A first-in-space active coronagraph will suppress a star's glare by a factor of one billion, opening the way to directly photograph planets like Jupiter orbiting stars like the Sun.
When light from a faint planet reaches a telescope, it is almost always drowned out by the blinding glare of the host star. A Jupiter-mass planet reflected light beside a Sun-like star is roughly one part in a billion — like trying to see a firefly next to a stadium floodlight. Coronagraphs solve this by blocking the star's light before it reaches the detector, but doing so from space, with a real mirror that has microscopic imperfections, is a far harder problem than it sounds.
NASA's Nancy Grace Roman Space Telescope, completed in late 2025 and due to launch in late August 2026 toward the Sun–Earth L2 point, will carry the first active coronagraph ever flown in space. Unlike a passive mask that simply shades a star, the Roman Coronagraph Instrument continuously measures the tiny amount of starlight that leaks around its mask and corrects it in real time — a process called active wavefront control.
The heart of the system is a pair of deformable mirrors. Each mirror holds a 48-by-48 grid of actuators — two thousand three hundred and forty-eight microscopic pistons beneath a thin sheet of glass. By pushing or pulling each piston by just a few picometres, the mirrors reshape themselves to cancel out the wavefront errors left behind by the telescope's optics and the mask's diffraction. The effect is a carved-out patch of darkness around the star, known to engineers as the dark hole, where contrast can reach parts per billion.
How active wavefront control works: a wavefront sensor measures the residual electric field inside the dark hole, the computer calculates how each piston should move, and the deformable mirrors reshape to push the leaked light outward. The loop runs continuously, fighting vibrations, thermal shifts, and the natural wander of a spacecraft.
Behind the mirrors sit the coronagraph masks. Roman uses several designs, including an innovative "silicon grass" mask — a dense forest of microscopic silicon spikes etched into a thin plate. When a stray photon hits the tips of the grass, it bounces between neighbouring spikes until it is trapped or scattered away, rather than being reflected straight toward the detector. The mask and the two shape-shifting mirrors work together: the mask handles the bulk of the star's light, and the mirrors clean up what the mask could not catch.
Roman's coronagraph is a technology demonstrator. Its target is a true Jupiter analogue — a giant planet orbiting a Sun-like star at a distance of a few times Earth's orbit — observed in reflected visible light for the first time. Success would prove that the chain of deformable mirrors, photon-trapping masks and active wavefront control can be made to work in the harsh environment of space, paving the way for the Habitable Worlds Observatory, the next flagship mission designed to directly image Earth-like planets around other stars.
Knowledge takeaway: a coronagraph blocks a star's glare so faint planets beside it can be seen directly; Roman's active coronagraph uses two deformable mirrors with 2,304 micro-actuators each to carve a billion-to-one "dark hole" in the star's light; silicon-grass masks trap stray photons by scattering them between microscopic spikes; the mission, launching in August 2026, is a pathfinder for future telescopes that will image Earth-like worlds.