James Webb’s hexagonal image array shows its mirror’s shape

By Georgina Torbet February 19, 2022

We won’t be seeing any stunning images of space from the James Webb Space Telescope just yet, as the telescope is still undergoing the months-long process of aligning its mirrors. However, there are still exciting updates to follow, and the latest step is a new image of the star HD 84406 shown 18 times in a hexagonal image array.

Webb’s first image was released last week, and in that image, you can see the 18 points of light which represent the same star imaged 18 times, due to the 18 hexagonal segments which make up the telescope’s primary mirror. As the segments are still being aligned, in that image they are dispersed across the image.

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Dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, called an “image array.” — This early Webb alignment image, with dots of starlight arranged in a pattern similar to the honeycomb shape of the primary mirror, is called an “image array.” NASA/STScI/J. DePasquale

Now, in this new image, you can see the 18 points of light arranged into a hexagonal shape which represents the hexagonal shape of the mirror. This is due to the completion of the first stage of the mirror alignment, called the “Segment Image Identification.” This allows the arrangement of the points of light into an image array.

“We steer the segment dots into this array so that they have the same relative locations as the physical mirrors,” explained Matthew Lallo, systems scientist and Telescopes Branch manager at the Space Telescope Science Institute, in a statement. “During global alignment and Image Stacking, this familiar arrangement gives the wavefront team an intuitive and natural way of visualizing changes in the segment spots in the context of the entire primary mirror. We can now actually watch the primary mirror slowly form into its precise, intended shape!”

The next stage of the mirror alignment process is called Segment Alignment. In this phase, each segment of mirror will have its position adjusted to allow for larger positioning errors, and the secondary mirror — a circular mirror located on the end of a boom — will be adjusted as well. With this done, each point of light representing a star will become more focused. Then the team can move onto the step of Image Stacking, in which each of the 18 separate dots of light is brought on top of each other to create one point.

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