“They can move in steps that are a fraction of a wavelength of light, or about 1/10,000th the diameter of a human hair,” explained Lee Feinberg, optical telescope element manager of the NASA James Webb Space Telescope. That sort of precision was only accomplished inside an isolated laboratory it was controlled all the way back on Earth, through well over a million kilometers of vacant space.
The technique of putting Webb’s segmented mirror into sharp focus is an unprecedented achievement in remote optical engineering. Its 6.5‑meter primary mirror consists of 18 hexagonal beryllium sections, which were folded to be launched, and were unfurled in orbit. Each segment needed to be placed in position and curve so that, together, they formed an individual flawless surface. Positioning and shape-change were accomplished by a system of seven actuators per segment-six to place and one to make subtle shape-change capable of movements on the nanometer scale, all of this in the cryogenic temperatures of approximately 40 kelvins.
To align and adjust for shape, engineers employed a specially constructed multi‑wavelength interferometer. This device divided laser light into two beams: one bounced off the mirror, the second used as a reference. Interference between them showed position and shape deviations. Fine phasing, the finest level of alignment, aligned all optical parameters to or beyond spec, with no detectable contamination or blockage present in Webb’s optical path. “We have fully aligned and focused the telescope on a star, and the performance is beating specifications,” stated Ritva Keski‑Kuha, deputy optical telescope element manager, NASA Goddard.
The calibration continued beyond the primary mirror. With the AOS Source Plate Assembly (ASPA) acting as an artificial star, engineers conducted “half‑pass” and “pass‑and‑a‑half” tests to confirm the alignment of the tertiary and the fine‑steering mirrors, and the secondary and primary. These iterative tests confirmed the whole optical train, from starlight entering the secondary mirror to photons arriving at the science instruments, was in sync. Algorithms subsequently reviewed each instrument’s performance and computed final corrections, bringing Webb into alignment across all of its cameras and spectrographs.
Such spatial precision in optics mirrors the requirements of gravitational wave observatories, where the observation of spacetime distortions necessitates precise control of light. The Laser Interferometer Gravitational-wave Observatory observes shifts thousands of times smaller than the proton width, and future detectors, such as LISA, will be tens of millions of kilometers apart in space. Webb’s wavefront sensing and phasing and control apply the same principles to phasing and maintaining large, segmented optical systems at vast distances, an ability immediately applicable to future space-based interferometers.
The interplay of these technologies extends into cosmology. Gravitational wave detectors have already verified Einstein’s century‑old prophesy, uncovering black hole mergers and neutron star collisions. Space‑based detectors will probe lower frequencies, possibly observing signals of the early universe. Theory in de Sitter space, a model of an expanding universe, predicts that gravitational wave signals have the ability to carry information of primordial fluctuations and the very birth of cosmic structure. Webb’s diffraction‑limited imaging from afar complements these efforts, providing electromagnetic context to those events observed first through the ripples of spacetime.
The challenge of phasing Webb remotely, as an engineer, also has the same DNA as future compact detectors of gravitational waves coming for the milli‑hertz band. These tabletop‑sized instruments utilize ultrastable optical cavities and atomic clocks to detect tiny phase shifts of laser light, similarly to Webb’s phasing by interferometry but tuned to pick up passing gravitational waves. They both depend on controlling the optical paths to extreme tolerances, rejecting noise of the environment, and seeking weak signals buried by fluctuations.
Here, the environment is the space vacuum, and thermal stability is achieved through its sunshield and passive cooling. For gravitational wave detectors, the most important requirement is isolation of the system from seismic and Newtonian noise. However, the same underlying principle of measuring light’s phase, or even just the intensity, to deduce distance or shape change applies. The successful alignment of Webb bears testament to the viability of the same methods in remote, extreme conditions. As Webb transitions to the science operations, its sharp eyes will scan galaxies put together only hundreds of millions of years after the Big Bang, probe the atmospheres of exoplanets, and follow the outlines gravitational wave astronomy promises but cannot see. Astrophysics and optical engineering, applied together in the case of space telescopes or gravitational wave observatories, are gradually increasing the reach of mankind into the universe’s most distant eras.
