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

Abstract

Aims. SPIRou is a near-infrared (nIR) spectropolarimeter at the CFHT, covering the YJHK nIR spectral bands (980−2350 nm). We describe the development and current status of the SPIRou wavelength calibration in order to obtain precise radial velocities (RVs) in the nIR. Methods. We make use of a UNe hollow-cathode lamp and a Fabry-Pérot étalon to calibrate the pixel-wavelength correspondence for SPIRou. Different methods are developed for identifying the hollow-cathode lines, for calibrating the wavelength dependence of the Fabry-Pérot cavity width, and for combining the two calibrators. Results. The hollow-cathode spectra alone do not provide a sufficiently accurate wavelength solution to meet the design requirements of an internal error of < 0.45 m s−1, for an overall RV precision of 1 m s−1. However, the combination with the Fabry-Pérot spectra allows for significant improvements, leading to an internal error of ∼0.15 m s−1. We examine the inter-night stability, intra-night stability, and impact on the stellar RVs of the wavelength solution.