Regaining the FORS: making optical ground-based transmission spectroscopy of exoplanets with VLT+FORS2 possible again

TL;DR

This paper discusses restoring the capability of the VLT+FORS2 instrument for high-quality ground-based exoplanet transmission spectroscopy by addressing previous systematic errors, and provides strategies for optimal observations.

Contribution

The authors identify and solve systematic errors in FORS2 caused by the LADC, enabling its renewed use for exoplanet transmission spectroscopy.

Findings

Systematic errors in FORS2 were successfully mitigated.

FORS2 can now perform high-precision transmission spectroscopy.

Guidelines for optimal observational strategies are provided.

Abstract

Transmission spectroscopy facilitates the detection of molecules and/or clouds in the atmospheres of exoplanets. Such studies rely heavily on space-based or large ground-based observatories, as one needs to perform time- resolved, high signal-to-noise spectroscopy. The FORS2 instrument at ESO's Very Large Telescope is the obvious choice for performing such studies, and was indeed pioneering the field in 2010. After that, however, it was shown to suffer from systematic errors caused by the Longitudinal Atmospheric Dispersion Corrector (LADC). This was successfully addressed, leading to a renewed interest for this instrument as shown by the number of proposals submitted to perform transmission spectroscopy of exoplanets. We present here the context, the problem and how we solved it, as well as the recent results obtained. We finish by providing tips for an optimum strategy to do transmission spectroscopy with FORS2, in the hope that FORS2 may become the instrument of choice for ground-based transmission spectroscopy of exoplanets.