Engineers and scientists for the James Webb Space Telescope have completed two more steps in the telescope’s primary mirror alignment process, and in a briefing today, officials said JWST’s optical performance appears to be better than even the most optimistic predictions.
The team released a new engineering image, showing the star 2MASS J17554042+6551277 in crisp clarity. This image demonstrates that all 18 mirror segments have been precisely aligned to act as one giant, high-precision 6.5-meter (21.3-foot) primary telescope mirror.
“We now have achieved what’s called ‘diffraction limited alignment’ of the telescope,” said Marshall Perrin, deputy project scientist for Webb at the Space Telescope Science Institute. “The mirrors are focused together as finely as the laws of physics allow, and this is the sharpest image you can get from a telescope of this size.”
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As evidence of how well the telescope works, if you look closely at the image, background galaxies are visible, much like the “Deep Field” images taken by the Hubble Space Telescope.
“Basically, everywhere ever we look, it’s a Deep Field,” said Jane Rigby, Webb operations project scientist. “These engineering images are as sharp and crisp as images that Hubble can take, but at a wavelength of light that Hubble can’t see.”
Perrin said the team still needs to “dial in very small adjustments to bring the telescope to even more exquisite sharpness,” and added, “It’s an absolute thrill to say that everything has worked. At no point the process did we have any issues with any of the deployments, and while there were some surprises in the data, the outcomes are far closer to our hoped-for predictions than we could have expected.”
Webb has now completed steps four and five of the seven-step, three-month long alignment process.
“All 18 mirror segments are now aligned into a single mirror,” said Lee Feinberg, Webb’s optical telescope element manager. “The images came down over the weekend, it was a very emotional moment. We can see the optical performance of the telescope is absolutely phenomenal.”
Feinberg said the image is a 2,100 second exposure, and taking an image over that length of time allows the team to assess several aspects of the telescope’s performance. Not only are the optics working perfectly, but other systems are working well too. This includes the fine guidance sensors and reaction wheels that allow the telescope to point precisely and stay on target.
“We know it’s working because we have a picture of star that looks like star,” Feinberg said. “We’re getting close to the point where we can turn this observatory over to the scientific community.”
Here’s a great comparison of this star in approximately the same field of view as seen by another infrared telescope, Spitzer and then Webb:
The star 2MASS J17554042+6551277, is a “generic, anonymous, average star” chosen for its brightness – or lack thereof.
“We plucked this star out of obscurity,” Rigby mused, “It is 100 times fainter than what the human eye can see, but here it looks blindingly bright.”
The mirror alignment process, called phasing, began in early February and the seven different steps takes the mirrors’ initial placements after they were deployed to doing a “coarse” and then “fine” alignment, and then making sure the mirror works with all four of Webb’s instruments and their various fields of view.
“So far, we’ve nailed it,” Rigby said. “Webb is seeing back in time without really breaking a sweat. We’re still commissioning and have to align the telescope to the four science instruments, and get those instruments ready for prime time. We’ve already selected a year of compelling and demanding science with this telescope, and we’re all excited to get started.”
The scientists expect Webb to be fully commissioned by the end of June. One instrument, the MIRI or mid-infrared instrument still needs to continue cooling down to 7 K, or 7 degrees above absolute zero. It is currently at about 90 K.