4 K cryocooling for space VLBI with the black hole explorer

The Black Hole Explorer (BHEX) is a space–Earth very long baseline interferometry (VLBI) mission concept currently in formulation. The mission addresses fundamental black hole physics while aiming to detect and study the theoretically predicted ‘photon ring’ of light orbiting a black hole and produce the sharpest images from space in the history of astronomy. To achieve the required angular resolution, the BHEX instrument will extend the interferometer baseline of the Event Horizon Telescope beyond the diameter of the Earth, allowing for a hybrid VLBI observatory. The scientific objectives demand the ability to reach milliJansky sensitivity at frequencies sufficiently high to mitigate the effects of synchrotron self-absorption and strong interstellar scattering. This necessitates a sophisticated instrument comprising a 76–320 GHz receiver system integrated with a spaceflight cryocooling unit, coupled to a large lightweight antenna, fast digital processing, high-stability frequency reference, and an ultra-high-speed laser downlink. The BHEX receiver front-end will observe simultaneously in two bands. The instrument’s dual-band receivers, operating at frequencies between 76–106 GHz and 228–320 GHz, require cooling to 20 K and 4.5 K, respectively. Operating at 4.5 K is a driving requirement at 228–320 GHz due to the use of niobium-based superconducting detectors. The BHEX cryocooler design requirements for the instrument are discussed.