The VLT/ERIS grating vector Apodizing Phase Plate coronagraph

Design and on-sky performance

¹ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
² ETH Zurich, Institute for Particle Physics & Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁶ Max Planck Institute for Intelligent Systems, Max-Planck-Ring 4, 72076 Tübingen, Germany
⁷ ETH Zurich, Department of Earth and Planetary Sciences, Sonneggstrasse 5, 8092 Zurich, Switzerland
⁸ National Solar Observatory, 3665 Discovery Dr., Boulder, Co 80303, USA
⁹ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
¹¹ School of Natural Sciences, Center for Astronomy, University of Galway, Galway H91 CF50, Ireland
¹² Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹³ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
¹⁴ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy
¹⁶ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
¹⁷ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741, Garching, Germany
¹⁸ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, 64100, Teramo, Italy
¹⁹ INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
²⁰ STFC UK ATC, Royal Observatory Edinburgh, Blackford Hill. Edinburgh EH9 3HJ, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 December 2025
Accepted: 6 March 2026

Abstract

Aims. We describe the design, laboratory manufacture, and on-sky testing of the grating vector apodizing phase plate (gvAPP) coronagraph for the Enhanced Resolution Imager and Spectrograph (ERIS) on the Very Large Telescope.

Methods. We used both laboratory measurements and on-sky observations to characterise the gvAPP in several different filters, from the K to the L band.

Results. In testing, the gvAPP reaches its design specification in the transmission of the optic with 90% in the K bands and 60% in the L band. While the gvAPP reaches its designed raw contrast performance of 1 × 10⁻⁵, it does not reach the post-processed contrast of 5 × 10⁻⁵ in on-sky observations. Electronic detector noise, due to the Airy core of the coronagraphic point spread function inducing cross-talk between the readout amplifiers, produces a repeated pattern within the coronagraphic regions of the gvAPP.

Conclusions. Despite these limitations, we recommend the gvAPP as a tool for characterising substellar companions with known separations and position angles, which allow them to be placed in the coronagraphic dark holes for observations. The ERIS gvAPP’s leakage term can also be used as a photometric reference for time series observations; however, we caution that the contrast performance may limit such studies to only the brightest targets. ERIS gvAPP data quality may be improved further with better modelling of detector electronic noise. This work is a pathfinder for Extremely Large Telescope instruments including METIS, which will include gvAPP coronagraphs with improved designs based on these results.