STELES: the high-resolution Échelle spectrograph for the 4.1-m SOAR Telescope

Abstract

The SOAR Telescope Echelle Spectrograph (STELES) is a high-resolution, near-UV/optical spectrograph for the 4.1 m SOAR Telescope. STELES is a Nasmyth-fed, dual-channel (blue/red), VPH cross-dispersed echelle spectrograph operating in quasi-Littrow white-pupil configuration. The instrument provides simultaneous wavelength coverage from ∼300 to 960 nm at mean resolving powers approaching R ∼ 50, 000 and is optimized for high throughput, particularly in the near-UV. The bench-mounted spectrograph is fed by a dedicated fore-optics system, and is permanently mounted below the Nasmyth platform. Installed and commissioned since August 2025, STELES is currently available for scientific observations. In this work, we present an overview of the instrument design, control and data-reduction systems, and its initial on-sky performance, including measurements of throughput, wavelength calibration, spectral resolution, and representative science observations.