  Over the past decades hollow-cathode lamps have been calibration standards
for spectroscopic measurements. Advancing to cm/s radial velocity precisions
with the next generation of instruments requires more suitable calibration
sources with more lines and less dynamic range problems. Fabry-Perot
interferometers provide a regular and dense grid of lines and homogeneous
amplitudes making them good candidates for next generation calibrators. We
investigate the usefulness of Fabry-Perot etalons in wavelength calibration,
present an algorithm to incorporate the etalon spectrum in the wavelength
solution and examine potential problems. The quasi periodic pattern of
Fabry-Perot lines is used along with a hollow-cathode lamp to anchor the
numerous spectral features on an absolute scale. We test our method with the
HARPS spectrograph and compare our wavelength solution to the one derived from
a laser frequency comb. The combined hollow-cathode lamp/etalon calibration
overcomes large distortion (50 m/s) in the wavelength solution of the HARPS
data reduction software. Direct comparison to the laser frequency comb bears
differences of only maximum 10 m/s. Combining hollow-cathode lamps with
Fabry-Perot Interferometers can lead to substantial improvements in the
wavelength calibration of echelle spectrographs. Etalons can provide economical
alternatives to the laser frequency comb, especially for smaller projects.
