James Webb telescope completes optical alignment, ready for final instrument calibration phase of commissioning

Date:
Thu, 28 Apr 2022 18:16:37 +0000

Description:
The James Webb Space Telescope (JWST) has entered a new phase of its commissioning process The post James Webb telescope completes optical alignment, ready for final instrument calibration phase of commissioning appeared first on NASASpaceFlight.com .

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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 NASAs Goddard Space
Flight Center.

To discuss this milestone, NASASpaceflight spoke with Charlie Atkinson,
Chief Engineer for Northrop Grummans 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, whos 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, Were 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 telescopes
mandatory systems, which for Webb includes the observatorys reaction control wheels and cryocooler, both of which induce disturbances, or jitters, into
the system. See Also JWST Mission Updates Space Science coverage L2 Future Spacecraft Click here to Join L2

With all those factors in place, the team has been able to compare predictions to reality. When you put it all together, its 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 Telescopes primary mirror had to be aligned. A process that is more complicated than it might sound.

( Click here to for NASASpaceflights coverage archive on the James Webb Space Telescope )

To begin with, James Webbs 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. NASAs James Webb Space Telescope. (NASA)

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 [hadnt] really started to be aligned, said Atkinson. We needed to go find out where those 18 individual images were. And we did whats 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 didnt 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 weve got the telescope completely aligned, that was about two months, said Atkinson.

Overlapping operations

While this work with the mirror progressed, other teams were busy bringing Webbs science instruments online and putting them through their initial checkouts.

For Northrop Grumman, this period also included their cryocoolers main task of chilling the observatory down to just 6 Kelvin, the required maximum operating temperature of the MIRI (mid-infrared instrument). The cryocooler for the MIRI. The MIRI requires a lower operating temperature than Webbs
other instruments, and the cryocooler accommodates this requirement. (Credit: NASA/JPL-Caltech)

The temperatures have basically stopped changing, noted Atkinson when speaking about the cryocoolers performance. Theyre still changing a little bit, but for the most part were 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 telescopes 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 [couldnt] pack the whole thing into one chamber because
there [wasnt] 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 youll never know until you go through the whole thing, said Atkinson.

Part of the cryocoolers operation involved running its coolant lines up a deployable boom from the telescopes main control bus up to the MIRI instrument, a deployment that stretched the cryocoolers 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. While the purpose of this image was to focus on the bright star at the center for alignment evaluation, Webbs optics and NIRCam are so sensitive that the galaxies and stars seen in the background show up. (Credit: NASA/STScI)

And it has.

Future with JWST for Northrop Grumman

As Webb moves through the final phases of its commissioning, Northrop Grummans 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 theres 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 well 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)

The post James Webb telescope completes optical alignment, ready for final instrument calibration phase of commissioning appeared first on NASASpaceFlight.com .



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Link to news story: https://www.nasaspaceflight.com/2022/04/jwst-update-april-2022/


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