SAN FRANCISCO — An initial set of images taken by NASA’s James Webb Space Telescope shows that the spacecraft’s primary mirror is performing as expected during its months-long alignment process.
NASA released Feb. 11 an image mosaic taken by one of the JWST’s instruments, the Near Infrared Camera (NIRCam). The images came from a search of a region of the sky the area of the full moon, looking for light from the same star, HD 84406. Because the segmented primary mirror of JWST is not yet aligned, there are 18 images of the star in the mosaic, one for each segment.
That result matched what project officials expected to see at this early phase in the mirror alignment effort. “The initial results are very similar to our simulations,” said Lee Feinberg, JWST optical telescope element manager at NASA’s Goddard Space Flight Center, in a call with reporters. “At this point we’ve been able to analyze multiple engineering images that help us understand the alignments and the mirrors themselves, and we don’t see anything of concern.”
The performance so far doesn’t mean they can rule out any kind of serious flaw with the mirrors, like the infamous spherical aberration problem with the Hubble Space Telescope’s primary mirror. “It’s too early to say that there’s definitely not a major flaw,” he said. “But had there been a large, major flaw, I think it is possible we would have seen it now, and we don’t.”
That mosaic is only the first step in a three-month process to align the 18 segments of the primary mission. Later steps will involve moving the mirror segments so that that the images they produce stack up in a single location and then are phased within a single wavelength of light, followed by calibrations for the telescope’s other instruments.
“We are just about right on track” on the schedule for mirror alignment, said Marshall Perrin, JWST deputy telescope scientist at the Space Telescope Science Institute. Some steps have taken a little longer than expected while others have been completed faster than planned. “We’re within a day or so of the initial pre-flight schedule.”
The initial alignment images also served as a test for NIRCam itself. “My team and I have worked on NIRCam for over 20 years and the excitement of finally getting some light through the telescope onto NIRCam’s detector was really hard to express,” said Marcia Rieke, principal investigator for NIRCam at the University of Arizona.
Those images confirmed that NIRCam is working as designed. “We got beautiful — at least beautiful to a person who has worked on NIRCam for a long time — images. None of the issues or problems were realized. NIRCam was working,” she said. “We were super-duper happy.”
NIRCam provided a bonus image: a “selfie” of the primary mirror, taken using a special lens in the instrument to help with alignment. The image showed one of the mirrors illuminated with starlight, as that was the one segment in alignment at the time.
Feinberg said that imaging mode wasn’t tested on the ground before launch, so engineers didn’t know what to expect. “Pretty much the reaction was, ‘holy cow,’” he said of what the project team thought of that image. “We were pretty much wowed by it, and that’s one of the reasons, I think, people wanted to share it.”