A diamond AGPM coronagraph for VISIR

摘要

In recent years, phase mask coronagraphy has become increasingly efficient in imaging the close environment of stars,enabling the search for exoplanets and circumstellar disks. Coronagraphs are ideally suited instruments, characterized byhigh dynamic range imaging capabilities, while preserving a small inner working angle. The AGPM (Annular GroovePhase Mask, Mawet et al. 2005sup1/sup) consists of a vector vortex induced by a rotationally symmetric subwavelength grating. This technique constitutes an almost unique solution to the achromatization at longer wavelengths (mid-infrared). For this reason, we have specially conceived a mid-infrared AGPM coronagraph for the forthcoming upgrade of VISIR, themid-IR imager and spectrograph on the VLT at ESO (Paranal), in collaboration with members of the VISIR consortium.The implementation phase of the VISIR Upgrade Project is foreseen for May-August 2012, and the AGPM installed willcover the 11-13.2 μm spectral range. In this paper, we present the entire fabrication process of our AGPM imprinted on adiamond substrate. Diamond is an ideal material for mid-infrared wavelengths owing to its high transparency, smalldispersion, extremely low thermal expansion and outstanding mechanical and chemical properties. The design processhas been performed with an algorithm based on the rigorous coupled wave analysis (RCWA), and the micro-fabricationhas been carried out using nano-imprint lithography and reactive ion etching. A precise grating profile metrology hasalso been conducted using cleaving techniques. Finally, we show the deposit of fiducials (i.e. centering marks) withAerosol Jet Printing (AJP). We conclude with the ultimate coronagraph expected performances.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.