MARVEL: development and strategies of the calibration system

MARVEL is a new state-of-the-art facility at the Mercator Observatory (La Palma, Spain) targeting high precision radial velocity measurements. The facility consists of an array of four 80-cm telescopes feeding a single stabilized high-resolution echelle spectrograph. It will provide essential ground-based RV follow-up on transit measurements by previous and upcoming space missions with an RV precision of 1 m/s. This precision requires simultaneous wavelength calibration monitoring on a reliable wavelength reference calibration source. The MARVEL calibration system will be powered by a continuum white light source spanning the target wavelength range of 380-950 nm, and both a ThAr hollow-cathode lamp and a Fabry-Pérot etalon, whose light will be used as a wavelength reference. The light from the continuum source will be divided for use in spectral flat fielding, and for the illumination of the Fabry-Pérot etalon. The wavelength comb of transmission peaks produced by the etalon will be tracked against the hyperfine lines of rubidium, for continuous evaluation of residual etalon drift. Furthermore, the ThAr lamp will serve as an absolute reference for the etalon spectrum. Optical fiber switches will select one of the three calibration sources, as well as transport the light to its destination: the spectrograph, the science fibers at the focus of each telescope, or to a flux monitoring system. In this contribution, we describe the MARVEL wavelength calibration system and the strategy that will be deployed to maximize calibration precision and long-term stability.