Small UV imager for hydrogen Lyman-alpha onboard ultra-small spacecraft

A small UV imager named HI (Hydrogen Imager) is under development to observe the hydrogen coma of a long-period comet or Interstellar object from space. The instrument will be aboard one of the probes for ESA's Comet Interceptor mission, scheduled for launch in 2029. This mission will remain at the Sun-Earth Second Lagrange Point (SEL2) until an optimal target (a reachable long-period comet or interstellar object) is detected through ground observation. During the cruise from SEL2 to the target, HI will observe the cometary hydrogen coma, which emits Lyman-alpha (wavelength 121.6 nm) through the resonance scattering of solar light. Additionally, during the closest approach phase, lasting several tens of hours, HI will measure Lyman-alpha emissions from both hydrogen and deuterium in the coma using switchable gas filters. The optical design, filters, and detector of HI are optimized for observing Lyman-alpha, with dimensions smaller than 100 mm × 100 mm × 250 mm and power consumption less than 9 W. The mirrors are coated with Al/MgF2 to enhance UV reflectance and maintain reflectivity during ground operations before launch. Two gas filters containing hydrogen and deuterium molecules, respectively, are installed to deduce the hydrogen/deuterium brightness ratio of the coma. A Z-stacked MCP detector assembled with a resistive anode is used, without a photocathode, to prevent degradation during ground operations.