The TSST potassium channel f/12 optical path. L1 is the 80 mm objective lens (top). L2, L3, L4 and L5 are achromatic doublet lens. M1 and M2 are flat folding mirrors. QWP and HWP are quarter-wave plates and half-wave plates, respectively. P1, P2 and P3 are linear polarizers. CL is a group of corrective lenses. After the P2 element the instrument passbands, typical of MOF instruments, is the one shown in the bottom panel. The measured passband is shown at different MOF cell temperatures. Thicker solid line corresponds to the chosen working temperature of the cell. Only one of the two wing passbands is later selected by the WS section, as shown in Figure 2. It is possible to select one of the wing passbands rotating the appropriate retarders.