Optimizing optics and opto-mechanical mounting to minimize static aberrations in high-contrast instruments

Abstract

One of the goals of high-contrast imaging is to reach contrasts of 10^(-10) at small inner working angles to directly image Earth-like exoplanets around solar-type stars. In most imaging systems, a deformable mirror (DM) in the pupil plane can correct for phase errors but surface errors in out-of-pupil optics get coupled into amplitude errors which can only be controlled with a second, out of pupil, DM. Furthermore, correcting static errors introduced by the optics can take up valuable DM stroke. Thus, minimizing the wavefront error within the system is critical to reaching high contrast levels. For example, the High Contrast Spectroscopy for Segmented Telescopes Testbed (HCST) in the Exoplanet Technologoy lab at Caltech aims to develop exoplanet imaging technologies down to small inner working angles (< 3λ/D) which requires an RMS wavefront error of less than 10 nm per optic to achieve a contrast of 10^(−5) with the DM flattened. While aligning HCST, it was determined that despite the excellent surface quality of all the optics, the mounts were introducing significant wavefront errors. Here we assess the effect of mount-induced wavefront errors that can rapidly consume the wavefront budget of a high-contrast system. We also present the method used to mitigate this effect within HCST such that a mean contrast of 6 × 10^(-6) from 3-10λ/D was achieved with a vortex coronagraph and flattened DM.