Cryogenic characterization of the grating vector apodizing phase plate coronagraph for the enhanced resolution imager and spectrograph at the Very Large Telescope

Abstract

We present results on the laboratory characterization of the grating vector apodizing phase plate (gvAPP) coronagraph that will be included in the upcoming instrument enhanced resolution imager and spectrograph (ERIS) at the VLT. ERIS will include a 1 to 5 μm adaptive-optics-fed imager, NIX, that will greatly improve the capability of the VLT to perform high-contrast imaging of exoplanets especially in the 3 to 5 μm wavelength range. The gvAPP, one of the coronagraphs in the NIX suite, is a pupil plane coronagraph that uses a thin film of patterned liquid crystals to create two images of a star with a D-shaped dark hole on either side. The gvAPP is manufactured using an innovative direct-write system that produces precise patterns of liquid crystals. We utilized the upgraded infrared cryogenic test bench run by the Exoplanets and Habitability Group at ETH Zurich to measure the morphology of the gvAPP PSF and to test the accuracy of the liquid crystal manufacturing technique in the lab for the first time at contrast levels of ∼10 − 5. We find that the gvAPP can reach raw contrasts below ∼10 − 5 between ∼10 and 13 λ / D. This contrast upper limit translates to a writing accuracy of the orientation of the liquid crystal’s fast axis of better than 0.3 deg for the spatial frequencies corresponding to those separations. This is a sufficient accuracy such that the gvAPP will not be the limiting factor in achieving the required contrasts to image exoplanets.