Experimental demonstration of binary shaped pupil mask coronagraphs for telescopes with obscured pupils

Abstract

Abstract We present the fabrication and experimental demonstration of three free-standing binary shaped pupil mask coronagraphs, which are applicable for telescopes with partially obscured pupils. Three masks, designed to be complementary (labeled Mask-A, Mask-B, and Mask-C), were formed in 5 μm thick nickel. The design of Mask-A is based on a one-dimensional barcode mask. The design principle of Mask-B is similar, but has a smaller inner working angle and a lower contrast than Mask-A. Mask-C is based on a concentric ring mask and provides the widest dark region and a symmetric point spread function. Mask-A and Mask-C were both designed to produce a flexibly tailored dark region (i.e., non-uniform contrast). The contrast was evaluated using a light source comprising a broadband super-luminescent light-emitting diode with a center wavelength of 650 nm, and the measurements were carried out in a large vacuum chamber. Active wavefront control was not applied in this work. The coronagraphic images obtained by experiment were mostly consistent with the designs. The contrast of Mask-A within the ranges 3.3–8 λ/D and 8–12 λ/D was ∼ 10−4–10−7 and ∼ 10−7, respectively, where λ is the wavelength and D is the pupil diameter. The contrast of Mask-B was ∼ 10−4 and that of Mask-C over an extended field of view (5–25 λ/D) was ∼ 10−5–10−6. The effect of tilting the masks was investigated, and found to be irrelevant at the ∼ 10−7 contrast level. Therefore the masks can be tilted to avoid ghosting. These high-contrast free-standing masks have the potential to enable coronagraphic imaging over a wide wavelength range using both ground-based and space-borne general-purpose telescopes with pupil structures not specifically designed for coronagraphy.