The VLT/ERIS grating vector Apodizing Phase Plate coronagraph

TL;DR

This paper details the design, testing, and performance of the gvAPP coronagraph for ERIS on the VLT, highlighting its capabilities and limitations for high-contrast imaging of substellar companions.

Contribution

It presents the first on-sky testing of the gvAPP coronagraph for ERIS, demonstrating its performance and identifying electronic noise limitations for future improvements.

Findings

Achieved 90% transmission in K band and 60% in L band.

Reached a raw contrast of 1×10⁻⁵, but not the post-processed contrast.

Electronic noise affects the coronagraphic regions, impacting contrast performance.

Abstract

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. We used both laboratory measurements and on-sky observations to characterise the gvAPP in several different filters, from the K to the L band. 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 $1 \times 10^{-5}$, it does not reach the post-processed contrast of $5 \times 10^{-5}$ 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. 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.