  Precise wavelength calibration is a critical issue for high-resolution
spectroscopic observations. The ideal calibration source should be able to
provide a very stable and dense grid of evenly distributed spectral lines of
constant intensity. A new method which satisfies all mentioned conditions has
been developed by our group. The approach is to actively measure the exact
position of a single spectral line of a Fabry-Perot etalon with very high
precision with a wavelength-tuneable laser and compare it to an extremely
stable wavelength standard. The ideal choice of standard is the D2 absorption
line of Rubidium, which has been used as an optical frequency standard for
decades. With this technique, the problem of stable wavelength calibration of
spectrographs becomes a problem of how reliably we can measure and anchor one
etalon line to the Rb transition. In this work we present our self-built module
for Rb saturated absorption spectroscopy and discuss its stability.
