Cross-section of FSP modulator package. The main components are two ferro-electric liquid crystal (FLC) half-wave retarders (FLC1 and FLC2); a quarter-wave static retarder (SR1); a half-wave, static retarder (SR2); and a polarizer beam splitter cube. The two FLCs and SRs are mounted inside rotating cases to permit the manual modification (by means of the four labeled knobs at the bottom) of their optical axes orientation. The latter allows the optimization of the efficiencies for a given wavelength and/or Stokes parameter in case a specific scientific program requires it.