The Band 3 cold optics. In the foreground, the two horns located symmetrically on either side of the polarization diplexing wire grid (seen edge on). Behind each horn sits one local oscillator injection coupler, and between the two couplers, diagonally, the LO power splitter that also serves as a mechanical support for the other elements. That sub-assembly, which is cooled to 4 K, is mechanically linked to, and thermally insulated from the mirror sub-assembly by fiberglass-epoxy (G11) supports.