Abstract: In order to measure stellar radial velocities at high precision, an accurate wavelength solution is of paramount importance. To achieve such a solution, hollow-cathode (HC) lamps have frequently been used (e.g. on HARPS, SOPHIE). By identifying and cross-matching their spectral lines with very precise catalogues, a pixel-wavelength correlation can be generated. However, these spectral lines are irregularly spaced and of highly varying intensity, and the resulting wavelength solutions are accurate only to the meter-per-second level.
Therefore, to improve the wavelength calibration, different solutions have been sought. One is the use of a Fabry-Pérot (FP) etalon; this provides a multitude of lines which are evenly spaced but whose wavelength is not accurately known. Combined with a hollow-cathode lamp to supply an absolute wavelength identification of the FP lines, it serves to better constrain the wavelength solution. Such a HC-FP combination is used by e.g. CARMENES, ESPRESSO.
In this contribution, we will present the development of the SPIRou wavelength solution. SPIRou makes use of a UNe hollow-cathode lamp, with a plethora of catalogued rays in the near infrared, and a Fabry-Pérot etalon which generates lines covering the whole domain. We describe the different approaches tested, challenges encountered and lessons learned.