New techniques for accurate and stable wavelength calibration

Key science drivers for ELT/ANDES, in particular the search for a possible change of the fine-structure constant and the direct observation of cosmic expansion in real time (a.k.a the Sandage test), require exquisite wavelength calibration. To ensure that ANDES can actually benefit from the unprecedented photon-gathering power and not be limited by systematics, an order-of-magnitude improvement in wavelength calibration compared to current instruments like VLT/ESPRESSO is needed. As a first step, substantial improvements in the ESPRESSO wavelength calibration accuracy could be achieved by a careful modeling of the line-spread function, utilizing the rich information of laser frequency comb spectra. This reduced the instrument-related systematics from over 30 m/s to 2 m/s. In addition, the feasibility of using iodine absorption cells for an end-to-end validation of the wavelength calibration, probing the full optical path from sky to detector, was recently demonstrated. Still, achieving the ambitious goals for the ANDES wavelength calibration of 1 m/s accuracy and 1 cm/s stability over decades will require further active and dedicated research in the next years.