Stars are getting sharper in the James Webb Space Telescope’s field of view.
The team recently completed the third of seven planned steps to align the 18 hexagonal segments of Webb’s mirror, marking nearly the halfway point in a complex, weeks-long process.
The second and third stages were respectively called segment alignment and image stacking, representing larger movements of the main mirror. Subsequent stages will make more minute adjustments to take an image of a distant star and gradually bring it to a single, precise point, NASA said in a statement Friday (Feb. 25).
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“We still have work to do, but we are increasingly pleased with the results we’re seeing,” Lee Feinberg, optical telescope element manager for Webb at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in the same statement. “Years of planning and testing are paying dividends, and the team could not be more excited to see what the next few weeks and months bring.”
During the segment alignment stage, Webb engineers refined an initial image of a star rendered 18 times. Engineers made minor adjustments to the main mirror and changed the alignment of Webb’s secondary mirror. These repositionings were key to “overlapping the light from all the mirrors so that they can work in unison,” Webb officials said in the update.
Then the third stage, image stacking, saw the focused dots reflected by each mirror stacked on top of one another. Photons of light from the individual segments were each rendered to the same location of a sensor on the telescope’s near-infrared mirror (NIRCam).
“The team activated sets of six mirrors at a time and commanded them to repoint their light to overlap, until all dots of starlight overlapped with each other,” Webb officials said of image stacking.
Next will come the fourth phase of mirror alignment, called coarse phasing. That phase is already underway. NIRCam will be used to receive the light spectra (or wavelengths) from 20 pairings of the mirror segments. The process, Webb officials said, will allow engineers to correct small differences in heights between mirror segments.
“This will make the single dot of starlight progressively sharper and more focused in the coming weeks,” NASA officials said, noting that the segments will gradually align to achieve an accuracy smaller than a single wavelength of light.