NASA’s James Webb Space Telescope is nearing completion of the first phase of the months-long process of aligning the observatory’s primary mirror using the University of Arizona-designed and led Near Infrared Camera, or NIRCam, instrument.
The Webb team’s challenge was twofold: Confirm that NIRCam was ready to collect light from celestial objects, and then identify starlight from the same star in each of the 18 primary mirror segments.
The result is an image mosaic of 18 randomly organized dots of starlight – the product of Webb’s unaligned mirror segments all reflecting light from the same star back at Webb’s secondary mirror and into NIRCam’s detectors.
What looks like a simple image of blurry starlight now becomes the foundation to align and focus the telescope so that Webb can deliver unprecedented views of the universe this summer. Over the next month or so, the Webb team will gradually adjust the mirror segments until the 18 images become a single star.
“The entire Webb team is ecstatic at how well the first steps of taking images and aligning the telescope are proceeding. We were so happy to see that light makes its way into NIRCam,” said Marcia Rieke, principal investigator for the NIRCam instrument and a UArizona Regents Professor of Astronomy.
Launched on Dec. 25, Webb is NASA’s top science priority, and UArizona astronomers played key roles in designing and developing the telescope’s infrared eyes, which will allow Webb to peer deeper into the cosmos than ever before and collect light from the earliest stars, galaxies, nebulous stellar nurseries, planetary atmospheres and more.
The engineering images
The mosaic image, known as the segment identification mosaic, was created by pointing the telescope at a bright isolated star in the constellation Ursa Major, known as HD 84406. This star was chosen specifically because it is easily identifiable and not crowded by other stars of similar brightness, which helps to reduce background confusion. Each dot within the mosaic is labeled by the corresponding primary mirror segment that captured it. These initial results closely match expectations and simulations.
NIRCam also captured a “selfie” of Webb’s primary mirrors. The image, which shows the 18 mirror segments as they would look from the secondary mirror was created using a specialized pupil imaging lens inside NIRCam. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In the image, one of the mirror segments glows more brightly than the rest because it was pointed at a bright star, while the 17 others weren’t in the same alignment. The image gave an early indication of the primary mirror alignment to the instrument.
While these photos, known as “engineering images,” mark a big moment and confirm that Webb is a functional telescope, there is much to be done in the coming months to prepare the observatory for full scientific operations using all four of its instruments.
The first “pretty” images, taken for scientific purposes, are expected in the summer.
Capturing starlight
During the image capturing process, which began Feb. 2, Webb was repointed to 156 different positions around the predicted location of the target star and generated 1,560 images using NIRCam’s 10 detectors, amounting to 54 gigabytes of raw data. The entire process lasted nearly 25 hours, but the observatory was able to locate the target star in each of its mirror segments within the first six hours and 16 exposures. These images were then stitched together to produce a single large mosaic that captures the signature of each primary mirror segment in one frame. The mosaic image released by NASA today shows only a center portion of that larger mosaic, a huge image with over 2 billion pixels.
“This initial search covered an area about the size of the full moon, because the segment dots could potentially have been that spread out on the sky,” said Marshall Perrin, deputy telescope scientist for Webb and astronomer at the Space Telescope Science Institute. “Taking so much data right on the first day required all of Webb’s science operations and data processing systems here on Earth working smoothly with the observatory in space right from the start. And we found light from all 18 segments very near the center early in that search. This is a great starting point for mirror alignment.”
Each unique dot visible in the image mosaic is the same star as imaged by each of Webb’s 18 primary mirror segments and is a treasure trove of detail that optics experts and engineers will use to align the entire telescope.
The UArizona-developed NIRCam was designed to sense and correct any errors in the telescope’s optics and serve as the telescope’s primary science imager. It was intentionally selected to be used for Webb’s initial alignment steps because it has a wide field of view and the unique capability to safely operate at higher temperatures than the other instruments. It is also packed with customized components that were designed to specifically aid in the process. NIRCam will be used throughout nearly the entire alignment of the telescope’s mirrors.
Because NIRCam is operating far above its ideal temperature while capturing the initial engineering images, visual artifacts can be seen in the mosaic. The impact of these artifacts will lessen significantly as Webb draws closer to its ideal operating temperature.
“Launching Webb to space wa
s of course an exciting event, but for scientists and optical engineers this is a pinnacle moment when light from a star is successfully making its way through the system down onto a detector,” said Michael McElwain, Webb observatory project scientist at NASA’s Goddard Space Flight Center.
Moving forward, Webb’s images will become clearer and more detailed and as its other three instruments arrive at their intended operating temperatures and begin capturing data.
