Frequency diplexing functions. Top and bottom: the two sky ports (actually in the Nasmyth focal plane) and the cryostat signal windows, one per band. Dnn: dichroic mirror, with reference to band numbers. MR21: solid refocusing mirror used only in conjunction with D12. R3/4: solid mirrors in a permanent position. See also Table 2. All elements are represented in their working position, which does not correspond to a real situation. The four cryostat windows B1-4 are represented in a plane, while actually they are a the corners of a square 130 mm on a side. Example 1: frequency diplexing of B1/2: D12 is used jointly with MR21; D13 and D24 are outside the beams. Example 2: B3 alone (minimum loss) a solid mirror replaces D13; R3 is a fixed mirror; all other mirrors are outside the beam.