip/tilt sensing in reflection of the vector vortex coronagraph (VVC)

With the identified objective of enabling Earth-like exoplanets direct detection, and characterization of their atmospheric content, the Astro2020 report has placed the maturation of exoplanet imaging technology as a key priority for the coming decade. The High Contrast Spectroscopy Testbed (HCST) within the Caltech Exoplanet Technology laboratory serves as an in-air coronagraphic testbed demonstrator, integrating a high order deformable mirror for wavefront control and a vector vortex coronagraph (VVC). HCST has demonstrated excellent in-air contrast performance, achieving 1 × 10⁻⁸ raw contrast in broadband light, for both the apodized off-axis segmented pupil configuration and using single mode fiber planet injection. By introducing a low-order wavefront sensor (WFS) that utilizes either the in-band or out-of-band reflected light from the VVC coupled with a tip/tilt mirror, our objective is to address dynamic errors, thereby enhancing the wavefront stability of the experiment. We present in this proceeding the first steps towards a full tip/tilt control loop, starting with the optical design of our low-order camera. We performed a drift test overnight as a diagnostic of the coronagraphic performance stability and to possibly identify causes for the drift. Conclusions show that HCST demonstrate a remarkably stable environment to perform high-contrast imaging experiments, at the level of 1 × 10⁻⁸ contrast.