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How engineers are getting James Webb’s NIRSpec instrument ready for science

While engineers for the James Webb Space Telescope continue the long and delicate process of aligning its mirrors in order to get the telescope ready for science operations this summer, other teams are working on preparing the telescope’s four science instruments for operations. One of the instruments, the Near-Infrared Spectrograph (NIRSpec), recently hit a milestone when it completed initial check-outs for three of its mechanisms. Now, members of NASA and the European Space Agency (ESA) have shared more information about NIRSpec and how it is being prepared to investigate targets including some of the oldest galaxies in the universe.

There are three mechanisms crucial to the operation of NIRSpec: A Filter Wheel Assembly (FWA), a Grating Wheel Assembly (GWA), and a Refocus Mechanism Assembly (RMA). These work together to allow the operation of the spectrograph, which splits light into a color spectrum. By looking at the spectrum of light from distant objects, scientists can tell what those objects are made of, as different elements absorb light in different wavelengths.

To make sure only light within the wavelengths being investigated gets to the instrument, NIRSpec uses filters to block out unwanted wavelengths, and these filters are controlled by the FWA. To focus the light, the instrument uses the RMA. And the light is separated into a spectrum using gratings, a prism, and a mirror in the GWA.

The engineers described how they checked each of these components: “We operated the Filter Wheel Assembly first, cycling it through all eight of its positions in both forward and reverse directions… At each position, we recorded a set of reference data. This data showed us how well the wheel was moving and how accurately it settled into each position… The data showed that the wheel moved very well even in the first attempt.”

The operation of the GWA was similarly successful. “We then used a very similar procedure for the Grating Wheel Assembly, which also performed excellently the first time,” they wrote. And finally, the RMA mechanism, which will help to focus the instrument, was moved through a few hundred steps to check it could be positioned correctly. These tests went well too, with the team writing, “successful completion of this test showed us that the RMA is a well-behaved and healthy mechanism.”

Everything is looking good for NIRSpec, so now the instrument can continue being tested and calibrated ahead of its first science data collection in a few months. “In the coming months, the NIRSpec team will continue their commissioning efforts,” the team wrote. “The whole team is very much looking forward to the start of science observations this summer!”

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