High-precision calibration of spectrographs

Abstract We present the first stringent tests of a novel calibration system based on a laser frequency comb (LFC) for radial velocity measurements. The tests were obtained with the high-resolution, optical spectrograph, High Accuracy Radial velocity Planet Searcher. By using only one echelle order, we obtain a calibration repeatability of 15cms−1 for exposures that are several hours apart. This is comparable with a simultaneous calibration using a Th–Ar lamp that makes use of all 72 echelle orders. In both cases, the residuals are compatible with the computed photon noise. Averaging all LFC exposures, recorded over a few hours, we could obtain a calibration curve with residuals of 2.4ms−1. Thanks to the adjustable and optimally chosen line density of the LFC, we resolve a periodicity of 512 pixels in the calibration curve that is due to the manufacturing process of the CCD mask. Previous Th–Ar calibration was unable to resolve these systematic deviations, resulting in a deviation of up to 70ms−1 from the true calibration curve. In future, we hope to be able to make use of all echelle orders in order to obtain a calibration repeatability below 1cms−1 and absolute calibration within a few ms−1.