The James Webb Space Telescope: Prepare for a New Way to See the Universe

The James Webb Space Telescope (JWST) is the next of NASA’s Great Observatories; in line with the Hubble Space Telescope, the Compton Gamma-ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope. JWST combines the qualities of two of its predecessors, observing in infrared light, such as Spitzer, with fine resolution, such as Hubble. Credit: NASA, SkyWorks Digital, Northrop Grumman, STScI

James Webb Space Telescope is finally ready to do science – and it’s seeing the universe more clearly than even its own engineers hoped for.

NASA is scheduled to release the first images taken by the James Webb Space Telescope on July 12, 2022. They’ll mark the beginning of the next era in astronomy as Webb – the largest space telescope ever built – begins collecting scientific data that will help answer questions about the earliest moments of the universe and allow astronomers to study exoplanets in greater detail than ever before. But it has taken nearly eight months of travel, setup, testing, and calibration to make sure this most valuable of telescopes is ready for prime time. Marcia Rieke, an astronomer at the University of Arizona and the scientist in charge of one of Webb’s four cameras, explains what she and her colleagues have been doing to get this telescope up and running.

1. What has happened since the launch of the telescope?

After the successful launch of the James Webb Space Telescope on December 25, 2021, the team began the long process of moving the telescope into its final orbital position, expanding the telescope and, while everything cooled down, installing the onboard cameras and sensors. to calibrate.

The launch was as smooth as a rocket launch can go. One of the first things my colleagues at NASA noticed was that the telescope had more fuel left on board than predicted to make future adjustments to its orbit. This allows Webb to operate much longer than the mission’s original 10-year goal.

The first task during Webb’s month-long journey to its final location in orbit was to unfold the telescope. This went off without a hitch, starting with the lightning-quick placement of the sunshade that helps cool the telescope, followed by aligning the mirrors and turning on sensors.

Once the sunshade was open, our team began monitoring the temperatures of the four cameras and spectrometers on board, waiting for them to reach the temperature low enough that we could begin testing each of the 17 different modes in which the instruments can work.


The NIRCam, seen here, will measure infrared light from extremely distant and ancient galaxies. It was the first tool to go online and helped align the 18 mirror segments. Credit: NASA/Chris Gunn

2. What did you test first?

The cameras on Webb cooled down as the engineers predicted, and the first instrument the team turned on was the Near Infrared Camera — or NIRCam. NIRCam is designed to study the faint infrared light produced by the oldest stars or galaxies in the universe. But before it could do that, NIRCam had to help align the 18 separate segments of Webb’s mirror.

Once NIRCam cooled to minus 280 F, it was cold enough to detect light bouncing off Webb’s mirror segments and produce the telescope’s first images. The NIRCam team was ecstatic when the first light image arrived. We were in business!

These images showed that the mirror segments all pointed to a relatively small area of ​​the sky, and the alignment was much better than the worst-case scenarios we had planned.

Webb’s Fine Guidance Sensor was also commissioned at that time. This sensor helps to aim the telescope stably at a target – much like image stabilization in consumer digital cameras. Using the star HD84800 as a reference point, my colleagues on the NIRCam team helped set the alignment of the mirror segments until it was near perfect, much better than the minimum needed for a successful mission.

3. Which sensors came to life next?

When the mirror alignment was completed on March 11, the Near Infrared Spectrograph – NIRSpec – and the Near Infrared Imager and Slitless Spectrograph – NIRISS – finished cooling off and joined the party.

NIRSpec is designed to measure the strength of different wavelengths of light coming from a target. This information can reveal the composition and temperature of distant stars and galaxies. NIRSpec does this by looking at its target through a slit that keeps out other light.

NIRSpec has multiple slots that allow it to look at 100 objects at once. Team members began testing the multi-target mode, ordering the slots to open and close, and confirming that the slots responded correctly to commands. Future steps will measure exactly where the slits point and check if multiple targets can be observed simultaneously.

NIRISS is a slitless spectrograph that will also refract light into its various wavelengths, but it is better at observing all objects in a field, not just those in slits. It has several modes, including two specifically designed for studying exoplanets that are especially close to their parent stars.

So far, the instrument’s checks and calibrations are running smoothly and the results show that both NIRSpec and NIRISS will provide even better data than engineers predicted before launch.

Webb MIRI and Spitzer comparison image

The MIRI camera, image on the right, allows astronomers to see through dust clouds with incredible sharpness compared to previous telescopes such as the Spitzer Space Telescope, which produced the image on the left. Credit: NASA/JPL-Caltech (left), NASA/ESA/CSA/STScI (right)

4. What was the last instrument turned on?

The last instrument to boot on Webb was the Mid-Infrared Instrument, or MIRI. MIRI is designed to take pictures of distant or newly formed galaxies and of faint, small objects such as asteroids. This sensor detects the longest wavelengths of Webb’s instruments and should be kept at minus 449 F – just 11 degrees F above that[{” attribute=””>absolute zero. If it were any warmer, the detectors would pick up only the heat from the instrument itself, not the interesting objects out in space. MIRI has its own cooling system, which needed extra time to become fully operational before the instrument could be turned on.

Radio astronomers have found hints that there are galaxies completely hidden by dust and undetectable by telescopes like Hubble that captures wavelengths of light similar to those visible to the human eye. The extremely cold temperatures allow MIRI to be incredibly sensitive to light in the mid-infrared range which can pass through dust more easily. When this sensitivity is combined with Webb’s large mirror, it allows MIRI to penetrate these dust clouds and reveal the stars and structures in such galaxies for the first time.

5. What’s next for Webb?

As of June 15, 2022, all of Webb’s instruments are on and have taken their first images. Additionally, four imaging modes, three time series modes and three spectroscopic modes have been tested and certified, leaving just three to go.

On July 12, NASA plans to release a suite of teaser observations that illustrate Webb’s capabilities. These will show the beauty of Webb imagery and also give astronomers a real taste of the quality of data they will receive.

After July 12, the James Webb Space Telescope will start working full time on its science mission. The detailed schedule for the coming year hasn’t yet been released, but astronomers across the world are eagerly waiting to get the first data back from the most powerful space telescope ever built.

Written by Marcia Rieke, Regents Professor of Astronomy, University of Arizona.

This article was first published in The Conversation.The Conversation

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