Sunshield Membrane Coatings
The James Webb Space Telescope's primary science comes from infrared light, which is essentially heat energy. To detect the extremely faint heat signals of astronomical objects that are incredibly far away, the telescope itself has to be very cold and stable. This means we not only have to protect JWST from external sources of light and heat (like the sun and the earth), but we also have to make all the telescope elements themselves very cold so they don't emit their own heat energy that could swamp the sensitive instruments. The temperature also must be kept constant so that materials aren't shrinking and expanding, which would throw off the precise alignment of the optics.
To accomplish all of this, JWST deploys a tennis-court sized Sunshield made of five thin layers of Kapton E with aluminum and doped-silicon coatings to reflect the sun's heat back into space. The Kapton is a commercially available polyimide film from Dupont, while the coatings are applied to a specialized JWST specification.
The Layer 1 membrane (facing the sun) is 0.002" thick, the other four layers are each 0.001". The higher emissivity doped-silicon coating is ~50 nanometers thick, and is applied to the sun-facing side of the two hottest layers (Layer 1 & 2) to maximize stoppage of the sun's heat. Doping is a process whereby a small amount of conductive material is mixed in during the silicon coating process, so that the coating is electrically conductive. The highly-reflective aluminum coating is ~100 nm thick and is applied to all the other surfaces, helping to "bounce" the remaining energy out the gaps between the layers.
Flight Layer 2 membrane of the James Webb Space Telescope sunshield during shape testing, in Jan 2016. All 5 flight membranes will be complete by late 2016. Image credit: Northrop Grumman.
Early shape testing of sunshield membranes. Credit: Northrop Grumman
The size, position, spacing and shape of the Sunshield layers are also very important - more information on these aspects can be found on our sunshield page.