Chromatic Drift of the Espresso Fabry-P\'erot Etalon

TL;DR

This study investigates the chromatic drift in the Espresso Fabry-Pérot interferometer over 2.5 years, revealing wavelength-dependent variations likely due to mirror aging, which can be measured and corrected with standard calibration methods.

Contribution

It provides the first detailed analysis of chromatic drift in the Espresso Fabry-Pérot, demonstrating its characteristics and potential correction techniques.

Findings

Chromatic drift amplitude of a few cm/s per day.

Quasi-oscillatory dependence on wavelength.

Possible cause: aging of dielectric mirror coatings.

Abstract

In the last decade, white-light illuminated Fabry-P\'erot interferometers wave been established as a widely used, relatively simple, reliable, and cost-effective way to precisely calibrate high-resolution echelle spectrographs. However, Terrien et al. (2021) recently reported a chromatic drift of the Fabry-P\'erot interferometer installed at the Habitable-zone Planet Finder spectrograph. In particular, they found that the variation of the etalon effective gap size is not achromatic as usually assumed but in fact depends on wavelength. Here, we present a similar study of the Espresso Fabry-P\'erot interferometer. Using daily calibrations spanning a period of over 2.5 years, we also find clear evidence for a chromatic drift with an amplitude of a few cm/s per day that has a characteristic, quasi-oscillatory dependence on wavelength. We conclude that this effect is probably caused by an aging of the dielectric mirror coatings and expect that similar chromatic drifts might affect all Fabry-P\'erot interferometers used for calibration of astronomical spectrographs. However, we also demonstrate that the chromatic drift can be measured and in principle corrected using only standard calibrations based on hollow cathode lamp spectra.