Stabilizing a Fabry–Perot Etalon Peak to 3 cm s⁻¹for Spectrograph Calibration

We present a method of frequency stabilizing a broadband etalon that can serve as a high-precision wavelength calibrator for an echelle spectrograph. Using a laser to probe the Doppler-free saturated absorption of the rubidium D 2 line, we stabilize one etalon transmission peak directly to the rubidium frequency. The rubidium transition is an established frequency standard and has been used to lock lasers to fractional stabilities of < 10 -12 , a level of accuracy far exceeding the demands of radial velocity (RV) searches for exoplanets. The stabilized bandwidth depends on the dispersion characteristics of the etalon. We describe a simple setup designed specifically for use at an observatory and demonstrate that we can stabilize the etalon peak to a relative precision of < 10 -10 ; this is equivalent to 3 cm s -1 RV precision.