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The Juno mission is a NASA New Frontiers mission, orbiting Jupiter since 4 July 2016 and placed on a 53-day period, highly elliptical, polar orbit. The Ultraviolet Spectrograph onboard Juno (Juno-UVS) is a photoncounting imaging spectrograph, designed to cover the 68-210 nm spectral range.1 This range includes the H2 bands and the Lyman series produced in Jupiter’s far-ultraviolet (FUV) auroras. The purpose of Juno-UVS is to study Jupiter’s auroras from the unique vantage point above both poles allowed by Juno’s orbit, and to provide a wider auroral context for the in-situ particle and field instruments on Juno. Because of the 2 rpm spin of Juno, UVS nominally observes 7.5°x360° swaths of the sky during each spin of the spacecraft. The spatial resolutions along the slit and across the slit, i.e. in the spin direction, are respectively 0.16° and 0.2° , while the filled-slit spectral resolution is ∼1.3 nm.2 UVS borrows heavily from previous instruments led by Southwest Research Institute (New-Horizons and Rosetta Alices, LRO-LAMP), major improvements are: (i) an extensive radiation shielding; (ii) a scan mirror which allows targeting specific auroral features; and (iii) an improved cross-delay line readout scheme of the microchannel plate (MCP) detector. The ability offered by the scan mirror combined with Juno’s spin allows UVS access to half of the sky during every spacecraft rotation. This pointing flexibility, combined with the changing spin-axis of the spacecraft since launch, has allowed UVS to map 99 % of the sky in the 68-210 nm range. This paper describes the substantial number of spectra that have been used to monitor the health of the instrument over the course of the mission. More than 5800 spectra of mainly O, A, and B spectral-type stars in the V-magnitude range of ∼0-7 have been extracted to date. Selected stars among this list are used to calibrate the UVS instrument. This paper describes how previous spectral databases from the International Ultraviolet Explorer have been refined and adapted for UVS’ calibration purposes, in combination with observations from the Hubble Space Telescope. The retrieved effective area of the instrument peaks around 0.28 at ∼125 nm, with uncertainties lower than 10%.
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