Overall detection efficiency of the Mini-EUSO detector (black curve) as a function of wavelength (Bacholle et al. 2021). This is the result of the transmittance of the UV transparent window of the ISS (green curve), the optics (purple curve), the BG3 bandpass filter (red curve) and the MAPMT photon detection efficiency (blue curve). The system has been designed to maximise observations of the fluorescence light emitted by nitrogen atoms excited by the extensive air shower of cosmic rays (grey histogram). The total efficiency of Mini-EUSO is higher than 50% of the maximum in the wavelength range 290-430 nm. The efficiency values plotted as the black curve need to be rescaled to the results of the end-to-end calibration of Mini-EUSO thanks to observational campaigns with UV flashers (see details in the text of Sect. 2).