Photons received: Webb sees its first star – 18 times
Credit: NASA
The 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 Near Infrared Camera (NIRCam) instrument.
The 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 in order for Webb to deliver unprecedented views of the universe this summer. Over the next month or so, the team will gradually adjust the mirror segments until the 18 images become a single star.
This image 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 reduce background confusion. Each dot within the mosaic is labelled by the corresponding primary mirror segment that captured it. These initial results closely match expectations and simulations. (Credit: NASA)
During the image capturing process that began on , Webb was repointed to 156 different positions around the predicted location of the 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 notably 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 images shown here are only a centre portion of that larger mosaic, a huge image with over 2 billion pixels.
Each unique dot visible in the image mosaic is the same star as imaged by each of Webb's 18 primary mirror segments, a treasure trove of detail that optics experts and engineers will use to align the entire telescope. This activity determined the post-deployment alignment positions of every mirror segment, which is the critical first step in bringing the entire observatory into a functional alignment for scientific operations.
NIRCam is the observatory's wavefront sensor and a key 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. It is, however, important to note that NIRCam is operating far above its ideal temperature while capturing these initial engineering images, and visual artifacts can be seen in the mosaic. The impact of these artifacts will lessen significantly as Webb draws closer to its ideal cryogenic operating temperatures.
This "selfie" was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of space. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this case, the bright segment was pointed at a bright star, while the others aren't currently in the same alignment. This image gave an early indication of the primary mirror alignment to the instrument. (Credit: NASA)
Moving forward, Webb's images will only become clearer, more detail-laden, and more intricate as its other three instruments arrive at their intended cryogenic operating temperatures and begin capturing data. The first scientific images are expected to be delivered to the world in the summer. Though this is a big moment, confirming that Webb is a functional telescope, there is much ahead to be done in the coming months to prepare the observatory for full scientific operations using all four of its instruments.
Abridged text is reprinted courtesy of NASA