Optical Verification Experiments of Sub-Scale Starshades

Starshades are a leading technology to enable the detection and characterization of Earth-like exoplanets. In this paper we report on optical experiments of sub-scale starshades that advance critical starlight suppression technologies in preparation for the next generation of space telescopes. These experiments were conducted at the Princeton starshade testbed, an 80 m long enclosure testing 1/1000$^\text{th}$ scale starshades at a flight-like Fresnel number. We demonstrate 10$^{-10}$ contrast at the starshade's inner working angle across 10\% of the visible spectrum, with an average contrast at the IWA of $2.0\times10^{-10}$ and contrast floor of $2\times10^{-11}$. In addition to these high contrast demonstrations, we validate diffraction models to an average accuracy of 15\% through tests of intentionally flawed starshades. Overall, this suite of experiments reveal a deviation from scalar diffraction theory due to light propagating through narrow gaps between petals. We provide a model that accurately captures this effect at contrast levels below $10^{-10}$. The results of these experiments demonstrate that there are no optical impediments to building a starshade that provides sufficient contrast to detect Earth-like exoplanets. This work also sets an upper limit on the effect of unknowns in the diffraction models used to predict starshade performance and set tolerances on the starshade manufacture.