The James Webb Space Telescope is regarding to complete the first phase of a months-long process: aligning the observatory’s main mirror using the Near-Infrared Camera (NIRCam).
The team’s challenge was twofold: confirm that NIRCam was ready to capture light from celestial objects, and then identify starlight coming from the same star in each of the 18 primary mirror segments. The result is an image mosaic of 18 randomly arranged starlight points, created by Webb’s misaligned mirror segments reflecting light from the same star onto Webb’s secondary mirror and NIRCam’s detectors.
What appears as a simple blur of starlight will form the basis for aligning and focusing the telescope so that this summer Webb can provide unprecedented views of the universe. Over the next month, the telescope team will gradually adjust the mirror segments until all 18 images become a single star.
“The Webb team is elated with how well the first steps in imaging and aligning the telescope are going. We were very happy to see light reaching NIRCam,” said Marcia Rieke, principal investigator for the NIRCam instrument and Regents Professor at astronomy at the University of Arizona.
This image mosaic was created by pointing the telescope at HD 84406, a bright, isolated star in the constellation Ursa Major. This star was specifically chosen because it is easily identifiable and has no nearby stars of similar brightness, which helps reduce background interference. Each point within the mosaic is labeled with the primary mirror segment that captured it. These initial results closely match expectations and simulations. Image credits: NASA
During the image collection process that began on February 2, Webb was redirected to 156 different positions around the star’s predicted location and generated 1,560 images using NIRCam’s 10 detectors, equivalent to 54 gigabytes of raw data. . The entire process took nearly 25 hours, but it’s worth noting that the observatory was able to locate the target star on each of its mirror segments in the first six hours and 16 exposures. Later, these images were put together to produce a single large mosaic that captures the imprint of each primary mirror segment in one frame. The images shown here are just the center of that larger mosaic, a huge image with more than 2 billion pixels.
“This initial search covered an area the size of the full Moon — the segment points might have been that spread out in space,” said Marshall Perrin, deputy scientist at the Webb Telescope and an astronomer at the Space Telescope Science Institute. “Collecting so much data correctly on day one required that, from the beginning, all of Webb’s science operations and data processing systems here on Earth work seamlessly with the observatory in space. And we found light from all 18 segments very close to the center at the beginning of that search! This is a great starting point for mirror alignment.”
Each individual point visible in the image mosaic is the same star captured by each of the 18 segments of Webb’s primary mirror, providing a bounty of detail that engineers and optics experts will use to align the telescope. This activity determined the alignment positions of each mirror segment following deployment, which is the first essential step for the observatory to obtain a functional alignment for its science operations.
NIRCam is the observatory’s wavefront sensor and its main imager. It was intentionally selected for use in the early stages of the Webb alignment due to its wide field of view and its unique ability to operate at higher temperatures than other instruments. It’s also packed with components specifically designed to aid in this process. NIRCam will be used during almost the entire process of aligning the telescope’s mirrors. However, it is important to note that NIRCam is operating well above its ideal temperature while capturing these early engineering images, so there are visual artifacts in the mosaic. The impact of these artifacts will diminish considerably as Webb approaches the ideal cryogenic temperatures for its operation.
“Sending Webb into space was, of course, an exciting event, but for optical scientists and engineers, this is a crowning moment, where light from a star makes its way through the system to a detector,” he said. Michael McElwain, Webb’s program scientist at NASA’s Goddard Space Flight Center.
This “selfie” was made using a specialized pupil lens inside the NIRCam instrument that was designed to image segments of the primary mirror rather than images of space. This setting is not used during science operations and is used strictly for engineering and alignment purposes. In this case, the segment was pointed at a bright star, while the others have the same alignment. This image told the instrument what the initial alignment of the primary mirror is. Credits: NASA
From now on, Webb’s images will become clearer, more detailed, and more complex as the other three instruments reach the cryogenic temperature set for their operation and begin collecting data. The first science images are expected to be released this summer. Although this is a great moment confirming that the Webb telescope is a functional telescope, there is still much to be done in the coming months to prepare the observatory for full science operations using all four of its instruments.
