A Fibre Scrambling Unit for the Laser Frequency Comb of ESPRESSO

ESPRESSO (Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) is a fiber-fed, cross-dispersed, high resolution and ultra-stable echelle spectrograph is designed to enable a new quality of quantitative science on ESO's Very Large Telescope (VLT). Its science cases, range from exo-planets to the study of the physical constants call for a precision of 10 cm/s in radial velocity. To this end the ESPRESSO spectrograph, recently installed at ESO's Paranal observatory, uses a laser frequency comb (LFC) to provide a grid of equidistant lines to deliver a high precision, high accuracy wavelength calibration traceable to the SI frequency standard. The unit is inserted in the light path between the laser frequency comb and the calibration unit delivering the calibration light to the ESPRESSO spectrograph. The scrambler ensures that the injection fibre of the spectrograph is homogenously illuminated preserving the excellent qualities of the LFC light and thus avoiding introducing systematics that could compromise the wavelength calibration and the science performance. In this context it is essential that photon-noise limited performance is achieved. The ESPRESSO spectrograph will be available to the astronomical community starting October 1st, 2018.