NASA is developing mirrors that could double the sensitivity of X-ray telescopes

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Hubble telescope. Image by Ondřej Šponiar from Pixabay

The US National Aeronautics and Space Administration (NASA) is developing mirrors that could double the sensitivity of X-ray telescopes, the agency said on 29 July.

Imaging systems based on x-rays use mirrors to reflect x-rays off an object at incidental angles, in the same way that more traditional optics or imaging systems reflect light off objects so that they can be viewed with naked eye or photographed. They are typically made of glass, ceramic, or metal foil, coated by a reflective layer – the most commonly used materials are gold and iridium.

Recent testing has shown that super-thin, lightweight X-ray mirrors made of a material commonly used to make computer chips can meet the stringent imaging requirements of next-generation X-ray observatories.

They are fifty times lighter – a two orders-of-magnitude leap in sensitivity – than those currently fitted in NASA’s flagship Chandra X-ray observatory and the European Space Agency’s Advanced Telescope for High-Energy Astrophysics, or Athena.

The mirrors could be fitted into the conceptual Lynx X-ray Observatory which is expected to launch at some point in the 2030s – one of four potential missions that scientists vetted as worthy pursuits under the 2020 Decadal Survey for Astrophysics.

If selected and ultimately launched in the 2030s, Lynx could potentially carry tens of thousands of the mirror segments. Chandra itself offered a significant leap in capability when it launched in 1999. It can observe X-ray sources — exploded stars, clusters of galaxies, and matter around black holes —100 times fainter than those observed by previous X-ray telescopes.

The mirrors in question are being developed by Will Zhang and his team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Zhang and his team have secured a nearer-term flight opportunity than Lynx, aboard a sounding rocket mission scheduled for 2021, which would represent the new technology’s first demonstration in space.

Seven years in the making

Efforts to develop the new mirrors began seven years ago when Zhang started to experiment with mono-crystalline, a single-crystal silicon that had not previously been used to create x-ray mirrors.

His goal — given the cost of building space observatories, which only increase in price as they get larger and heavier — was to develop easily reproducible, lightweight, super-thin mirrors, without sacrificing quality.

“What we’ve done is shown from a scientific perspective and empirically that these optics can be built using an inexpensive, abundantly available material that is immune from the internal stresses that can change the shape of X-ray mirrors made of glass, the more traditional mirror-making material”, Zhang said in a statement.

According to a NASA-commissioned panel of 40 experts, Zhang’s mirrors made from the brittle, highly stable silicon are capable of producing the same image quality as the four larger – and heavier – pairs currently flying on Chandra. The panel also deemed two other technologies – full-shell mirrors and adjustable optics – as able to fulfil the requirements of the conceptual Lynx Observatory.

Not only could Zhang’s mirrors provide an image resolution comparable to the quality of an ultra-high-definition television screen, they also met his low-mass requirements. But, Zhang said, he and his team are still “far, far away from flying our optics”.

Next steps

Zhang and his team now have to figure out how to bond these fragile mirror segments inside the canister that protects the entire mirror assembly during a rocket launch and maintains their “nested alignment”.

“We have a lot to do, and not a lot of time to do it,” Zhang said. “This is now an engineering challenge.”

He added that “time is of the essence” because in two years, he and his team are expected to deliver a 288-segment mirror assembly to Randall McEntaffer, a professor at Pennsylvania State University in State College who is developing a sounding rocket mission called the Off-plane Grating Rocket Experiment (OGRE), expected to launch from the Wallops Flight Facility in 2021.

In addition to the mirrors, OGRE will carry a “university-developed spectrograph equipped with next-generation X-ray diffraction gratings used to split X-ray light into its component colours or wavelengths to reveal an object’s temperature, chemical makeup, and other physical properties”.

Zhang expects that OGGRE will “do much to advance the mirror assembly” and that the mission will help to determine if its design will be able to protect the delicate mirrors from the extreme launch forces during lift-off and ascent through the Earth’s atmosphere.

Even if Lynx isn’t chosen for development by the 2020 Decadal Survey, Zhang envisions a bright future for the team’s optics. Other proposed missions could benefit, he said, including a couple X-ray observatories now being investigated as potential astrophysics Probe-class missions and another now being considered by the Japanese.

“Five years ago, people said it couldn’t be done, but we proved our ideas,” Zhang said. “My team is grateful to Goddard’s Internal Research and Development program for giving us the seed money. We couldn’t have achieved this without it.

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