Instrument design and laboratory evaluation of the OSAS-B heterodyne spectrometer for sounding atomic oxygen in the MLT

The Oxygen Spectrometer for Atmospheric Science on a Balloon (OSAS-B) is dedicated to the remote sounding of atomic oxygen in the mesosphere and lower thermosphere (MLT) region of Earth's atmosphere, where atomic oxygen is the dominant species. OSAS-B is a heterodyne receiver for the thermally excited ground state transition of atomic oxygen at 4.75 THz. Due to water absorption, this line can only be observed from high-altitude platforms such as a balloon. A combined Helium/nitrogen dewar comprises the detector of the instrument, a hot-electron bolometer mixer, as well as a quantum-cascade laser, which serves as the local oscillator for heterodyning. A turning mirror allows for measurements at different vertical inclinations and for radiometric calibration against two blackbody sources. The first flight will take place in autumn 2022 within the HEMERA2020 program. We will present the instrument design and results of the laboratory evaluation of the instrument.