WASHINGTON — NASA has completed the next step in the commissioning of the James Webb Space Telescope by fine-tuning the alignment of its primary mirror segments, confirming the telescope’s optics will meet or exceed expectations.
The agency announced March 16 that spacecraft team had finished the “coarse phasing” and “fine phasing” steps of alignment of the telescope’s optics. Those steps involved very small changes in the positions of JWST’s 18 primary mirror segments such that their images were aligned on top of each other and then matched to within a wavelength of infrared light.
The completion of the fine phasing means that the telescope is now fully aligned for one of its main instruments, the Near-Infrared Camera, and that there are no flaws like the spherical aberration in the primary mirror of the Hubble Space Telescope that diminished its performance after launch until corrective optics were installed.
“The optical performance of the telescope is absolutely phenomenal,” said Lee Feinberg, optical telescope element manager for JWST at NASA’s Goddard Space Flight Center, during a media briefing. “The performance is as good, if not better, than our most optimistic predictions.”
Others at the briefing echoed his sentiments. “The telescope performance so far is everything that we dared hope,” said Jane Rigby, JWST operations project scientist at NASA Goddard.
“We now have achieved what’s called diffraction-limited alignment of the telescope. The images are focused together as finely as the laws of physics allow,” said Marshall Perrin, JWST deputy telescope scientist at the Space Telescope Science Institute. “This is as sharp an image that you can get from a telescope of this size.”
The completion of fine phasing comes as JWST nears the halfway mark in its six-month post-launch commissioning. Officials said at the briefing they will now check the alignment in three other instruments on the telescope, tweaking the mirror positions to optimize alignment for all the instruments. The instruments themselves will undergo tests to confirm they are ready to begin operations.
Project officials stuck to the original schedule for commissioning at the briefing. “We will start science operations when we’re done with commissioning,” Rigby said. “Commissioning is nominally six months long. We launched on Christmas Day, so that would be the end of June.”
Officials were pleased as much as they were relieved that the telescope alignment work confirmed there were no problems with the optics. “We do know now that we’ve built the right telescope in terms of the optics,” said Feinberg. “That’s a big deal.”
Rigby said that, at this point, if something goes wrong, such as a problem with one of the instruments, there would be “partial degradation” of the science but not a total loss, which would have been the case in earlier phases of commissioning. “There were parts on this mission where this was going to work or we’re done. We’re past those points now.”
“This is one of the most magnificent days in my whole career at NASA,” said Thomas Zurbuchen, NASA’s associate administrator for science since 2016. He said he had “sleepless nights” during the earlier phases of commissioning the telescope. “They’re all behind us now.”