How a telescope’s mirror stability makes or breaks exoplanet detection

Finding life beyond our solar system is a major goal of modern astronomy. NASA’s planned Habitable Worlds Observatory (HWO) aims to take direct images of Earth-sized planets around stars other than our sun. This task, however, is extraordinarily difficult, given that these planets are roughly 10 billion times fainter than their host stars. To detect them, scientists must find ways to suppress nearly all of the nearby starlight, which would otherwise overwhelm the faint planetary signal.

Even when using a coronagraph—a device designed to block the host star’s light—a small amount of residual starlight remains in the image. Astronomers thus employ advanced image-processing methods to estimate and remove this leftover light. These methods depend on the telescope’s optics remaining stable over time. Small changes in mirror shape or alignment, measured in picometers, can alter the star’s light pattern and reduce the effectiveness of starlight subtraction. However, the level of wavefront drift that future planet-imaging missions can tolerate remains unclear.

As reported in the Journal of Astronomical Telescopes, Instruments, and Systems, Natalia Sanchez-Soria and colleagues investigated how wavefront drift affects the performance of common postprocessing techniques used for exoplanet imaging. Using simulations of a coronagraph-equipped space telescope with a segmented primary mirror, they examined how optical instabilities influence the detection of faint planets around nearby stars.

The team used computer simulations of a space telescope equipped with a coronagraph and a segmented primary mirror similar to those being considered for future missions. They evaluated three commonly used postprocessing approaches, namely reference star differential imaging (RDI), angular differential imaging (ADI), and coherent differential imaging (CDI). The simulations included both large-scale optical distortions and small misalignments between mirror segments, and the researchers introduced different levels of wavefront drift to determine how each method responded.

One of the main findings was that not all sources of instability had the same impact. Changes in large-scale optical aberrations degraded performance, but the effects were relatively manageable at lower drift rates. In contrast, small shifts between mirror segments proved far more damaging. The simulations revealed that Earth-like and Venus-like planets quickly became difficult to detect as segment misalignments increased, even when the changes were extremely small. “All the simulated image-processing techniques needed segment alignment stability below two picometers per 10 minutes to detect close-in exoplanets. This indicates that segment stability will be crucial for planet detection,” said Sanchez-Soria.

Among the techniques tested, ADI generally achieved the strongest starlight suppression under the simulated conditions. However, the study also showed that increasing wavefront drift reduced the effectiveness of every method to some extent. Larger planets located farther from their host stars, such as Jupiter-like worlds, remained detectable under a wider range of conditions, whereas Earth-like planets were much more sensitive to segment alignment instability.

Overall, the findings of this study provide valuable guidance for the design of future instruments. Rather than focusing only on improving the coronagraph itself, engineers will also have to carefully consider how mirror stability affects the image-processing techniques that ultimately help reveal distant planets. “Understanding the limitations of these techniques under wavefront drift will inform telescope stability requirements during HWO’s early architecture trade studies,” said Sanchez-Soria.

The researchers note that their simulations represent a simplified scenario and do not account for active wavefront correction or several real-world sources of optical instability. Future studies will incorporate more realistic observing conditions to refine telescope stability requirements and support the design of missions such as NASA’s HWO.

Read the Gold Open Access paper by Sanchez-Soria et al., “Wavefront drift effects on postprocessing of coronagraph images,” Journal of Astronomical Telescopes, Instruments, and Systems 12(4) 041013 doi: 10.1117/1.JATIS.12.4.041013

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