Engineers and technicians at NASA's Johnson Space Center in Houston, Texas prepare for the James Webb Space Telescope's Thermal Pathfinder (practice model) test. On the right the practice model stands fully deployed outside the massive thermal vacuum chamber, called Chamber A. The actual telescope will look like this before it gets tested inside the Chamber A next year, except with flight mirrors instead of placeholder ones.

There are two flight spare mirrors on the Pathfinder, the rest are gold-coated aluminum. They look this way to emulate thermally the real mirrors and so will cool-down and warm-up like the real telescope. Gold-coated aluminum emulates the heat absorbing and emitting properties of the real mirrors, and with the addition of some heaters they emulate the thermal inertia of the real mirrors.

Credit: NASA/Desiree Stover

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