The James Webb Space Telescope has returned its first images, a “selfie” showing the 18 hexagonal segments of its 6.5-meter (21.3-foot) primary mirror and 18 corresponding, misaligned views of a star near the bowl of the Big bear. And that’s exactly what they hoped to see in the early stages of a complex mirror alignment process.
“This amazing telescope has not only spread its wings, it has now opened its eyes,” said Lee Feinberg, Webb’s head of optics.
A mosaic showing the 18 fuzzy reflections shows their current post-launch alignment and will serve as a guide for engineers, controlling actuators at the rear of each segment, to ultimately direct all beams to the same point to deliver sharply focused light to all four of Webb. scientific instruments.
So far, Feinberg said, the work has gone smoothly, but “I want to warn…it’s still very early days.”
“We don’t have detailed assessments of everything at this point,” he told reporters on a Feb. 11 conference call. “But based on our preliminary assessments, everything matches the models and we expect the models to work at this point.”
Webb’s mirrors and his four instruments cool further to the ultra-low temperatures, near absolute zero, needed to detect the faint infrared light emitted by the first stars and galaxies to form following the Big Bang.
But one of the instruments, the near-infrared camera – NIRCam – is already cool enough for its detector to register light from a target star in Ursa Major known as HD 84406.
“As expected, we pointed the telescope at a bright, isolated star, and found and identified 18 points for the 18 primary mirror segments,” Feinberg said. “At this point, we have been able to analyze several engineering images that help us understand the alignments and the mirrors themselves.
“And we don’t see anything to worry about. This is the first time we’ve gotten data on mirrors that are actually weightless and use starlight to illuminate the primary mirror. And again, so far the data matches our patterns and expectations.
Engineers expected the mirror segments to be misaligned by up to a millimeter early in the alignment process. To ensure that the target star could be seen by all 18, more than 1,500 images were taken of a region of sky the size of a full moon.
It turned out that all 18 images were clustered in the top 10 percent of the search area, indicating that the telescope had been launched with the expected coarse alignment.
Over the next few weeks, engineers will move the segments in small increments, check alignment, and then adjust them again as needed in an iterative process designed to move reflected beams toward the center of Webb’s optical axis. The goal is to stack or merge them into a single beam, achieving the sharpness of a single 6.5 meter mirror.
After that, Webb’s scientific instruments will be tested and calibrated, setting the stage for the first scientific observations in the June period.