The observatory is dominated visually by the sunshield subsystem, which separates
the observatory into a warm sun-facing side and a cold anti-sun side.
The James Webb Space Telescope will observe primarily the infrared light from faint and very distant objects.
But all objects, including telescopes, also emit infrared light in the form
of heat energy. To avoid swamping the very faint astronomical signals with
radiation from the telescope, the telescope and its instruments must be
very cold.
The observatory will be pointed so that the Sun, Earth and Moon are always on one side, and the sunshield will act like a parasol, keeping the Optical Telescope Element and the Integrated Science Instrument Module (ISIM) cool by keeping them in the shade and protecting them from the heat of the sun and warm spacecraft bus electronics.
The sunshield will allow the telescope to cool down to a temperature below 50 Kelvin (equal to -370 degree F, or -223 degree C) by passively radiating its heat into space. The near-infrared instruments (NIRCam, NIRSpec, FGS/NIRISS) will work at about 39 K (-389 degree F, -234 degree C) through a passive cooling system. The mid-infrared instrument (MIRI) will work at a temperature of 7 K (-447 degree F, -266 degree C), using a helium refrigerator, or cryocooler system.
In addition to providing a cold environment, the sunshield provides a thermally stable environment. This is essential to maintaining proper alignment of the primary mirror segments as the telescope changes its orientation to the Sun.
When fully deployed, the sunshield that will be about the size of a regulation
tennis court.
Why does the sunshield have five layers instead of just a single thick one? Each successive layer of the sunshield is cooler than the one below. The heat radiates out from between the layers, and the vacuum between the layers is a very good insulator. One big thick sunshield would conduct the heat from the bottom to the top more than 5 layers separated by vacuum.
These are photos of another engineering model of the sunshield, called the pathfinder. Read more...
Below are two videos showing the sunshield tests on the engineering model of Webb.
