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The combination on large ground-based telescopes of extreme adaptive optics (ExAO), coronagraphy and high-dispersion spectroscopy is starting to emerge as a powerful technique for the direct characterisation of giant exoplanets. High spectral resolution not only brings a major gain in terms of accessible spectral features, but it also enables to better disentangle between the stellar and planetary signals thanks to the much higher spectral content. On-going projects such as KPIC for Keck, REACH for Subaru and HiRISE for the VLT base their observing strategy on the use of a few science fibres, one of which is dedicated to sampling the PSF of the planet, while the others sample the stellar residuals in the speckle field. The main challenge in this approach is to blindly centre the science fibre on the planet’s PSF, with typically a tolerance of less than one resolution element (0.1 λ/D). Several possible centring strategies can be adopted, either based on calibration fibres retro-injecting signal to mark the position of the science fibres or based on the use of focal-plane features introduced by the ExAO system. In this proceeding, we describe different possible approaches and we compare their centring accuracy using the MITHiC high-contrast imaging testbed. For this work, MITHiC has been upgraded to reproduce a setup close to the one that will be adopted in HiRISE, the coupling system that will soon be implemented between VLT/SPHERE and VLT/CRIRES+. Our results demonstrate that reaching a specification accuracy of 0.1 λ/D is extremely challenging regardless of the chosen centring strategy. It requires a high level of accuracy at every step of the centring procedure, which can be reached with very stable instruments. We studied the contributors to the centring error in the case of MITHiC and we quantified some of the most important terms.
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