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Fig. 1

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Characterization of the Espresso Fabry-Pérot effective gap size as function of time and wavelength, decomposed as described in Eq. (2). Top panel: just the wavelength dependence of the cavity length Deff (λ) obtained by time-averaging all measurements. Right panel: achromatic drift ∆Deff(t) as function of time. A linear trend was fitted for the period before the shutdown in March 2020. The central panel shows the residual variation of the Deff(t, λ) measurements after subtraction the average spectral shape and the achromatic drift. The values are expressed as velocity shift. Horizontal red lines correspond to interventions to the system.

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