Today NASA shared a picture indicating that the image alignment phase of the commissioning of the James Webb Space Telescope was successfully completed. Webb’s primary mirror is made up of 18 individual segments, and with today’s update, all of those segments are aligned so that a single star appears as a single object. A few more focusing steps are necessary, but the way to get the telescope up and running is getting shorter and shorter.
Immediately after launch, the focus was on unfolding all parts of the telescope that needed to be kept in a compact configuration to fit in the launch vehicle. This process included realigning and lengthening the primary mirror, lowering the secondary mirror into place, and stretching out the multi-layer sunscreen that helps keep the imaging hardware cool.
To the surprise and delight of many people, everything went incredibly smoothly. Since then, the focus has shifted to… well, focus. The Webb’s main mirror consists of 18 separate mirrors in a hexagonal array, each of which can be controlled separately. When the mirror was first unfolded, these initially created 18 individual streaks scattered across the secondary mirror.
However, changes will be made to the mirrors earlier this month creates a hexagonal array of smears that replicated the arrangement of the primary mirror segments. In today’s announcement, the segments were shifted so that each of the swabs was partially focused and moved to the center of the secondary mirror. The result? The star imaged for this process is now a single point in the center of the telescope’s field of view.
However, NASA is not done yet. Even though all the images are in the same place, they are simply overlaid there. The ultimate goal is to have the segments behave like a single mirror, which requires more careful focusing. To do this, the engineers will map the spectra of the light and look for slight shifts in the image locations at different wavelengths. From this it can be deduced in which direction the mirrors must be shifted in order to fine-tune the mirror segments.
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