NIRPS fiber-link design, performances and modal noise mitigation performances tested on sky

Abstract

NIRPS (Near Infra-Red Planet Searcher) is an AO-assisted and fiber-fed high-resolution spectrograph operating in the YJH-bands at the ESO 3.6m telescope in La Silla Observatory, Chile. The optimal geometrical scrambling and the minimization of the modal noise, requested to reach 1 m s−1 precision in radial velocity, is obtained by combining octagonal fibers, a fiber stretcher, a double-scrambler, and a tip-tilt scanning of the 29-µm fiber core. We tested the performance of the fiber-link design on sky and evaluated the modal noise mitigation via near and far-field images taken at the fiber-link output. Without the inclusion of the stretcher and tip-tilt scanning, an extreme injection position at the edge of the fiber should induce an apparent change in radial velocity of ~20 m s−1 with respect to a well-centered injection. Observations with the entire instrument of fast-rotating hot stars show that the stretcher and tip-tilt scanning significantly reduce the modal noise from 1.6% to 0.7%. Optimizing the tip-tilt scanning pattern can further minimize the modal noise, thereby improving the precision in radial velocity.