Today, NASA shared an image indicating that it had successfully completed the image alignment stage of commissioning the James Webb Space Telescope. The Webb’s primary mirror is composed of 18 individual segments, and, as of today’s update, all of those segments are aligned so that a single star shows up as a single object. While there are still several more focusing steps required, the path to commissioning the telescope keeps getting shorter.
Immediately after launch, the focus was on unfolding all the pieces of the telescope that had to be held in a compact configuration to fit inside the launch vehicle. This process included reorienting and extending the primary mirror, lowering the secondary mirror into place, and stretching out the multi-layered sunscreen that helps keep the imaging hardware cold.
To the surprise and delight of many people, that all went incredibly smoothly. Since then, the focus has shifted to… well, focus. The Webb’s primary mirror consists of 18 separate mirrors in a hexagonal array, each of which can be controlled separately. Initially, when the mirror was first unfolded, these produced 18 individual smears scattered across the secondary mirror.
Earlier this month, however, tweaks to the mirrors created a hexagonal array of smears that replicated the arrangement of the primary mirror segments. Today’s announcement saw the segments shifted so that each of the smears was partly focused and moved to the center of the secondary mirror. The result? The star that’s being imaged for this process is now a single dot at the center of the telescope’s field of view.
NASA isn’t done just yet, however. Although all the images are in the same place, they’re simply superimposed there. The ultimate goal is to have the segments behave as a single mirror, which requires more careful focusing. To do so, engineers will image the spectra of the light, looking for slight shifts of the image locations at different wavelengths. From that, it’s possible to figure out which way the mirrors must be shifted to fine-tune the mirror segments.
