The cryostat is designed around the successful NRAO, closed-cycle, 4 K refrigerator design. The unit to be used in the array was constructed in a collaborative effort with the NRAO staff. The sky beams enter the cryostat through a Teflon coated, crystalline quartz window, chosen for its strength and low loss. The Teflon layers serve as an anti-reflection coating. Smaller versions of this window are currently being used on the SMTO 492 GHz and AST/RO 492/810 GHz receivers. Most of the remaining IR radiation is absorbed by 4 layers of 10 mil thick Zytex stretched across the 77 K radiation shield. (Zytex also serves as the IR-blocking filter in the SMTO 492 GHz and AST/RO receivers.) The sky beams then pass through the SSB and LO diplexing optics described earlier. The optics package is cooled to 4 K to minimize its noise contribution. The LO beams enter through the side of the dewar through a window. The pressure window is formed by the lenslet array collimating the beams from the LO power splitter. A 77 K sheet of Zytex is again used as the IR blocking filter. In order to simplify the optical alignment of the system, the optics package and receiver rocket are designed to be a modular, integrated unit. The dewar internal support structure consists of two cylindrical sections of G-10; one between the 300 K and 77 K stations and the other beween the 77 K and 4 K stations. Holes will be drilled into the sidewalls of the G-10 to reduce thermal loading. The G-10 cylinder will serve as a rigid, stable platform for the optics package. G-10 feedthroughs will be used to position the MPI mirrors. Optical alignment will be performed at room temperature with a laser. The optics package + rocket will then be mounted to the 4 K support ring. The vacuum vessel and radiation shield are designed to split at a position permitting easy access to the 4 K working surface.