A specialized observing mode aboard NASA’s James Webb Space Telescope has overcome a major technical setback. After years of work to address unexpected detector problems, astronomers have demonstrated that the technique can deliver high-resolution images of compact cosmic targets.
The James Webb Space Telescope (JWST) has transformed astronomy since beginning scientific operations in 2022, largely thanks to its ability todetect faint objects across the universe. Its 6.5-meter mirror provides extraordinary sensitivity, allowing researchers to study galaxies that formed not long after the Big Bang.
Yet astronomers have also been exploring ways to improve another aspect of the observatory’s performance: resolution. While Webb can detect extremely faint sources, distinguishing objects that appear very close together remains a challenge in some observations. To tackle that problem, scientists have turned to a little-known instrument mode that intentionally blocks much of the telescope’s light.
A Technique That Sacrifices Light To Gain Clarity
The method relies on interferometry, a technique that combines light collected from different points to extract fine details from astronomical targets. It is widely used in radio astronomy, where networks of antennas can operate together as a single giant instrument.
For Webb, the principle is applied differently. A device known as the Aperture Masking Interferometer (AMI), installed within the telescope’s Near-Infrared Imager and Slitless Spectrograph (NIRISS), uses a metal mask perforated with seven holes. Light entering through those openings creates interference patterns that can be analyzed to reconstruct high-resolution images.
As reported by Science, the concept was introduced into the telescope’s design in 2008 after Anand Sivaramakrishnan of the Space Telescope Science Institute successfully advocated for its inclusion. The mask itself is a metal disk measuring about 5 centimeters across.
The approach does not increase the telescope’s size or extend its baseline. Instead, it reduces noise and allows astronomers to extract more detailed information from bright, compact objects that might otherwise be difficult to study.
Early Observations Exposed An Unexpected Weakness
When Webb began operations, the first results obtained with the AMI mode fell short of expectations. Researchers discovered that the technique was pushing the NIRISS infrared detectors beyond their practical limits. Charge leaked between adjacent pixels, subtly altering the interference patterns recorded during observations. Those distortions reduced image quality and limited the gains in resolution that astronomers had anticipated.
“The initial results we got using this mode were not as good as we predicted,” Sasha Hinkley of the University of Exeter told Science, where the team discussed the challenges encountered during early testing.
Correcting the issue proved far from straightforward. The detector effects were intertwined with the observational data, making conventional post-processing techniques ineffective. For scientists who had waited years to see the instrument in action, the setback was difficult.
“Nothing was working,” Anand Sivaramakrishnanrecalled when describing the initial attempts to make the system perform as intended.
A Detailed Model Unlocks The Instrument’s Potential
The breakthrough came when researchers abandoned efforts to simply remove the distortions after observations had been completed. Instead, they developed an extensive forward model that reproduces the behavior of the entire observing system.
A study published in the Publications of the Astronomical Society of Australiaexplains how the model incorporates telescope optics, detector physics, electronic readout processes and the very distortions that had been limiting performance. Scientists begin with an estimated image of an object, run it through the simulation and compare the result with real observations. The process is repeated until the simulated and observed data align closely.
“I can’t think of another way to solve the problem than to just model it, which is what we did,” said Max Charles of the University of Sydney, a member of the research team.
The technique has already produced several notable demonstrations. The researchers reconstructed images of Io, revealing volcanic hot spots across the surface of Jupiter’s moon. They also observed a binary star system that shapes the dust surrounding it and examined the central region of a distant galaxy, where a spiraling jet of glowing material emerges from a black hole. Those results suggest that the long-troubled observing mode is finally delivering on its promise.
