Optics Block Diagram of the High Resolution Telescope path. M1 and M2 are the mirrors of the Ritchey-Chrétien telescope; LC1 and LC2 are the liquid crystal cells; L1, L2, L3 and L4 are the lenses of the magnifying Barlow lens system, which can be shifted to act as the refocus compensator. M3 (feed select mirror) and M4 are the folding mirrors; the Filtergraph oven includes the two lenses FG1 and FG2, the two components of the prefilter (PF1 and PF2) and the etalon; finally, the camera lenses (C1, C2 and C3) provide the image at the scientific focal plane. The beam splitter providing light to the Image Stabilisation System (ISS) is not shown.