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JWST's Secondary Mirror Is Now Deployed. We Have A Telescope


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

webb close-up

The JWST's secondary mirror (left) is now in position. The primary mirror, right, is not fully unfurled, but could operate at reduced capacity in its current form if anything went wrong. Image Credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Now it is escaping the baking heat of the Sun, the JWST has successfully deployed its secondary mirror, marking the last stage of deployment that needed to go perfectly. If future stages fail the JWST will not live up to its full potential, but will still far exceed Hubble as the most powerful space telescope ever built.

Reflecting telescopes have come a long way since Newton invented them, but on the most basic level they remain the same. A large curved mirror focuses light onto a smaller one, which reflects it to a point where it can be collected – an eyepiece for an amateur's backyard 'scope, or cameras and spectroscopes in modern professional devices. The JWST adds a tertiary mirror which will remove astigmatism created by the secondary.


The extent to which images are magnified depends, in part, on the distance between the two mirrors. Since The JWST had to be packed up tightly to fit on the Ariane 5 rocket that took it into space, the mirrors needed to be launched too close together to operate and unfold in space. Indeed, in a further effort at compaction for transport, the JWST's main mirror has two wings, each made up of three of its hexagonal components, that will be deployed separately.

According to NASA, it is supported by three struts, each almost 8 meters (25 feet) long. Like an old dog on a winter's day, the struts required heating systems to allow their joints and motors to move, now the sun shields have started the process of cooling the telescope down.


“The world’s most sophisticated tripod has deployed,” Lee Feinberg, the JWST's optical telescope element manager, said. “That’s really the way one can think of it. Webb’s secondary mirror had to deploy in microgravity, and in extremely cold temperatures, and it ultimately had to work the first time without error. It also had to deploy, position, and lock itself into place to a tolerance of about one and a half millimeters, and then it has to stay extremely stable while the telescope points to different places in the sky – and that’s all for a secondary mirror support structure that is over 7 meters in length.”

The primary's wings are still to unfurl in the next few days, starting with the Port Wing, followed by the Aft. However, if these were to fail the fixed part of the main mirror would still operate, albeit with reduced light-gathering capacity.


Bill Ochs, the JWST's Project Manager called it “Another banner day for the JWST. This is unbelievable…We’re about 600,000 miles from Earth, and we actually have a telescope.”


As with each passing milestone, scientists and astronomers are celebrating.

As the tension eases, some even feel able to joke. 



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