The detection of non-radial oscillations on late-type dwarf stars may require the measurement of Doppler shifts of order 10 cm s−1. We report observations using the coudé echelle spectrograph of the 3.9 m Anglo-Australian Telescope which reveal a noise level in a 1 minute integration about a factor of one hundred greater than this. We then discuss a method of imposing a set of stable reference features on stellar spectra to permit Doppler measurements of the required accuracy. The method exploits a voltage-tuned solid Fabry-Perot interferometer whose passbands are fixed by a servo-control system to the light from a HeNe laser whose frequency is polarization-stablized. The desired wavelength stability demands careful control of the voltage, temperature and tilt of the etalon.