Design of an atmospheric dispersion corrector and double scrambler for improved fiber coupling and illumination uniformity for the HET HRS

Abstract

The Hobby--Eberly Telescope High Resolution Spectrograph (HET--HRS) is undergoing a major instrument upgrade to improve its performance over the 360--1000 nm wavelength range. As part of this effort, a new fiber-feed system has been developed to increase throughput and enhance illumination stability. This paper presents the optical and opto-mechanical designs of the upgraded HRS Input Head Module Package and lens-based Double Scramblers. The Input Head incorporates a static Atmospheric Dispersion Corrector (ADC) and relay optics optimized for the HET operational zenith-angle range, reducing the residual atmospheric dispersion to less than 0.42 arcsec while improving wavelength-dependent fiber coupling. Separate Double Scrambler designs were developed for the 260 \textmu m and 320 \textmu m science fibers to exchange the near-field and far-field illumination distributions. Geometrical image analyses demonstrate stable near-field and far-field illumination, reduced coupling losses, and acceptable angular acceptance across both fiber geometries. The opto-mechanical designs supporting the Input Head and Double Scrambler are also presented to provide the alignment accuracy and mechanical stability required for the upgraded fiber-feed system. These results establish the proposed architecture as a practical solution for improving the optical performance and illumination stability of HET--HRS.