Design, manufacturing and prototype testing of the METIS ring apodizer

The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of the three first-generation instruments of the Extremely Large Telescope (ELT). METIS will enable high-contrast imaging through different coronagraphic modes, one of which is implemented with a vortex coronagraph. However, the complex pupil of the ELT with its large central obscuration limits the performance of a classical vortex coronagraph. Using a grayscale ring apodizer in a pupil plane upstream of the vortex phase mask can correct for the effect of the central obscuration and partly restore the coronagraphic performance of the vortex for the ELT pupil. The coronagraphic performance of the ring-apodized vortex coronagraph relies on the precise control of transmission in the grayscale region, which can be implemented using a technology based on chromium microdots. Here, we present the ring apodizer for the METIS instrument exploiting the microdots technology. We first describe the design process of microdot patterns and perform simulations for both transmission and phase response of the microdots with various geometric parameters. We then describe the manufacturing of three prototypes featuring different design parameters, as well as their testing in terms of transmission and phase response. Finally, capitalizing on the lessons learned, we report on the current status of manufacturing and testing of the final ring apodizer for METIS.