Precision near-infrared radial velocity instrumentation II: Non-Circular Core Fiber Scrambler

TL;DR

This paper presents a novel non-circular core fiber scrambler for near-infrared radial velocity measurements, demonstrating improved stability and precision in on-sky tests, with potential applications in future spectrographs and smaller telescopes.

Contribution

Introduction of a new agitated non-circular core fiber scrambler for enhanced near-infrared spectroscopic stability and radial velocity precision, validated through on-sky performance tests.

Findings

Improved blaze function stability and line spread function stability.

Enhanced radial velocity precision with the fiber scrambler.

Feasibility of applying the technique to next-generation spectrographs.

Abstract

We have built and commissioned a prototype agitated non-circular core fiber scrambler for precision spectroscopic radial velocity measurements in the near-infrared H band. We have collected the first on-sky performance and modal noise tests of these novel fibers in the near-infrared at H and K bands using the CSHELL spectrograph at the NASA InfraRed Telescope Facility (IRTF). We discuss the design behind our novel reverse injection of a red laser for co-alignment of star-light with the fiber tip via a corner cube and visible camera. We summarize the practical details involved in the construction of the fiber scrambler, and the mechanical agitation of the fiber at the telescope. We present radial velocity measurements of a bright standard star taken with and without the fiber scrambler to quantify the relative improvement in the obtainable blaze function stability, the line spread function stability, and the resulting radial velocity precision. We assess the feasibility of applying this illumination stabilization technique to the next generation of near-infrared spectrographs such as iSHELL on IRTF and an upgraded NIRSPEC at Keck. Our results may also be applied in the visible for smaller core diameter fibers where fiber modal noise is a significant factor, such as behind an adaptive optics system or on a small < 1 meter class telescope such as is being pursued by the MINERVA and LCOGT collaborations.