Optical characterisation and modelling of a UV-Vis Fabry-Perot etalon for wavelength calibration of EPRV spectrometers

Abstract

Stable broadband Fabry–Pérot etalons are essential calibrators for extreme-precision radial-velocity (EPRV) spectrographs, providing dense, uniform reference spectra with the line symmetry and long-term stability required to achieve centimeter-per-second sensitivity. The ultimate spectral stability and uniformity of these devices are governed by the mirror coatings, cavity substrate figure, and geometric precision. In this work, we present a method to spatially resolve the cavity surface with nanometer precision. This not only allows us to characterise etalons, but also provides the opportunity to test measures to mitigate cavity deformation on both short- and long-term timescales. This will provide new insights into the performance limits and optimisation pathways for next-generation EPRV calibration sources. Here we present the measurement setup with which this precision was achieved. This method will also provide the opportunity to test countermeasures against cavity deformation and evaluate long-term stability in future work.