Author(s): , ,
Institution(s): 1. Institute for Astrophysics Goettingen
Hollow-cathode lamps (HCLs) are used as default wavelength standard for spectroscopic measurements but have a number of well-known shortcomings. Advancing to cm/s precision in radial velocity experiments requires more stable calibration sources with more uniform line distributions. Fabry Perot Interferometers (FPI) are a practical alternative with a well-suited line distribution at relatively low cost. We present a simple method to characterize FPIs using standard HCLs and including the FPI spectrum in the wavelength calibration process. We propose to use the HCL wavelength solution to define a rough wavelength scale that is used to approximate the FPI peak positions. We assume that the FPI mirror distance is a smooth function of wavelength and utilize the large number of FPI peaks (typically 10^4) to consistently model all FPI peak wavelengths. With this approach, we anchor the dense FPI lines with the absolute HCL-scale combining their precision and accuracy. We test our method with the HARPS spectrograph and compare our wavelength calibration to one derived from a laser frequency comb (LFC) spectrum. Our combined HCL/FPI wavelength calibration removes the known, large-amplitude distortions of 50 m/s that occur in the HCL solution. Direct comparison with the LFC solution bears only small differences between the LFC and the HCL/FPI solutions and demonstrates that the HCL/FPI solution can overcome the most important shortcomings in HCL wavelength solutions. An FPI can provide an economical alternative to LFCs in particular for smaller projects.