Ultraviolet detectors for astrophysics missions: a case study with the star-planet activity research cubeSat (SPARC)

Here we discuss high-performance UV detectors to be used with the planned Star-Planet Activity Research CubeSat (SPARCS). SPARCS is a 6U cubesat designed to monitor M stars (0.1 – 0.6 solar masses) in two photometric bands in the near UV and far UV (S-NUV, 260-300 nm; S-FUV, 150-170 nm). SPARCS targets range in mass and age, including young stars (10-20 Myr), which are likely forming terrestrial planets, to old stars with known transiting planets, allowing us to map the evolution of UV emission and flare rates. The spectral slope, variability and evolution of a host star’s highenergy radiation would provide realistic input stellar fluxes to planet atmospheric models, which would aide in understanding the evolution and habitability of a planet and in interpreting its transmission and emission spectrum. The baseline S-NUV detector is a 2D-doped (delta-doped or superlattice-doped) charge coupled device (CCD) optimized with a custom antireflection (AR) coating to achieve quantum efficiency (QE)>70% throughout the S-NUV band. The SNUV detector would be coupled with a stand-alone red-blocking filter that provides at least three orders of magnitude (i.e., ≥OD3) out-of-band suppression, critical for the observations of such cool, red stars. Their combined throughput would be >25% (peak) in the S-NUV. The baseline S-FUV detector is a 2D-doped CCD optimized for the S-FUV band; it includes an integrated filter designed to maximize in-band throughput with good red-leak suppression. As designed, the solar-blind silicon detector achieves peak QE>35% in the S-FUV band and ≥OD2 out-of-band suppression. SPARCS has baselined a dichroic design that allows for simultaneous S-NUV and S-FUV observation. SPARCS would advance 2D-doped detectors and detector-integrated out-of-band-rejection filter technologies for their potential application in future mission concepts such as LUVOIR and HabEx.