Data analysis methods for laser frequency comb line position measurements with a Fourier transform spectrograph

Astrophysical calibration sources which can be used for high-precision radial-velocity spectroscopy require a calibration with even higher precision and accuracy. Calibration of these sources can be achieved with a high-resolution Fourier-Transform-Spectrograph (FTS). The precision (similar to 20 m/s) of the FTS is mainly driven by its reference, often a stabilized HeNe-laser. To reach an acceptable precision, either averaging over a large number of measurements or a better reference is needed. We developed a setup including a Laser-Frequency-Comb (LFC) for referencing a high-resolution FTS. Due to the pulsed source specific evaluation methods have to be used to retrieve the spectrum properly. We extend the sub-nominal method used in absorption spectroscopy by showing an algorithm to determine the interferogram cut points from the interferogram itself, rather than calculating them from the repetition rate and reference laser wavelength. Furthermore, we show that line position errors measured from comb spectra are associated with amplitude variability and phase noise. We give an estimate of the measured line position stability for different evaluation methods (truncation, shifting, apodization, zerofilling) on scales not dominated by these errors.