A dual cavity Fabry-Perot device for high precision Doppler measurements in astronomy

Abstract

We propose a dual cavity Fabry-Perot interferometer as a wavelength calibrator and a stability tracking device for astronomical spectrograph. The FPI consists of two adjoining cavities engraved on a low expansion monoblock spacer. A low-finesse astro-cavity is intended for generating a uniform grid of reference lines to calibrate the spectrograph and a high-finesse lock-cavity is meant for tracking the stability of the reference lines using optical frequency standards. The differential length changes in two cavities due to temperature and vibration perturbations are quantitatively analyzed using finite element method. An optimized mounting geometry with fractional length changes $\Delta L/L \approx 1.5\times 10^{-12}$ is suggested. We also identify conditions necessary to suppress relative length variations between two cavities well below 10$^{-10}$~m, thus facilitating accurate dimension tracking and generation of stable reference spectra for Doppler measurement at 10 cms$^{-1}$ level.