HARMONI: Characterising the line-spread-function with a tunable Fabry-Perot etalon

HARMONI is the first light visible and near-infrared (450 - 2450 nm,λ/Δλ = 3500 - 18000) integral field spectrograph (IFS) for the European Southern Observatory's Extremely Large Telescope (ELT). Spectroscopic observations from ground-based telescopes in these wavelengths are contaminated by emission from molecules in the Earth's atmosphere, called skylines. Conventional means of removing these skylines requires spending up to half of the telescope time on observing blank sky exposures. Observations with IFS can circumvent this inefficiency by making use of sky models from science exposure themselves to remove the background, called on- IFU sky-subtraction. However, this has not been achieved in practice as it requires an accurate knowledge on the Line Spread Function (LSF) of the spectrograph. The information on the LSF is also useful in telluric calibration of science observations, especially in case of telluric standards that underfill the slits. In this manuscript, we present a tunable Fabry-perot design which will be used to characterise the LSF of HARMONI. The Fabry- perot etalon will operate over HARMONI's entire wavelength range and spectral resolving powers. The design presented in this manuscript has the potential to be adapted for any spectroscopic instrument in the future, and fundamentally change the way we have been observing with IFS instruments.