James Webb begins careful, slow process of aligning mirrors

With the exciting process of deployment complete, the James Webb Space Telescope team is now embarking on their next challenge: Aligning the telescope’s mirror segments. This slow, months-long process is required to fine-tune the individual optics into one large, accurate telescope.

The telescope’s primary mirror consists of 18 gold-colored hexagons made of beryllium, which fit together to create a huge mirror 6.5 meters across. It has a secondary mirror as well, which is a smaller round shape and is located at the end of the boom arms. These all require careful tweaking to be in exactly the right position to allow the telescope to be as accurate as possible.

To achieve that, the engineers began by sending commands to the 126 actuators which will move the primary mirror segments as well as six devices that position the secondary mirror to ensure that they were working. With that confirmed, they could begin moving the segments off of the snubbers that they were sitting on during launch to absorb vibrations in a process that will take around 10 days.

The adjustment of the mirrors will take around three months in total, and will require many small, careful tweaks. “Getting there is going to take some patience: The computer-controlled mirror actuators are designed for extremely small motions measured in nanometers,” wrote Marshall Perrin from the Space Telescope Science Institute in a blog post. “Each of the mirrors can be moved with incredibly fine precision, with adjustments as small as 10 nanometers (or about 1/10,000th of the width of a human hair). Now we’re using those same actuators instead to move over a centimeter. So these initial deployments are by far the largest moves Webb’s mirror actuators will ever make in space.”

In addition, each actuator needs to work one at a time for safety reasons, and it can only work for a short period to limit how much heat it creates and spreads to the very cold mirrors. So this will be a long, slow process to get the mirrors tuned.

“This may not be the most exciting period of Webb’s commissioning, but that’s OK,” Perrin wrote. We can take the time. During the days that we’re slowly deploying the mirrors, those mirrors are also continuing to slowly cool off as they radiate heat away into the cold of space. The instruments are cooling, too, in a gradual and carefully controlled manner, and Webb is also continuing to gently coast outwards toward L2. Slow and steady does it, for all these gradual processes that get us every day a little bit closer to our ultimate goal of mirror alignment.”

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