On-sky demonstration of dual-field interferometry at the CHARA Array

To see the finest details of the universe, astronomers often need to link telescopes together, creating a virtual instrument as wide as the distance between them. This technique, known as interferometry, allows scientists to resolve tiny structures on distant stars that would otherwise appear as mere points of light. However, the Earth's atmosphere is a turbulent blanket that constantly distorts incoming starlight, causing the delicate interference patterns needed for these measurements to blur and vanish in a fraction of a second. Traditionally, this meant astronomers could only observe bright stars for very short bursts, limiting their ability to study fainter, more elusive objects like distant companions or exoplanets. The challenge has always been how to keep the telescope's "vision" steady long enough to gather enough light from these dim targets without the atmospheric noise washing out the signal.

A team of researchers at the CHARA Array, a collection of six telescopes perched on Mount Wilson in California, has now successfully demonstrated a new way to overcome this barrier. By using a bright nearby star as a real-time guide, they have shown that it is possible to track the atmospheric distortions on one star while simultaneously taking a long, steady look at a much fainter companion star. This "dual-field" approach effectively decouples the need for a bright target from the need for a long observation, opening the door to studying systems that were previously too dim to see in such detail.

The team put this new capability to the test in the summer of 2025 using a triple-star system known as alpha Piscium. This system is a hierarchical arrangement, meaning it contains a wide pair of stars, one of which is itself a tight binary pair. The researchers used two different instruments connected to the same six telescopes. One instrument, MIRC-X, locked onto the brighter primary star to act as a reference, measuring the atmospheric wobbles thousands of times per second. These measurements were used to correct the path of the light in real time. Meanwhile, a second instrument, MYSTIC, used those same corrections to observe the fainter companion star in the infrared part of the spectrum. Because the reference star kept the system stable, the second instrument could integrate the light from the faint companion for much longer than usual, allowing it to resolve details that would otherwise be lost.

The results of this first test were immediate and precise. The team successfully resolved the inner binary pair of the companion star, separating two stars that are only seven thousandths of an arcsecond apart. To put that distance in perspective, it is roughly the size of a coin seen from hundreds of miles away. Furthermore, by comparing the positions of the two main stars in the system, the team measured their separation with an uncertainty of just 234 microarcseconds. This level of precision confirms that the entire chain of operations—from capturing the light from two stars at once to stabilizing the fringes and recording the data—works as intended.

While the initial success used two stars of comparable brightness, the true goal of this technology is to observe objects that are significantly dimmer than their reference stars. The researchers are now working to push the limits of this sensitivity. They have developed software that can track the phase of the light in different colors of infrared light, ensuring the system remains stable even as the atmosphere shifts. Early tests show that the system can maintain a steady lock with very small errors, a crucial step for observing faint targets. However, the current setup is still limited by the brightness of the stars it can acquire and the thermal noise generated by the instruments themselves during long exposures.

To reach even fainter targets, the team is planning several hardware upgrades. They intend to replace the camera used to find and track the stars with a more sensitive model that can detect much dimmer objects, potentially extending the range of observable stars from the current limit to much fainter magnitudes. They also plan to cool the detectors and remove warm windows in the instrument to reduce the background heat that currently drowns out faint signals. Additionally, they aim to improve the adaptive optics systems on the telescopes, which correct for atmospheric blurring, to ensure that more light from the faint companion enters the instrument.

Computer simulations suggest that with these upgrades, the system could eventually observe stars as faint as magnitude 13 in the K-band, a significant leap from the current limit of around magnitude 8 or 9. This would allow astronomers to search for faint companions, such as brown dwarfs or exoplanets, orbiting stars that are too dim to be studied with current methods. The team has already begun gathering data to test these sensitivity limits, and while the full analysis is ongoing, the path forward is clear. By turning a bright star into a lighthouse that guides the view of a fainter neighbor, the CHARA Array is transforming how we can see the hidden, dim components of our cosmic neighborhood.