Calibrating echelle spectrographs with Fabry-Perot etalons

TL;DR

This paper explores using Fabry-Perot etalons combined with hollow-cathode lamps to improve wavelength calibration of echelle spectrographs, achieving high precision comparable to laser frequency combs at lower cost.

Contribution

It introduces an algorithm to incorporate Fabry-Perot etalons into wavelength calibration and demonstrates its effectiveness with HARPS data.

Findings

Overcomes 50 m/s distortion in calibration.

Achieves maximum 10 m/s difference compared to laser frequency comb.

Provides an economical calibration alternative for smaller projects.

Abstract

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.