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 .