A Harsh Test of Far-Field Scrambling with the Habitable Zone Planet Finder and the Hobby Eberly Telescope

TL;DR

This study tests the effectiveness of the HPF fiber scrambler in mitigating far-field illumination changes caused by pupil variations at the Hobby Eberly Telescope, demonstrating its robustness for precise radial velocity measurements.

Contribution

The paper provides the first on-sky test of HPF's fiber scrambler under extreme pupil variation conditions, confirming its effectiveness in stabilizing radial velocity measurements.

Findings

Radial velocities are decoupled from extreme pupil centroid offsets.

Fiber scrambler effectively suppresses illumination-induced RV variations.

Results suggest controllable effects of pupil changes for next-generation RV instruments.

Abstract

The Habitable zone Planet Finder (HPF) is a fiber fed precise radial velocity spectrograph at the 10 m Hobby Eberly Telescope (HET). Due to its fixed altitude design, the HET pupil changes appreciably across a track, leading to significant changes of the fiber far-field illumination. HPF's fiber scrambler is designed to suppress the impact of these illumination changes on the radial velocities -- but the residual impact on the radial velocity measurements has yet to be probed on sky. We use GJ 411, a bright early type (M2) M dwarf to probe the effects of far-field input trends due to these pupil variations on HPF radial velocities (RVs). These large changes ($\sim$ 2x) in pupil area and centroid present a harsh test of HPF's far-field scrambling. Our results show that the RVs are effectively decoupled from these extreme far-field input changes due to pupil centroid offsets, attesting to the effectiveness of the scrambler design. This experiment allows us to test the impact of these changes with large pupil variation on-sky, something we would not easily be able to do at a conventional optical telescope. While the pupil and illumination changes expected at these other telescopes are small, scaling from our results enables us to estimate and bound these effects, and show that they are controllable even for the new and next generation of RV instruments in their quest to beat down instrumental noise sources towards the goal of a few cm/s.