The star used for JWST’s first-light alignment was HD 84406. This relatively faint, seventh-magnitude star lies in the constellation Ursa Major, about 260 light-years from Earth.
After launch, Webb had to align its 18 hexagonal primary-mirror segments so they would function as one precise telescope. Engineers selected HD 84406 because it was bright enough for the Near Infrared Camera, or NIRCam, but isolated enough to identify clearly. Its position also kept it available throughout the early commissioning period.
In February 2022, NIRCam detected the star as 18 separate, blurry points—one image from each misaligned mirror segment. Teams matched those images to the corresponding segments, corrected their positions and focus, and gradually stacked the images into one. This painstaking process brought the mirror into alignment to an accuracy of roughly 50 nanometers.
HD 84406 was not JWST’s first major science target or first released deep-space image. It was a calibration and alignment target. Sirius, Polaris and Betelgeuse are prominent stars, but they were not the star chosen for this initial mirror-alignment procedure.