MOTH simplified optical scheme of one single channel. The polarization rotator λ/2 is shown at 45° with respect to the λ/4 retarder. The hot mirror, the UV filter and the narrowband interference filter, necessary to reduce the heat load on optical systems and to select the narrow bandwidth around the MOF spectral passbands, are present in the optical path but they are not shown in the figure. In this scheme we show CCD cameras, however, the latest version of the MOTH is equipped with CMOS cameras as discussed in Section 6.