Alignment of the Primary Mirror Segments of The James Webb Space Telescope

Engineers at NASA’s Johnson Space Center in Houston used light waves to align the James Webb Space Telescope’s mirror segments to each other, so they act like a single, monolithic mirror in the cryogenic cold of the center’s iconic Chamber A. Part of the Webb telescope’s ongoing cryogenic testing in Chamber A at Johnson includes aligning, or “phasing,” the telescope’s 18 hexagonally shaped primary mirror segments so they function as a single 6.5-meter mirror. All of these segments must have the correct position and correct curvature; otherwise, the telescope will not be able to accurately focus on its celestial targets. To measure the shape of the Webb telescope’s primary mirror, engineers use a test device called an interferometer, which shines a laser down onto the mirror. Because the mirror is segmented, it requires a specially designed interferometer, known as a multi-wavelength interferometer, which allows the engineers to use two light waves at once, explained Lee Feinberg, optical telescope element manager for the Webb telescope at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The interferometer splits the laser light into two separate waves. One of these waves goes through a lens and reflects off the primary mirror; the other wave acts as a reference. The reflected wave interferes with (meets) the reference wave, and engineers analyze the combined wave that results from that interference. “By analyzing the interference signal, engineers determine the mirror shape and the alignment of the mirrors,” explained Feinberg. When the engineers need to adjust the positions and shapes of the mirror segments to achieve precise alignment, they use the seven actuators (tiny mechanical motors) attached to the back of each one of the mirror segments. For each segment, six of these actuators are placed into groups of two, at three equally spaced points along the outside of the mirror (to adjust the segment’s position), and one is attached to six struts that are connected to each of the hexagonal mirror segment’s corners (to adjust the segment’s shape).