From synchrotron beams to solar flares: Japan’s new X-ray optics reach orbit

🔭✨🌌 Japanese researchers have combined synchrotron radiation science with space astronomy to build the sharpest X-ray telescope ever tested on the ground — and it has already observed a solar flare from space. This is a milestone for high-energy astrophysics and a preview of what compact satellites could soon carry into orbit. Read on to find out how a 60 mm nickel mirror is rewriting the rules of X-ray optics.

Why X-rays can’t be studied from the ground

X-rays released by solar flares, exploding stars, and matter falling into black holes carry some of the most extreme physics in the universe. But Earth’s atmosphere absorbs them completely before they reach the ground. To study them, instruments must travel to space aboard balloons, sounding rockets, or satellites.

That makes every component of an X-ray telescope extremely difficult and expensive to develop and validate. Any failure in the mirror geometry — even at the nanometer scale — can ruin the resolution of the entire instrument. Until now, evaluating the true sharpness of a high-resolution X-ray mirror before launch has been an unsolved engineering problem.

The two obstacles that blocked progress

Building a high-resolution X-ray space telescope has historically run into two hard walls. The first is the mirror itself. Unlike optical mirrors, X-rays can only reflect off surfaces at extremely shallow angles. This means mirror surfaces must be shaped with nanometer-level precision. The second obstacle is integration: even a perfectly fabricated mirror can lose that precision during the mechanical stress of being mounted inside a telescope assembly.

«The mirror is like a very precise funnel for X-rays. If any part of the funnel is even slightly out of place, the X-rays miss their target and the image blurs,» explained Isuyuki Mitsuishi, senior author and project leader from the Graduate School of Science at Nagoya University.

From synchrotron beams to a space mirror

The solution came from an unlikely collaboration. SPring-8, located in Hyogo Prefecture, Japan, is one of the world’s most powerful X-ray synchrotron radiation sources. Researchers there had developed electroforming techniques that produce extremely precise curved surfaces using synchrotron-grade X-ray beams as a calibration and shaping tool.

The team used this electroforming process to fabricate a nickel mirror 60 mm in diameter and 200 mm tall — cast as a single seamless shell. There are no joints or seams that could deflect X-rays away from the focal point, and nothing can shift out of place. The result is a mirror with a geometry stable enough to survive both the mechanical assembly process and the violent vibrations of a sounding rocket launch.

A ground-based test system that thinks like a star

Before flight, the researchers needed to prove the mirror worked. But testing a space telescope properly requires simulating starlight — and starlight arrives with rays so nearly parallel that reproducing them on the ground is extremely difficult.

The team solved this by building a dedicated evaluation system at SPring-8. A very small X-ray source, just 10 micrometers across, was placed 900 meters away from the mirror. At that distance, the X-rays remained parallel enough to closely mimic the rays arriving from a real astronomical source.

The result: the mirror could distinguish an object just 3.5 mm wide from a distance of one kilometer. This performance was sharp enough to meet the requirements of space astronomy, and it was achieved before the instrument ever left the ground.

«It’s the first ground-based system capable of accurately evaluating the performance of high-resolution X-ray space telescopes at hard X-ray energies, and it is available to researchers worldwide who want to develop and test similar technology,» said Ryuto Fujii, first author and former master’s student at Nagoya University.

FOXSI-4 and the first solar flare observation

The telescope flew as one of seven X-ray instruments aboard FOXSI-4, which launched from Alaska on April 17, 2024. FOXSI — the Focusing Optics X-ray Solar Imager — is a collaborative sounding rocket experiment designed to capture X-ray images of the Sun’s corona and active regions.

During the flight, the telescope successfully observed a solar flare in progress. For the research team, this was a historic moment: the first time a domestically developed Japanese high-resolution X-ray telescope had flown as part of an international sounding rocket mission. Dr. Mitsuishi and his students were present at the launch.

The researchers also identified what limits further improvements in sharpness: tiny imperfections distributed along the length of the mirror surface. This clear diagnosis gives them a specific target for refinement in the next-generation mirror design.

The road to FOXSI-5 and CubeSat astronomy

An improved version of the telescope is already being developed for the FOXSI-5 mission, currently scheduled for 2026. But the longer-term ambition goes well beyond sounding rockets.

The research team aims to scale this mirror technology down to fit inside CubeSats — satellites roughly the size of a shoebox. High-resolution X-ray optics have never flown aboard CubeSats. If successful, this miniaturization could make X-ray space observations dramatically more accessible and open a new chapter in compact high-energy astronomy.

The findings were published in Publications of the Astronomical Society of the Pacific.

Source Ryuto Fujii et al., «Development of Electroformed X-Ray Optics Bridging Synchrotron Radiation Technology and Space Astronomy.» Publications of the Astronomical Society of the Pacific (2026). DOI: 10.1088/1538-3873/ae3b74

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. Todos los derechos reservados. Prohibida la reproducción total o parcial sin autorización expresa. Fuente original: Publications of the Astronomical Society of the Pacific, DOI: 10.1088/1538-3873/ae3b74.