NIRCam helps align and focus the James Webb Space Telescope’s main mirror segments.
NIRCam, the Near Infrared Camera, is more than Webb’s primary imaging camera. It also serves as the observatory’s wavefront sensor. During commissioning, engineers used NIRCam images to determine how the telescope’s 18 hexagonal primary-mirror segments were positioned and then calculated the adjustments needed to make them act as one mirror.
The alignment process began with observations of the star HD 84406. Because the mirror segments initially pointed in slightly different directions, NIRCam recorded separate images. Engineers identified which image belonged to each segment, corrected focus errors, stacked the images and repeatedly refined the mirror positions. The final alignment had to reach extraordinary precision—within roughly 50 nanometers.
NIRISS, NIRSpec and MIRI are also important Webb instruments, but they have different primary roles. NIRSpec performs near-infrared spectroscopy, NIRISS performs imaging and spectroscopy, and MIRI observes at mid-infrared wavelengths. NIRCam’s special wavefront-sensing capability makes it central to mirror alignment as well as everyday imaging.