HiCIBaS project (the High Contrast Imaging Balloon System)

https://seti.org/it-possible-see-other-worlds-stratosphere-hicibas-first-proof-concept-fly-soon

« …The mission’s goal is to test the technology needed to fly a large telescope on a stratospheric balloon, and to complement the work of future space missions like WFIRST and HABEX/LUVOIR. At an altitude of 40-km, the maximum possible for sub-orbital balloons, the telescope will be only half-way to the Karman line—the official boundary between Earth’s atmosphere and outer space—but the atmospheric pressure is a thousand times less than on Earth’s surface, and radiation levels are very similar to those of outer space. True, it’s not outer space—but for electronic devices and hypersensitive instruments, it’s quite close!… »