  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.
