Optical transmission of the gvAPP measured in the laboratory (black curve) and the on-sky background transmission in the Br-α filter (orange point). Additional measurements with the ERIS internal calibration unit are shown by the blue points. The blue regions trace all filters available for observations with the gvAPP in ERIS. Note that for on-sky throughput, this transmission curve should be multiplied by 0.23 to account for the EE and division of the flux between coronagraphic PSFs (see Sect. 3.2).