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JWST’s First Star Sparkles 18 Times In New Image Array


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

JWST array

Images of the star HD 84406 from each of the JWST's 18 mirror components positioned to match the layout of the mirrors themselves. Image Credit: NASA/STScI/J. DePasquale

JWST has taken another step towards producing scientifically useful images in the form of a hexagonal array of images of a single star. The array represents each image taken by JWST's 18 mirrors, which are neither fully focused nor yet aligned so their reflected light combines at a single point.

Launching a single mirror 6.5 meters (21 feet) wide into space is beyond humanity's current capabilities – even making it would be a stretch. Consequently, JWST is composed of 18 mirrors in the now-familiar hexagonal shape, which unfolded like a flower a fortnight after launch.


Eventually, these mirrors will all be focused together so the telescope's instruments absorb their combined light. To do so, however, they need to be positioned with an error of less than 50 nanometers, or about a tenth of a wavelength of visible light. Getting to that is a slow process, one that will take far longer than deploying the sunshield and unfolding the mirrors in the first place.

Prior to combining the individual mirrors' focus, JWST's operators collected the first image of a star from each of them. The star chosen was HD 84406. It may not be bright enough to have been given a more catchy name, but there's nothing close enough to it in the sky of similar brightness to mess with the image further. There was also time for a quick selfie of the mirror.

Initially, the spots representing the star were strewn across the camera field, not randomly, but certainly messily, covering about a half degree of the sky, equivalent to a full moon. 

Not a star cluster, but 18 images captured of the same star with JWST's 18 mirror segments before they had been aligned. Image Credit: NASA

Identifying which image came from which mirror was a small but necessary step.

Although the images might look random to the untrained eye, those created by the two wings were clustered together on either side. Image Credit: NASA

From there, the next step was to create a neat array, a process known as Segment Image Identification. This is being done before any of the mirrors have been brought fully into focus, which is why none of these images are sharp. Although, as you can see, some are considerably closer to the goal than others.

“We steer the segment dots into this array so that they have the same relative locations as the physical mirrors,” Matthew Lallo of the Space Telescope Science Institute said in a statement. “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 team are now undertaking the next stage, known as Segment Alignment, which should lead to 18 sharp dots in the same relative locations. Once they are satisfied each mirror is perfectly focussed the telescope's operators will move to “Image Stacking”, where the light will be combined. There are still four more phases after that to make the combination work

The entire process is expected to take three months. However, while JWST was delayed repeatedly on the ground, in space its operations have often beaten predictions


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