A new way to make telescope mirrors could sharpen our view of the universe

A new way to make telescope mirrors could sharpen our view of the universe

Researchers have developed a new method for using femtosecond laser pulses to fabricate the ultra-thin, high-resolution mirrors needed for high-performance X-ray telescopes. Femtosecond laser resurfacing is applied to selectively remove areas of the stressed film on the mirror substrate, correcting the shape of the reflective mirror surface. Credit: Heng Zhou, MIT Kavli Institute of Astrophysics and Space Research

Researchers have developed a new method for using femtosecond laser pulses to fabricate the ultra-thin, high-resolution mirrors required for high-performance X-ray telescopes. This technology could help improve space-based X-ray telescopes used to capture high-energy cosmic events associated with the formation of new stars and supermassive black holes.

“The discovery of cosmic X-rays is an important part of our exploration of the universe that reveals high-energy events that permeate our universe but are not observable in other wavebands,” said lead researcher Heng Zhou, who conducted the research at MIT Kavli. Institute for Astrophysics and Space Research, now at the University of New Mexico. “The technologies developed by our group will help telescopes obtain accurate astronomical X-ray images that can answer many interesting scientific questions.”

X-ray telescopes orbit above the Earth’s atmosphere and contain thousands of thin mirrors that must each have a precisely curved shape and be carefully aligned with respect to all the others. in opticsIn the study, the researchers describe how they used femtosecond laser technology to bend these ultra-thin mirrors into a precise shape and correct errors that can arise in the manufacturing process.

“It’s difficult to make ultra-thin mirrors with a precise shape because the manufacturing process tends to bend the thinner material very severely,” Zuo said. “Also, telescope mirrors are usually coated to increase reflectivity, and these coatings usually distort the mirrors further. Our technologies can address both challenges.”

precise bending

New methods for fabricating ultra-high-resolution, high-performance X-ray mirrors for telescopes are needed as new mission concepts continue to push the boundaries of X-ray imaging. For example, NASA’s Lynx X-ray Surveyor concept will contain the most powerful X-ray optical instrument ever envisioned and will require the fabrication of a large number of ultra-resolution mirrors.

To meet this need, Zuo’s research group combined femtosecond laser technology with a previously developed technology called stress-based shape correction. Stress-based shape correction exploits the bendability of thinner mirrors by applying a deformable film to a mirror substrate to adjust their stress states and induce controlled bending.

A new way to make telescope mirrors could sharpen our view of the universe

Zuo’s research group combined femtosecond laser technology with a previously developed technology called stress-based shape correction. The experimental setup is displayed. Credit: Heng Zhou, MIT Kavli Institute of Astrophysics and Space Research

The technique involves the selective removal of areas of compressed film that have grown on the back surface of a flat mirror. The researchers chose femtosecond lasers to accomplish this because the pulses produced by these lasers can create very precise holes, channels, and marks with little collateral damage.

The higher repetition rates of these lasers also allow for faster processing speeds and throughput compared to conventional methods. This could help speed up the manufacture of the large numbers of ultra-thin mirrors required for the next generation of X-ray telescopes.

stress mapping

To implement the new approach, the researchers first had to determine exactly how precisely the microwave laser alters the mirror’s surface curvature and stress states. Then they measured the initial mirror shape and created a map to correct the pressure needed to create the desired shape. They have also developed a multi-lane correction scheme that uses a feedback loop to reduce errors iteratively until an acceptable mirror profile is achieved.

“Our experimental results show that typical removal of periodic holes leads to equiaxial (pot-shaped) stress states, while careful removal of periodic sinks generates unequal (potato-chip-shaped) stress components,” Zhou said.

“By combining these two features with the appropriate rotation of the trough orientation, we can create a variety of pressure states that, in principle, can be used to correct any kind of error in the mirrors.”

In this work, the researchers demonstrated the new technique on flat silicon wafers using regular patterns. To correct for real X-ray astronomy telescope mirrors, which are bent in two directions, researchers are developing a more complex optical setup for the 3D motion of the pillars.


Development of a low-cost, high-precision manufacturing method for thin film mirrors and silicon wafers


more information:
Heng Zuo et al, Femtosecond Precision Laser Machine for Stress-Based Shape Correction for Thin Mirrors, optics (2022). DOI: 10.1364 / OPTICA.461870

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