A multibeam electron beam-induced deposition (EBID) system is presented, which aims at the fabrication of sub-10 nm structures with EBID. This system consists of a multibeam source (MBS) module, delivering 100 virtual sources and a standard scanning electron microscope (SEM) column to image the 100 sources onto a wafer. In the proposed concept, beamlets are traveling off-axis through the projection lenses, introducing off-axis aberrations. An analytical description of the projection lens aberrations is derived and the system is optimized by tuning a field lens, which only affects the direction of the beamlets as they enter the projection system. It is found that this lens must be excited such that all beamlets go through the center of the last projection lens. This is an important design rule for the total system.