BLUVES: calibrating astronomical spectrographs in the UV with laser frequency combs

Precise and accurate wavelength calibration of spectrographs is essential for key science cases, e.g. the search for extrasolar planets, a possible variation of fundamental constants and the direct observation of cosmic expansion. A crucial tool for this are laser frequency combs (LFCs), directly linking the accuracy of atomic clocks to optical laser lines. However, strong material dispersion and large spectral separation from the established infrared laser oscillators so far prevent the use of LFCs for spectrograph calibration in the blue and UV part of the spectrum. At OHP/SOPHIE, we demonstrated for the first time the calibration of an astronomical spectrograph using an astrocomb in the ultraviolet spectral range below 400nm. Key technology used were nano-fabricated, periodically-poled waveguides in lithium niobate photonic chips, fed by either a robust infrared electro-optic comb generator or a chip-integrated microresonator comb. In an end-to-end test, we could demonstrate stable and accurate LFC-based spectrograph calibration, showcasing a viable path towards precision wavelength calibration of spectrographs in the ultraviolet, crucial e.g. for the future ELT/ANDES.