A new infrared Fabry-Pérot-based radial-velocity-reference module for the SPIRou radial-velocity spectrograph

Abstract

The field of exoplanet research is moving towards the detection and characterization of habitable planets. These exo-Earths can be easily found around low-mass stars by using either photometric transit or radial-velocity (RV) techniques. In the latter case the gain is twofold because the signal induced by the planet of a given mass is higher due to the more favourable planet-star mass ratio and because the habitable zone lies closer to the star. However, late-type stars emit mainly in the infrared (IR) wavelength range, which calls for IR instruments. SPIRou is a stable RV IR spectrograph addressing these ambitious scientific objectives. As with any other spectrograph, calibration and drift monitoring is fundamental to achieve high precision. Our goal was to build, test and finally operate a Fabry-P\'erot-based RV-reference module able to provide the needed spectral information over the full wavelength range of SPIRou. We adapted the existing HARPS Fabry-P\'erot calibrator for operation in the IR domain. After manufacturing and assembly, we characterized the FP RV-module in the laboratory. We measured finesse, transmittance, and spectral flux of the system. The measured finesse value of F=12.8 corresponds perfectly to the theoretical value. The total transmittance at peak is of the order of 0.5%, mainly limited by fibre-connectors and interfaces. Nevertheless, the provided flux is in line with the the requirements set by the SPIRou instrument. Once installed on SPIRou, we will test the full spectral characteristics and stability of the RV-reference module. The goal will be to prove that the line position and shape stability of all lines is better than 0.3 m s$^{-1}$ between two calibration sequences (typically 24 hours), such that the RV-reference module can be used to monitor instrumental drifts.