Design, analysis, and testing of a microdot apodizer for the apodized pupil Lyot coronagraph - II. Impact of the dot size

Context. The Apodized Pupil Lyot Coronagraph (APLC) is a promising coronagraphic device for direct exoplanet detection with the European Extremely Large Telescope. This concept features amplitude apodization in the entrance aperture, and a small opaque Lyot mask in the focal plane. We present new near-IR laboratory results using binary apodizers – the so-called microdot apodizer – which represent a very attractive and advantageous solution for the APLC. Aims. Microdot apodizers introduce high-frequency noise whose characteristics depend on the pixel size. The aim of this work is to characterize the impact of the pixel size on the coronagraphic image. We aim to estimate both the noise intensity and its localization in the field of view. Methods. The microdot apodizer, consisting of an array of pixels with spatially variable density that are either opaque or transparent, was manufactured by lithography of a light-blocking metal layer deposited on a transparent substrate. A set of 5 masks has been designed with different pixel sizes, tested in the near-IR, and their behavior compared to theoretical models. Results. Stray light diffraction introduced by the finite pixel size was measured during experiments. The intensity decreases, and radial distance increases, when the pixel size gets smaller. Conclusions. The physical properties of these microdot apodizers have been demonstrated in the laboratory. The microdot apodizer is a suitable solution for any coronagraph using pupil amplitude apodization, if properly designed.