Taken from inside Chamber A at the Johnson Space Center in Houston in September 2017 while the combined optical and science instrument element of the James Webb Space Telescope was undergoing cryogenic testing, the temperature at the time this image was taken was approximately 50 kelvins (about -369.7 degrees Fahrenheit/-223.2 degrees Celsius). The camera that captured this image was placed inside the chamber to measure the telescope’s alignment, but engineers also used it to monitor the black DuPont™ Kapton® covering that outlines Webb’s primary mirror. Engineers used this and other images to assess the material’s slack as the telescope shrank ever so slightly in the extreme cold of the chamber.
Once Webb is fully deployed and in orbit at the second Lagrange point (L2), this Kapton® “wreath” around the primary mirror will block unwanted light from behind the telescope from interfering with its observations. There are five so-called "Lagrange Points" - areas where gravity from the Sun and Earth balance the orbital motion of a satellite. Putting a spacecraft at any of these points allows it to stay in a fixed position relative to the Earth and Sun with a minimal amount of energy needed for course correction.
In the photo, you can see each of Webb’s 18 hexagonal primary mirror segments, though the ones further from the camera quickly fade into darkness. The bright elements in the photo — the “stars” that seem to envelope Webb within the chamber — are targets that were used to measure extremely precise movements of the telescope as it cooled. Those targets appear so bright because this photo had a very long exposure time.
Webb’s combined optical and science instrument element completed cryogenic testing inside the chamber in November 2017.
Image Credit: NASA/Chris Gunn
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