Deep Contrast and Companion Detection Using the EvWaCo Test Bed Equipped with an Achromatic Focal Plane Mask and an Adjustable Inner Working Angle

Abstract

The evanescent wave coronagraph uses the principle of frustrated total internal reflection (FTIR) to suppress the light coming from the star and study its close environment. Its focal plane mask is composed of a lens and a prism placed in contact with each other to produce the coronagraphic effect. In this paper, we present the experimental results obtained using an upgraded focal plane mask of the Evanescent Wave Coronagraph (EvWaCo). These experimental results are also compared to the theoretical performance of the coronagraph obtained through simulations. Experimentally, we reach a raw contrast equal to a few 10-4 at a distance equal to 3 λ/D over the full I band (λ c = 800 nm, Δλ/λ ≈ 20%) and equal to 4 λ/D over the full R band (λ c = 650 nm, Δλ/λ ≈ 23%) in unpolarized light. However, our simulations show a raw contrast close to 10-4 over the full I band and R band at the same distance, thus confirming the theoretical achromatic advantage of the coronagraph. We also verify the stability of the mask through a series of contrast measurements over a period of 8 months. Furthermore, we measure the sensitivity of the coronagraph to the lateral and longitudinal misalignment of the focal plane mask and to the lateral misalignment of the Lyot stop.