Photon's Eye View: optical bench is the plate at the bottom, to be at 4K; the lower light-weighted structure is "500mK", thermally attached to the 3-stage refrigerator's intercooler; the upper lightweighted structure is the "250mK" stage, to which the feed horns with bolometric detectors are attached. The rest of the optics are attached directly to the optical bench, including the fold mirrors. You can see several of the fold mirror mounts in the photo.
The yellow columns (and similar ones hidden within the aluminum tubes) are made of a low-thermal conductivity material not formerly used in cryogenic systems to the author's knowledge. It is nearly a direct replacement for Dupont Vespel SP-1 though much less expensive; paper in preparation.