JWST entering “home stretch” of commissioning

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WASHINGTON — The James Webb Space Telescope is operating better than expected as the spacecraft enter the final stages of commissioning, project officials said May 9.

In a call with reporters, scientists and mission managers said they have completed the alignment of the telescope’s optics with all its instruments and now are moving into setting up the instruments for science operations, the final step in a commissioning process that started shortly after the telescope’s launch on Christmas Day last year.

“The performance is even better than we anticipated,” said Michael McElwain, JWST observatory project scientist at NASA’s Goddard Space Flight Center. “We’ve basically reached a perfect telescope alignment. There are no adjustments of the telescope optics that would make material improvements to our science performance.”

Asked later to quantify that “better than expected” performance, he said a parameter known as the static wavefront error is “significantly better” than planned for. “What that means is that we put the telescope mirrors into position with better accuracy and precision than what we had budgeted for, so we’re doing much better than requirements.” That reduced error, he said, improved both the sensitivity and resolution of the instruments.

Marcia Rieke, principal investigator of one of JWST’s instruments, the Near-Infrared Camera (NIRCam), said that images taken by the instrument show that the telescope resolution is diffraction limited at key wavelengths, meaning its images are as sharp as possible under the laws of physics. “It’s just amazing that the image quality is that excellent, and that’s going to help our science quite a bit,” she said.

With the telescope alignment completed, the project is moving the final phases of commissioning, which include preparing the instruments for science operations. “It’s the time we’re doing all the requisite checkouts and calibrations before we start science,” McElwain said.

“I would also call this the home stretch,” he added. “We’ve had about 1,000 activities planned for all of commissioning, and there are only about 200 activities left to complete.”

That process will take about two months. The mission will wrap up that commissioning with the public release of what it calls “early release observations,” an initial set of images designed to showcase the telescope’s capabilities.

“Their objective is to demonstrate, at the end of commissioning, to the world and to the public, that Webb is fully operational and that it produces excellent results,” said Klaus Pontoppidan, JWST project scientist at the Space Telescope Science Institute. Formal science operations through a program of observations called Cycle 1 will begin after the early release observations are made public, which he said is tentatively planned for mid-July.

Those early release observations will involve all four science instruments and cover a range of objects. Pontoppidan said a committee developed a ranked list of objects to include among the early release observations, but declined to say what objects are included in that list. One reason, he said, is that the objects selected could change depending on when observations can be scheduled. Also, he added, “we’d really like to be a surprise.”