The James Webb Space Telescope has sent back its first images, a “selfie” showing the 18 hexagonal segments of its 6.5-metre (21.3-foot) primary mirror and 18 matching, misaligned views of a star near the bowl of the Big Dipper. And that’s exactly what they were hoping to see in the initial stages of a complex mirror alignment process.
“This amazing telescope has not only spread its wings, but it has now opened its eyes,” said Lee Feinberg, Webb optical elements manager.
A mosaic showing all 18 blurry reflections shows their current post-launch alignment and will serve as a guide for engineers, commanding actuators on the back of each segment, to eventually aim all the beams at the same point to provide sharply focused light to Webb’s four science instruments.
So far, Feinberg said, the work has gone smoothly, but “I want to caution … that it’s still very early.”
“We don’t have detailed evaluations of everything at this point,” he told reporters during a teleconference on 11 February. “But from our preliminary evaluations, everything is matching the models as well as we would expect the models to work at this point.”
Webb’s mirrors and its four instruments are still cooling down to the ultra-low, near-absolute-zero temperatures required to detect the faint infrared light emitted from the first stars and galaxies to form in the wake of the Big Bang.
But one of the instruments, the Near Infrared Camera – NIRCam – already is cold enough for its detector to register the light of a target star in Ursa Major known as HD 84406.
“As planned, we pointed the telescope at a bright, isolated star, and we found and identified 18 spots for the 18 primary mirror segments,” Feinberg said. “At this point, we’ve been able to analyse multiple engineering images that help us understand the alignments and the mirrors themselves.
“And we don’t see anything of concern. This is the first time we’re getting data on mirrors that are actually at zero gravity and using starlight to illuminate the primary mirror. And again, so far the data is matching our models and expectations.”
Engineers expected the mirror segments to be misaligned by up to a millimetre at the start of the alignment process. To make sure the target star could be seen by all 18, more than 1,500 images were taken over a region of the sky the size of a full moon.
As it turned out, all 18 images were clustered within the first 10 percent of the search area, indicating the telescope came through launch with the expected coarse alignment.
Over the next several weeks, engineers will move the segments in tiny increments, check the alignment and then adjust them again as needed in an iterative process designed to move the reflected beams to the center of Webb’s optical axis. The goal is to stack, or merge, them into a single beam, achieving the razor-sharp focus of a single 6.5-metre mirror.
After that, Webb’s science instruments will be tested and calibrated, setting the stage for the first science observations in the June timeframe.
