Results of the JUICE ultraviolet spectrograph near-Earth commissioning

We present the results of the JUpiter ICy moons Explorer ultraviolet spectrograph (JUICE-UVS) near-earth commissioning performed between June 14th and 25th 2023. JUICE-UVS is a modest power (9 W), compact ultraviolet spectrograph that is the fifth in a series of six such spectrographs designed and built by Southwest Research Institute for ESA and NASA missions. JUICE-UVS includes several innovations in detector, electronics, optics, and software. These innovations include an atomic-layer deposition (ALD) coating on the detector microchannel plates to minimize gain sag as photons are detected during the mission, electronic components able to withstand the harsh Jovian radiation environment, low-scatter optics to minimize the “wings” of the detected interplanetary Lyman-alpha emission, and a programmable histogram data collection mode that maximizes science return while minimizing stored and telemetered data volume. JUICE-UVS also features a high-spatial resolution observation mode, where a small (1/4 the width of nominal) aperture swings into place at the aperture stop and sharpens the instrument focus. Near-earth commissioning activities included measurements of detector dark noise, optimizing detector HV level via observations of the interplanetary Lyman-alpha, and a “sky-spin” observation over the JUICE-UVS 7.5°-long slit while the spacecraft rolled about the high-gain antenna (a “great circle” swath 7.5° wide). Dark rates matched those measured on the ground (approximately 5 Hz/cm²). HV levels were nominal for the operational temperature (approximately 0°C). Observations with the high-spatial resolution aperture confirmed the sharpened focus when compared to nominal observations. The “sky-spin” observation passed through the galactic plane, allowing for the identification of many known UV-bright stars. These stars provide a rough estimate of instrument effective area that will be refined after JUICE passes >2 AU from the sun, enabling 3-axis stabilized pointed observations.