The MIRI solid hydrogen cryostat. The cryostat is designed to hold about 1000 liters of solid hydrogen at less than 7 degrees Kelvin (i.e., 7 degrees above absolute zero).

MIRI Cryostat

To operate at the natural background sensitivity limit between 5 and 10 microns, the MIRI optical bench assembly must be cooled below the temperature of the JWST Integrated Science Instrument Module (ISIM). Furthermore, the MIRI focal planes must be cooled to 7 Kelvin to work correctly. The necessary cooling will be supplied by a solid hydrogen cryostat, to be built by Lockheed Advanced Technology Center for JPL. In the concept to the right, there are two hydrogen vessels - the large one is for the optical bench and the small one for the detectors. To provide good heat transfer, the vessels are filled with an aluminum foam, and the filling is denser in the small vessel

Here is the cryostat(Dewar) mounted on the telescope along with MIRI and the other instruments.

to get the best possible cooling for the detectors. The pre-launch cooler helps preserve the solid hydrogen on the launch pad. The two heat switches are for the optical bench and focal planes, respectively.

The heat switches are connected to the instrument with thermal straps that carry the heat into the switches and, if they are closed, into the hydrogen.

The heat switches are kept open until the instrument has cooled to about 70 Kelvin, and then closed to bring it and the focal planes down to their final operating temperatures. The strategy will be optimized to conserve the solid hydrogen as much as possible. All of these operations are controlled through the Dewar Control Electronics (DCE).