Dual-wavelength extreme ultraviolet camera (EUC) for the Chang’E-7 mission: Instrument development and calibration

The dual-wavelength extreme ultraviolet camera (EUC) for the Queqiao-2 relay satellite of the Chang’E-7 (CE-7) mission operates at 30.4 and 83.4 nm independently to simultaneously image the plasmasphere, magnetosheath, and ionospheric outflow from a lunar orbit. Each channel of the EUC is consisted of a concave multilayer mirror and a photon-counting imaging detector. This simple system achieves a large field of view (FOV), high spatial resolution, and optimized photon transmission efficiency to capture high-quality images of very weak extreme ultraviolet emissions in Earth space. Here we present the detailed design, tests, and calibrations of the EUC. Ground geometrical tests showed that the FOV was 20.2° for the 30.4 nm channel and 20.3° for the 83.4 nm channel, and the spatial resolution was 0.09° for both channels. Geometric distortion was corrected to be less than 1%. Pixels can be further binned on the ground to achieve higher sensitivity. Radiometric calibration results demonstrate that the sensitivity is 0.103 counts s⁻¹ Rayleigh⁻¹ pixel⁻¹ at 30.4 nm channel and 0.061 counts s⁻¹ Rayleigh⁻¹ pixel⁻¹ at 83.4 nm channel, with a calibration accuracy of ∼12%. The exposure can be flexibly set between 1–1500 s, with a longer exposure time achievable through time-delay integration during ground processing. The performance of the EUC fulfills the requirements of the scientific targets for the CE-7 mission and could significantly contribute to investigations of the solar wind-magnetosphere-ionosphere coupling system.