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Hubble’s Flaw Informs Webb’s Perfection

Nancy Atkinson
Tristan Hanks
February 22, 202211:00 AM UTC (UTC +0)

It’s been an amazing ride, and the science hasn’t even started yet. 

Since the Christmas morning launch of the James Webb Space Telescope, we’ve witnessed remarkable success in every single step of the nail-biting, complicated, and highly orchestrated set of sequences to unfold and deploy the giant telescope. The achievement is a testament to the dedication of the thousands of designers, engineers, technicians, and scientists who have worked on the observatory over the past 30 years.  

Now in its halo orbit at the Sun-Earth Lagrange 2 (L2) point, Webb has begun its formal commissioning process. This includes activating its four science instruments and initiating the three-month-long procedure to align the 18 separate mirror segments to work as one perfectly aligned 6.5-meter (21.3-foot) primary telescope mirror.

"Those segments need to be aligned precisely,” said Lee Feinberg, the Optical Telescope Element Manager for the mission. “We're not talking in terms of within microns, we're talking a fraction of a wavelength."

"That's what's tricky about Webb."

Feinberg has been part of Webb for over 20 years and his experience working with space telescope mirrors goes back to the Hubble Space Telescope. He was part of the team that developed the optical correction and upgrades for the science instruments to compensate for the flaw in Hubble’s primary mirror.

While Webb has always been billed as the successor to Hubble, Feinberg said there’s a remarkable thread, a crossover between the two missions: The same computer algorithms used to correct Hubble’s flaw are now being used to align Webb’s mirror segments.

“We adapted what was done to fix Hubble, in a way that helped us figure out how to move the mirror segments to properly align them,” Feinberg explained. These learnings allowed for the ability to have a segmented, foldable primary mirror. “So, there is a real continuity there. And it’s pretty neat.” If not for the flaw in Hubble’s mirror, a spherical aberration, there’s a chance that the design of the James Webb Space Telescope could be very different from what it is today.

Hubble’s spherical aberration story is well-known: Within weeks of the Hubble Space Telescope’s launch, the first images indicated a serious problem with the telescope’s mirror. Astronomers determined the mirror had been ground incorrectly, by 2 microns, less than the width of a human hair. 

Teams of astronomers and engineers then developed computer algorithms to figure out how to fix it, and created Hubble’s "eyeglasses," the Corrective Optics Space Telescope Axial Replacement (COSTAR). 

“There was a whole team of folks who developed algorithms for taking the defocused images from Hubble and determining what the wavefront of the optics were, which is the equivalent of a prescription for fixing the mirror,” Feinberg said.

“My job was to make sure we had the right prescription.” 

Of course, you know the rest of the story: COSTAR worked, and Hubble has gone on to change our understanding of the Universe. This hallmark tale has come to represent the resiliency and ingenuity of NASA, and Hubble’s legacy is one of redemption and overcoming the odds to provide spectacular views and insights into the cosmos. 

And now, Hubble’s legacy lives on in the algorithms –– developed over 30 years ago –– to align Webb’s mirror segments into the perfect shape. The process, called phasing, uses the algorithms to determine how each primary mirror segment can be moved –– with adjustments as small as 1/10,000th the diameter of a human hair.

“When we first started developing Webb and came up with its design,” Feinberg said, “we realized that when the mirrors aren’t perfectly aligned, they actually represent an aberrated primary mirror that is a lot like the aberrated primary mirror of Hubble. So that’s why we can use a very similar set of algorithms for Webb.”

Engineers are now undergoing the process of using Webb’s near-infrared camera (NIRCam) instrument to help align the beryllium primary mirror segments. NIRCam has taken 18 out-of-focus images of a star,  one from each mirror segment. The engineers then use the computer algorithms to determine the overall shape of the primary mirror from those individual images and to determine how they must move the mirrors to align them.

The seven-step phasing process goes from the initial alignment using a sort of ‘best guess’ to align the segments, to coarse and then fine phasing, and then making sure the mirror works with all the instruments and their various fields of view.

The star that NIRCam is using is called HD 84406, a G-type main-sequence star that is a lot like our own Sun, located near the ‘bowl’ of the Big Dipper (Ursa Major).

Each primary mirror segment has six actuators or tiny mechanical motors attached to the back that can align the segments, along with an additional actuator at the center of each segment that adjusts its curvature.

These corrections are made through another process called wavefront sensing and control, which measures any imperfections in the alignment of the mirror segments that prevent them from acting like a single, mirror. The corrections are incredibly precise, especially considering each of the hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter.

“It’s a fairly complex process, and as is often the case with optical alignment, there’s an iterative process to it,” Feinberg explained. “You align the mirrors, then check them, and then you need to go back a few steps and adjust and recenter, and then go back through the entire process again.”

That’s why the process will take approximately three months. But this is not a “once and done” procedure. Even small temperature changes or movements of the spacecraft can alter the alignments, so the alignment process will be ongoing during Webb’s lifetime.

“Our plan is to do this every two weeks,” Feinberg said.

“But we will take data roughly every two days, and look at it. So, we have the ability to do it even more frequently, but it may be that we’ll find we don’t need to do it every 2 weeks. This is one of the things we are interested to learn — is how frequently we’ll have to update the mirror.”

So, for all the naysayers who worried about Webb being 1.5 million km (1 million miles away) from Earth, with no ability to service the telescope, Webb will actually be able to perfect its own vision, thanks to Hubble.

“If there was spherical aberration across the entire primary mirror of Webb (across all 18 segments when aligned), similar to Hubble, the actuators would allow us to remove most of it so that is not something we worry about,” Feinberg said, adding one caveat: 

“There are possible flaws in individual mirror segments that could not be compensated for,” he said. “So that is something we still need to prove.  This should be known at the end of fine phasing in March.”

With that –– even with all the successes thus far in unfolding and deploying the telescope –– Feinberg says he really hasn’t had the time or ability to celebrate. 

“While every one of these steps is a relief, to be honest, I am not there yet,” he said with a sigh. “On the one hand, there’s happiness but also, I’m thinking about everything we still have to get through, and all the things that can still go wrong, or if there’s anything we missed. So, to be honest, I’m still not at the point where I’m sleeping well yet!”

But if all goes well, Webb astronomers hope to be able to share the first science images by the summer solstice in June. 

Nancy Atkinson
Tristan Hanks
February 22, 202211:00 AM UTC (UTC +0)