Experimental demonstration of spectral linear dark field control at NASA’s high contrast imaging testbeds

In order to directly image Earth-like exoplanets (exoEarths) orbiting Sun-like stars, the Habitable Worlds Observatory coronagraph instrument(s) will be required to suppress the starlight to raw contrasts of ∼ 10⁻¹⁰. Coronagraphs use active methods of wavefront sensing and control (WFSC) such as pairwise probing (PWP) and electric field conjugation (EFC) to create regions of high contrast in the science camera image, called dark holes. Due to the low flux of these exoEarths, long exposure times are required to spectrally characterize them. During these long exposures, the optical system will drift resulting in degradation of the contrast over time. To prevent such contrast drift, a WFSC algorithm running in parallel to the science acquisition can stabilize the contrast in the dark hole. However, PWP cannot be reused to efficiently stabilize the contrast since it relies on strong temporal modulation of the intensity in the image plane that would interrupt the science acquisition. Conversely, spectral linear dark field control (LDFC) takes advantage of the linear relationship between the change in intensity of the post-coronagraph out-of-band image and small changes in wavefront to preserve the dark hole region during science exposures. In this paper, we show experimental results that demonstrate spectral LDFC stabilizes the contrast to levels of a few 10⁻⁹ on a Lyot coronagraph testbed which is housed in a vacuum chamber. Promising results show that spectral LDFC is able to correct for disturbances that degrade the contrast by more than 100×.