This hexagonal image array captured by the NIRCam instrument shows the progress
made during the Segment Alignment phase, further aligning Webb’s 18 primary
mirror segments and secondary mirror using precise movements commanded from
the ground. During the phase of alignment known as Image Stacking, individual
segment images are moved so they fall precisely at the center of the field to
produce one unified image instead of 18. In this image, all 18 segments are on
top of each other. After future alignment steps, the image will be even sharper.
Credit: NASA/STScI
Webb continues on its path to becoming a focused observatory. The team has successfully worked through the second and third out of seven total phases of
mirror alignment. With the completion of these phases, called
Segment Alignment and
Image Stacking, the team will now begin making smaller adjustments to the positions of Webb’s mirrors.
After moving what were 18 scattered dots of starlight into Webb’s signature
hexagonal formation, the team refined each mirror segment’s image by making minor adjustments, while also changing the alignment of Webb’s secondary mirror. The completion of this process, known as
Segment Alignment, was a key step prior to overlapping the light from all the mirrors so that they can work in unison.
Once Segment Alignment was achieved, the focused dots reflected by each mirror were then stacked on top of each other, delivering photons of light from each segment to the same location on
NIRCam’s sensor. During this process, called
Image Stacking, the team activated sets of six mirrors at a time and commanded them to repoint their light to overlap, until all dots of starlight overlapped with each other. ...
Although Image Stacking put all the light from a star in one place on NIRCam’s detector, the mirror segments are still acting as 18 small telescopes rather than one big one. The segments now need to be lined up to each other with an accuracy smaller than the wavelength of the light.
The team is now starting the fourth phase of mirror alignment, known as
Coarse Phasing, where NIRCam is used to capture light spectra from 20 separate pairings of mirror segments. This helps the team identify and correct vertical displacement between the mirror segments, or small differences in their heights. This will make the single dot of starlight progressively sharper and more focused in the coming weeks.
