Prototype development of broadband scalar vortex coronagraphs with phase dimples for exoplanet imaging

Abstract

Directly imaging Earth-like exoplanets around Sun-like stars with the future Habitable Worlds Observatory (HWO) will require coronagraphic focal plane masks able to suppress starlight to the 1 × 10⁻¹⁰ contrast levels. Furthermore, to collect enough photons for broadband imaging and detection and to minimize the number of parallel channels for spectroscopic characterization, this level of contrast must be achieved across a 20% bandwidth. Scalar vortex coronagraphs show promise as a polarization-independent alternative to polarizationsensitive vector vortex coronagraphs, but still face chromatic limitations. New scalar vortex mask designs incorporate radial phase dimples to improve the broadband performance. We present initial manufacturing results of prototype masks of these designs including phase metrology and microscope images, in preparation for broadband chromatic characterization and starlight suppression measurements, to be taken on a high contrast imaging testbed. We also present a preliminary narrowband (2%) dark hole result achieving 1.8 × 10⁻⁸ average contrast from 3.5-10λ₀/D on the High Contrast Spectroscopy Testbed at Caltech. This work aims to advance the technological maturity of scalar vortex coronagraphs as a viable option for consideration for HWO.