Fine phasing is the fifth of seven major alignment steps. It is conducted three times, directly after each round of coarse phasing, and measures and corrects remaining alignment areas. NASA writes, ‘These operations measure and correct the remaining alignment errors using the same defocusing method applied during Segment Alignment. However, instead of using the secondary mirror, we use special optical elements inside the science instrument which introduce varying amounts of defocus for each image (-8, -4, +4, and +8 waves of defocus).’
Following fine phasing alignment, every optical parameter of Webb’s Optical Telescope Element is performing at or above expectations. The team also found no critical issues or measurable contamination or blockages in the telescope’s optical path. While there are still months before Webb captures its first new view of the cosmos, the alignment milestone gives the team confidence that Webb’s first-of-its-kind optical system is working as expected.
‘More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals,’ said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. ‘Today we can say that design is going to deliver.’
— Dr Heidi B. Hammel (@hbhammel) March 16, 2022
The James Webb Space Telescope is the first space-based telescope to employ a segmented primary mirror design. Its primary mirror is 6.5m (21′ 4″) in diameter, which is too large to have fit inside a rocket fairing in one piece. As we’ve discussed before, the mirror is made up of 18 hexagonal, beryllium mirror segments. The mirror needed to be folded for launch and then unfolded in space. Each segment has been adjusted in space to within nanometers to form a single mirror surface.
‘In addition to enabling the incredible science that Webb will achieve, the teams that designed, built, tested, launched, and now operate this observatory have pioneered a new way to build space telescopes,’ said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Now that the team has completed the fine phasing stage of the telescope’s alignment, Webb’s primary imager, the Near-Infrared Camera (NIRCam), is fully aligned. ‘We have fully aligned and focused the telescope on a star, and the performance is beating specifications. We are excited about what this means for science,’ said Ritva Keski-Kuha, deputy optical telescope element manager for Webb at NASA Goddard. ‘We now know we have built the right telescope.’
The team remains on track to finish all aspects of Optical Telescope Element alignment by early May. Over the next six weeks, the team will proceed through the remaining alignment steps, and further align the telescope to include its other imagers, including the Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near InfraRed Imager and Slitless Spectrograph. Each instrument will need to be evaluated using specialized algorithms to inform final corrections. Before full scientific operations can commence, all instruments must be aligned and meet the team’s exacting standards. So far, so good with the James Webb Space Telescope. We should see its first full-resolution imagery and science data this summer.