a crucial step validated by superb shots

The James Webb Space Telescope has finally finished adjusting the various segments of its main mirror; it is now approaching the home stretch before its commissioning, expected this summer.

That’s it ! After long months of preparation, the James Webb Space Telescope has finally passed a crucial milestone, the most important since its launch on December 25th. Its 18 hexagonal mirrors are now perfectly aligned, and the telescope will soon be able to draw worms from the nose of the most intriguing objects in the cosmos.

Originally, this jewel of technology was launched completely folded in on itself, a great first in the history of aerospace. The first steps therefore consisted of completely unfolding the machine to give it its final shape. After some routine adjustments, the telescope had started aligning its famous mirrors.

Indeed, the main mirror of the JWST is not made of a single piece. To be able to send such a large reflector into space, the researchers chose to subdivide it into 18 hexagonal pieces; an approach which made it possible to doubt the machine of a gigantic main mirror of 6.5m in diameter in total.

This is a particularly significant element, because the size of this reflector has a direct bearing on the operational accuracy of the machine. But there is a flip side. It is also an infinitely complicated approach to implement in terms of engineering; it involves aligning all those mirrors with simply phenomenal precision so that they function as a single functional unit.

A long and delicate deployment

The problem is that the vibrations generated by the rocket trip are very problematic at this level, and the alignment of the mirrors had therefore necessarily been disturbed. Except that at this level, there is simply no margin for error. To observe objects located millions of light-years away, the slightest fraction of a millimeter shift on just one of the 18 mirrors would cause an unacceptable loss of precision in this context.

Precisely for this reason, on the JWST’s inaugural first optical test, we saw not one, but eighteen images of the same celestial body. If the telescope squinted like this, it was because each of the points represented the contribution of an individual mirror, with a more or less significant shift in relation to its neighbour. This image has since served as a reference point for the alignment process driven by the Fine Guidance System.

With the help of this galactic “compass”, initialized and calibrated last February (see our article), the JWST was able to finalize the alignment once and for all. The reason the process took so long was that the mirrors had to be moved very slowly to ensure that the expected level of precision was achieved.

Main mirror now perfectly aligned

But this crucial step is finally over. Alignment is now amazingly accurate, with variability less than a single wavelength of infrared light, or 100 µm. This means that the maximum deflection of each segment of this 6.5m long mirror is comparable to the thickness of a human hair!

We have reached what is called the diffraction-limited alignment of the telescope”, explains engineer Marshall Perrin. “We have now matched the images of the 18 mirrors as finely as the laws of physics allow.”, he adds. The patience of the engineers was therefore more than rewarded; not only is the JWST no longer squinting, but the result has even exceeded all the expectations of the staff.

The optical performance of the telescope is absolutely phenomenal” raves Lee Feinberg, one of the optics managers at NASA’s prestigious Goddard Space Flight Center. “. “They are at least as good, if not better than our most optimistic predictions.”, he adds, supported by his colleague Jane Rigby.

Indeed, even if they do not observe in the same wavelengths, the JWST was designed to be about 100x more sensitive than Hubble; a goal that has not only been achieved, but even exceeded according to the latest data synthesized by NASA.

It is therefore a major step for the telescope, NASA, the entire scientific community, and even for the general public. And to mark the occasion, NASA unveiled a superb image of the star HD 84406 in front of a crowd of other stars in the background. It’s official: the JWST’s main infrared camera is ready!

The last straight line before the start of operations

Enough to draw a big sigh of relief to all the teams involved. “All the sleepless nights, all the worries we’ve had, they’re behind us now”, explains Thomas Zurbuchen, scientific administrator at NASA. But they are not at the end of their sentences for all that.

Indeed, the JWST is much more than a simple ultra-precise infrared camera. The machine carries several other instruments; we can notably cite the which will make it possible to further refine the observations, and these will also have to be calibrated with extreme precision. However, the process will be much less laborious than the alignment of the main mirror, the slightest defect of which could have compromised this mission.

This process should still take a few weeks, the time to configure the various spectrographs and remaining infrared cameras. Once the set has been validated, which should happen at the beginning of May, all that remains is to finish the global calibration to ensure that all these instruments provide consistent results.

The telescope will then be officially commissioned, and should report its first real scientific contributions at the beginning of the summer. A deadline which, without a doubt, is already making astronomers and enthusiasts around the world salivate.

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