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

TL;DR

This paper presents experimental and simulated results of an upgraded evanescent wave coronagraph with an achromatic focal plane mask, demonstrating high contrast performance and stability over several months for potential exoplanet imaging.

Contribution

The study introduces an improved focal plane mask for the EvWaCo coronagraph, validating its achromatic performance and stability through experiments and simulations.

Findings

Achieved contrast of a few 10^{-4} at 3 λ/D in the I-band

Confirmed the theoretical achromatic advantage via simulations

Demonstrated mask stability over 8 months

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 ${\lambda}/D$ over the full I-band (${\lambda}_c =$ $800$ $nm$, ${\Delta}{\lambda}/{\lambda} \approx 20\%$) and equal to 4 ${\lambda}/D$ over the full R-band (${\lambda}_c =$ $650$ $nm$, ${\Delta}{\lambda}/{\lambda} \approx 23\%$) 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.