Schematic of POPO. The optics are composed of a liquid crystal variable retarder (LCVR) in front of the telescope focus (TF), three lenses (L1, L2a and L2b), a band-pass filter (BF; R_C band), and a polarizing beam splitter (PBS). A half-wave plate (HWP) and a quarter-wave plate (QWP) are inserted in front of the LCVR for Stokes $\hat{u}$ and $\hat{v}$ observations, respectively. Images are taken by an EM-CCD camera (ANDOR iXon) and a CMOS camera (ANDOR Zyla). A data acquisition device (DAQ) sends synchronized pulses to the LCVR and the two cameras.