ESPRESSO front end exposure meter: a chromatic approach to radial velocity correction

Abstract

This paper presents the Espresso Exposure Meter (EM) implementation. ESPRESSO,1-3 the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will be installed on ESOs Very Large Telescope (VLT). The light coming from the Telescope through a Coud Focus4 of all the Four Telescope Units (UTs) will be collected by the Front End Unit that provides Field and Pupil stabilisation and injects the beams into the Spectrograph fibers.5 An advanced Exposure Meter system will be used to correct Radial Velocity (RV) obtained from the scientific spectrum for the Earth relative motion. In this work we will present the perfor mance of an innovative concept for the Exposure Meter system based on a Charge Coupled Device (CCD) with a chromatic approach for the calculation of the Mean Time of Exposure (MTE). The MTE is a crucial quantity used for the correction of RV for the Earth relative motion during the exposure. In particular, splitting the light in different chromatic channels on the CCD, we will probe for potential chromatic effects on the calculation of the MTE in each channel and how they could be used in order to perform the correction of RV. The paper is accompanied by a fully described numerical analysis that keeps into view a key performance evaluation for different stellar spectral types (B to M spectral main sequence classes).