Spectral Point Spread Function, or in other words Line Spread Function (LSF), quantifies a spectrograph’s response to a monochromatic light source. Accurate knowledge of the LSF is needed to measure the velocity dispersion of the stars or gas from the spectra of galaxies, when these quantities are comparable or smaller than the width of the LSF. The LSF information is also crucial to subtract background sky emission in near-infrared integral field spectroscopic (IFS) instruments from ground-based telescopes. Accurate models of LSF cannot be obtained from the spectrograph data itself due to their coarse sampling to provide a wide instantaneous wavelength coverage. In the case of IFS instruments, the LSF can assume complex shapes and these shapes can change dependent on the location on the IFS field-of-view. In this manuscript, we derive accurate LSF models in the H-band grating of the SINFONI spectrograph, a near-infrared IFS on board the Very Large Telescope in Chile, using a dedicated calibration programme. We model the LSF profiles using Gauss-Hermite polynomials and we use the parameters from these models to predict the shape of the LSF profile at any location on the detector. We also demonstrate that the LSF can be derived from the curvature in the arc-lamp frames. Finally, we derive the LSF of the upcoming ELT/HARMONI spectrograph using the slit-curvature method for different resolutions and grating set-ups
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.
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