How NASA's James Webb Space Telescope Maintains Unmatched Optical Performance.

 

The left panel shows a NIRCam in-focus image at 2.12 microns, while the middle and right panels display defocused NIRCam images at two different positions, used to assess the telescope's alignment.

NASA’s James Webb Space Telescope (JWST) is the largest and most powerful space telescope ever launched, featuring a mirror made up of 18 individual segments. These segments have been meticulously aligned to function as a single, giant 21.6-foot (6.5-meter) reflector. Achieving this precise alignment was a complex process that required the expertise of engineers and optics scientists. Dr. Marcio B. Meléndez, principal astronomical optics scientist at the Space Telescope Science Institute, elaborates on the challenges of aligning the telescope after launch and maintaining its precision during ongoing scientific operations.

“Following Webb’s successful launch and deployment, the delicate task of aligning its massive, gold-coated mirrors began. It took nearly three months to transform the initially unfocused, individual mirror segments into a fully aligned system, with the optical design serving as the only limiting factor.

“While the alignment process was completed in early 2022, the mirror does not stay perfectly aligned on its own due to factors such as temperature fluctuations and tilt events. Therefore, a continuous maintenance program is essential. The wavefront sensing team, tasked with monitoring and maintaining the mirrors, operates from Webb’s Mission Operations Center at the Space Telescope Science Institute in Baltimore. They track, analyze, and occasionally adjust the primary mirror segments during Webb’s science operations.

This ongoing monitoring involves a series of observations using the Near Infrared Camera (NIRCam) onboard Webb, which employs special optical equipment to intentionally defocus star images. These defocused images reveal measurable features that allow the team to assess the alignment through a technique called phase retrieval, which calculates the ‘wavefront error.’ These monitoring observations occur every other day, integrated with Webb’s scientific observations, and last around 20 minutes. All data from these telescope monitoring sessions are publicly available through the MAST archive, enabling other researchers and astronomers to model and examine the optical performance using specialized tools.

The maintenance program also includes taking a "selfie" with a specialized "pupil imaging" lens, which captures images of the mirror segments rather than the sky, four times a year. These pupil images help assess the health of the primary mirrors. During each observation, the team measures Webb's pointing stability, or "jitter," which has exceeded design expectations by being six times more stable than required. The Fine Guidance Sensor uses a small steerable mirror to lock onto a target with extraordinary precision, maintaining stability within the thickness of a human hair from seven miles (11 kilometers) away.

The telescope's overall optical performance surpasses the original design requirements, making it more sensitive to faint objects and capable of detecting finer details than anticipated. Webb's optical design aimed for a wavefront error of 150 nanometers, accounting for both uncorrectable surface imperfections and correctable misalignments. Currently, the uncorrectable errors are very low, at just about 65 nanometers. To maintain this level of performance, the alignment program monitors and, when necessary, realigns the mirror segments if misalignments exceed predefined thresholds.

NASA's James Webb Space Telescope's wavefront error fluctuates due to small, correctable mirror misalignments (green arrows), with lower wavefront error values indicating better imaging. Larger misalignments result from "tilt events" in one or more segments. After corrections, such as the one on Oct. 3—following a record 186 days since the last update—the telescope is realigned to its optimal performance.

Each segment of the James Webb Space Telescope’s primary mirror can be repositioned in six "degrees of freedom," allowing for six types of movement. The mirror's curvature can also be adjusted to fine-tune its focal length. The mirrors maintain passive alignment through stable support from the backplane structure. As Webb points to different parts of the sky, the heat absorbed from the Sun causes tiny (0.1 kelvin) temperature fluctuations in the support structure, leading to small physical movements that result in mirror misalignments. These displacements are minuscule, typically only a few nanometers of change in the wavefront. In addition, "tilt events"—sudden offsets in the structure—can occur. These abrupt shifts do not self-correct, and are thought to be linked to the release of stored energy within the mirror's support structure.

Mirror control updates are required to occur less frequently than every two weeks. When misalignments are detected, the telescope team corrects them within 48 hours through a well-coordinated procedure across various flight systems. The corrective actions involve a set of mirror movements that are converted into commands, uploaded, and executed. After the corrective moves, new observations are taken to verify the alignment. Since science operations began, more than 25 corrective actions have been implemented. On October 3, a mirror correction was performed after a record 186 days since the last update.

Figure 3: A time-lapse showing NASA's James Webb Space Telescope's NIRCam in-focus image (left) alongside the map of mirror segment offsets (right) from all maintenance observations since July 12, 2022. Despite small movements in individual segments, as seen in the right panel, the in-focus image (left) shows minimal changes.

Dr. Marcio B. Meléndez's work in ensuring the James Webb Space Telescope’s (JWST) optical performance through its rigorous wavefront maintenance program is key to unlocking the universe’s deepest secrets. The precision in Webb's mirror alignment, which has required fewer corrections than expected, is a significant achievement. This not only maximizes the telescope’s observation time but also offers valuable insights for future space missions.

Webb's remarkable stability is particularly important for missions like NASA's upcoming Habitable Worlds Observatory (HWO), which will search for signs of life on Earth-like planets around Sun-like stars. The fact that Webb requires fewer adjustments than anticipated shows that the telescope is more stable than expected, which will be essential for missions that demand long-term precision in their observations. For instance, HWO will need to monitor distant exoplanets over extended periods, and its success depends on the ability to maintain optical precision over time.

It's worth noting that the careful and controlled adjustments made to Webb's mirrors are part of an ongoing process. While the telescope’s mirror segments are aligned with great precision, factors like temperature changes or small structural shifts can cause slight misalignments, which are corrected through a well-established procedure. This continual fine-tuning ensures that the telescope stays within the tight performance specifications needed for its sensitive observations.

Would you like to dive deeper into how Webb's optical system works, or explore the technical challenges that allow it to maintain such high precision over time? Understanding how these mechanisms relate to future missions could give you a better grasp of both the capabilities of Webb and the challenges of designing next-generation telescopes.