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Researchers have created a new method for fabricating the high-precision, ultrathin mirrors needed for telescopes.
Researchers have created a new method for fabricating the high-precision, ultrathin mirrors needed for high-performance x-ray telescopes using femtosecond laser pulses. The method might assist advance the capabilities of space-based x-ray telescopes that are used to observe high-energy cosmic phenomena like the formation of supermassive black holes and new stars.
“Detecting cosmic x-rays is a crucial piece of our exploration of the universe that unveils the high-energy events that permeate our universe but are not observable in other wavebands,” said research team leader Heng Zuo, who performed the research at MIT Kavli Institute for Astrophysics and Space Research and is now at the University of New Mexico. “The technologies our group developed will help telescopes obtain sharp images of astronomical x-rays that can answer many intriguing science questions.”
Thousands of thin mirrors make up the X-ray telescopes, which orbit above the Earth’s atmosphere. Each mirror in an X-ray telescope must be properly curved and positioned in relation to the others. The researchers detail how they employed femtosecond laser micromachining to bend these ultrathin mirrors into a precise shape and fix mistakes that can occur during the fabrication process in Optica, Optica Publishing Group’s publication for high-impact research.
“It is difficult to make ultra-thin mirrors with an exact shape because the fabrication process tends to severely bend the thin material,” said Zuo. “Also, telescope mirrors are usually coated to increase reflectivity, and these coatings typically deform the mirrors further. Our techniques can address both challenges.”
Precision bending
As new mission concepts continue to push the boundaries of x-ray imaging, it is necessary to develop new techniques for manufacturing ultra-precise and high-performance x-ray mirrors for telescopes. For instance, the Lynx X-ray Surveyor idea from NASA will have the most potent x-ray optic ever created and demand the production of numerous ultra-high-resolution mirrors.
In order to address this demand, Zuo’s research team combined stress-based figure rectification with femtosecond laser micromachining. Through the application of a deformable film to the mirror substrate, stress-based figure correction takes use of the thin mirrors’ capacity to bend by inducing controlled bending.
The technique involves selectively removing regions of a stressed film grown onto the back surface of a flat mirror. The researchers selected femtosecond lasers to accomplish this because the pulses produced by these lasers can create extremely precise holes, channels and marks with little collateral damage. Also, the high repetition rates of these lasers allow faster machining speeds and throughput compared to traditional methods. This could help speed up fabrication for the large numbers of ultra-thin mirrors required for next-generation x-ray telescopes.
Mapping stress
The researchers had to pinpoint precisely how laser micromachining altered the surface curvature and stress states of the mirror in order to use the novel strategy. After taking measurements of the original mirror shape, they developed a map showing the stress adjustment needed to achieve the required shape. Additionally, they created a multi-pass correction method that use a feedback loop to repeatedly cut faults until a suitable mirror profile is attained.
“Our experimental results showed that patterned removal of periodic holes leads to equibiaxial (bowl-shaped) stress states, while fine-pitched oriented removal of periodic troughs generates non-equibiaxial (potato-chip-shaped) stress components,” said Zuo. “Combining these two features with proper rotation of the trough orientation we can create a variety of stress states that can, in principle, be used to correct for any type of error in the mirrors.”
In this work, the researchers demonstrated the new technique on flat silicon wafers using regular patterns. To correct real x-ray astronomy telescope mirrors, which are curved in two directions, the researchers are developing a more complex optical setup for 3D movement of the substrates.
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