Accurate wavelength calibration to test the fine-structure constant

The wavelength calibration of ESPRESSO has been studied in detail and it has been demonstrated to be extremely accurate and suitable to serve its main scientific goals.

This study used the highly comprehensive set of calibration sources ESPRESSO is equipped with, namely a classical ThAr hollow cathode lamp, a Fabry-Pérot interferometer and a laser frequency comb. The results demonstrated that ESPRESSO data will allow a significant improvement on constraints on a possible variation of the fine-structure constant across the history of the Universe. This test is based on measuring differential shifts of specific spectral lines of metal absorption systems, for which a very precise wavelength calibration is needed. This study showed ESPRESSO’s wavelength calibration to be up to two orders of magnitude better than for spectrographs previously utilised for fine-structure studies.