CHARA Array delay lines: upgrades, performance, and future directions

Long baseline optical and infrared interferometric arrays achieve high angular resolution and enable detailed astrophysical measurements. Interferometers have enabled observations of stars at various stages of evolution, as well as studies of binary stars, circumstellar disks, and active galactic nuclei. The Center for High Angular Resolution Astronomy (CHARA) Array is a long-baseline interferometric array at the Mount Wilson Observatory, United States. At the core of CHARA operations are the delay lines, which equalize the optical path length for all telescopes as the Earth rotates and compensate for optical path variations induced by atmospheric turbulence. We report recent upgrades and performance of the CHARA Array optical delay lines for high-precision interferometric observations. The legacy system had been operational for over two decades, and it was increasingly difficult to acquire replacement parts. Beginning in mid-2021, the control system underwent a major upgrade, replacing the aging Versa Module Eurocard-based architecture with a modern hybrid field-programmable gate array and Linux-based system; this modernization continued through the end of 2024. We describe hardware/software changes, the servo architecture, and lab/on-sky performance. The upgraded system achieves residual delay line cart tracking errors of ∼12 nm, the same level as the legacy system, and a control bandwidth of 100 to 130 Hz, allowing fringe tracking across the R-, H-, and K-bands. Initial commissioning revealed key issues such as metrology time-tick jitter and vibration-induced visibility loss, which were diagnosed and resolved. We note ongoing and future efforts to extend baselines up to 1 km and support advanced observing modes such as dual-field interferometry and nulling. This is a reference for current and future use of the CHARA Array and for next-generation instrument design.