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

Abstract

Context. 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. Aims. 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. However, the IR domain suffers from a lack of suitable reference spectral sources. Our goal was to build, test and finally operate a Fabry-Pérot-based RV-reference module able to provide the needed spectral information over the full wavelength range of SPIRou. Methods. We adapted the existing HARPS Fabry-Pérot calibrator for operation in the IR domain. After manufacturing and assembly, we characterized the FP RV-module in the laboratory before delivering it to the SPIRou integration site. In particular, we measured finesse, transmittance, and spectral flux of the system. Results. 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. Although we could test the stability of the system, we estimated it by comparing the SPIRou Fabry-Pérot with the already operating HARPS system and demonstrated a stability of better than 1 m s −1 during a night. Conclusions. 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. In principle, the system is also intrinsically stable over longer time scales such that it can also be used for calibration purposes.

The SPIRou wavelength calibration for precise radial velocities in the near infrared

“…The design of the SPIRou FP étalon is described in Cersullo et al (2017). It has a finesse value of F = 12.8, and was designed to cover the entire 980 − 2350 nm wavelength range of SPIRou.…”

Frequency stability of the mode spectrum of broad bandwidth Fabry-Pérot interferometers

“…However the desired short-term stability and drift is comparable to the state of the art, representing a relatively unexplored region of design parameter space. While recent technological demonstrations and in situ (at the telescope, 'on-sky') measurements with low finesse FPs have made evident the improved precision capabilities they can offer for RV spectrographs, there have been few published studies (though see [8,[27][28][29]) quantifying the performance of these FP calibrators in detail and verifying theoretically predicted behavior. Based on these criteria for an RV spectrograph calibrator, we have designed, built and characterized two FP etalons whose mechanical and optical parameters are given in Table 1.…”

Development of a stabilized Fabry–Perot etalonbased calibrator for Hanle echelle spectrograph

“…Effective finesse of the etalon depends upon reflectivity finesse, contribution from surface finish, parallelism finesse and imperfect collimation due to finite size of the light source [19]. The beam divergence resulting from the finite size of the fiber core introduces loss in effective finesse and hence a shift in transmitted peaks towards the shorter wavelengths.…”

“…Misalignment or decentering of the fiber will cause line broadening and loss in effective finesse. Using the method discussed by F. Cersullo et al in [19], we have simulated the decentering and effect of fiber size for the FP we are using and the RV error induced by each, as shown in Fig. 2 and Fig.…”