The James Webb Space Telescope (JWST) has entered a new phase of its commissioning process as NASA announces that the observatory has completed its mirror alignment and focusing process and is ready to move into the instrument calibration ahead of full science operations later this year.
“These remarkable test images from a successfully aligned telescope demonstrate what people across countries and continents can achieve when there is a bold scientific vision to explore the universe,” said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center.
To discuss this milestone, NASASpaceflight spoke with Charlie Atkinson, Chief Engineer for Northrop Grumman’s JWST Program, regarding the status of the telescope, its performance so far, and what remains for the prime contractor as the telescope moves toward operational status.
Optical alignment of James Webb Space Telescope
“Every step of the way, things have been going either exactly as planned or better than planned, which has just been an incredibly great feeling,” said Atkinson, who’s been with the Webb program for Northrop Grumman since 1998.
One of those better-than-expected performance points comes from the optical performance of the telescope. As Atkinson explained, “We’re getting an indication that the actual optical performance of the telescope is actually better than we expected.”
Atkinson elaborated, commenting on the fact that all space telescopes carry an optical error budget to account for certain parts of the manufacturing process that can contribute to the degradation of the optics, such as imperfect polishing of the mirrors and imperfect alignment of those mirrors.
Additionally, some optical degradation can come from the telescope’s mandatory systems, which for Webb includes the observatory’s reaction control wheels and cryocooler, both of which induce disturbances, or “jitters,” into the system.
With all those factors in place, the team has been able to compare predictions to reality. “When you put it all together, it’s actually performing a lot better than those allocations we were given,” noted Atkinson.
But to get to the point where the optics could be tested, the 18 individual segments of the James Webb Space Telescope’s primary mirror had to be aligned. A process that is more complicated than it might sound.
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To begin with, James Webb’s primary mirror had to be assembled from 18 smaller individual mirror segments. And those individual mirror segments could not be fully integrated and aligned on the ground because the 8.4-meter diameter mirror was too large to fit into any payload fairing for launch. So the mirror had to be folded up in sections and then unfolded in space.
Once the unfolding process was complete, the next step was to move each of the 18 mirror segments out of their launch lock configurations by about half an inch — a process that took several days to complete because of how slow the motors are designed to move for fine alignment procedures.
Then the team had to figure out where each of those mirror segments was pointing so they could align them all together into what is an effective single mirror.
“At this point, we [had] a telescope that [hadn’t] really started to be aligned,” said Atkinson. “We needed to go find out where those 18 individual images were. And we did what’s called a mosaic pattern.”
This involved spinning the observatory in a spiral pattern to allow the desired imaging target to be seen by all 18 mirror segments.
“And the good news is that it didn’t take long to do that because that collection of 18 images was not very far from ideal alignment,” said Atkinson.
Once those images were collected, the team then needed to figure out which mirror was looking at which image. To do this, they slowly moved one mirror at a time and watched to see which image moved.
This was necessary to provide one piece of the information needed to move each mirror segment to align them all together.
Once those identifications were complete, the team then performed the initial focus sweep, changing the focus of the secondary mirror and changing its distance from the primary mirror to see how the images changed.
“That allowed us to do two things. One it gave us the initial location of where we want to put the secondary mirror, to find kind of the optimum position,” related Atkinson.
The second item provided was the final piece of information on how to move each primary mirror segment for alignment.
“The entire process to get it from when we first moved them out off of their launch locks to the point where we’ve got the telescope completely aligned, that was about two months,” said Atkinson.
While this work with the mirror progressed, other teams were busy bringing Webb’s science instruments online and putting them through their initial checkouts.
For Northrop Grumman, this period also included their cryocooler’s main task of chilling the observatory down to just 6 Kelvin, the required maximum operating temperature of the MIRI (mid-infrared instrument).
“The temperatures have basically stopped changing,” noted Atkinson when speaking about the cryocooler’s performance. “They’re still changing a little bit, but for the most part we’re pretty stable at this point.”
The cryocooler was a significant portion of the observatory that could not be fully tested end-to-end on the ground due to the telescope’s operational size.
“There were a number of situations like that where just because of ground environments with gravity, with temperature cases, with such a large observatory, you [couldn’t] pack the whole thing into one chamber because there [wasn’t] one that [was] big enough to go test it.”
“So we tested the cryocooler itself as an entity to make sure that it was going to be able to get the right amount of coolant, the right pressure, the right temperature conditions. And we tested the infrared instrument to make sure that it was going to react appropriately. But you’ll never know until you go through the whole thing,” said Atkinson.
Part of the cryocooler’s operation involved running its coolant lines up a deployable boom from the telescope’s main control bus up to the MIRI instrument, a deployment that stretched the cryocooler’s lines like a Slinky.
“That had never been checked out,” added Atkinson. “But all the testing we did on the ground give us high confidence that was going to work.”
And it has.
Future with JWST for Northrop Grumman
As Webb moves through the final phases of its commissioning, Northrop Grumman’s work with the observatory will not end when it enters science operations.
The testbed that has been built on the ground will continue to be used to test software upgrades and to evaluate and analyze anything that might go wrong with Webb that requires a fix.
“If there’s an anomaly and we can go test that out on the testbed to say ‘Yep, that kind of behavior results if this is what happened on the observatory’, and then we can test out, “Okay well, how do we want to fix that?’, noted Atkinson.
“So that kind of a maintenance operation is something that we’ll definitely be doing for James Webb for its foreseeable life.”
(Lead image: Engineering images of sharply focused stars in the field of view of each instrument demonstrate that the telescope is fully aligned and in focus. Credit: NASA/STScI)