X-Ray Image of the Cosmos

"The really important thing about focusing x-rays is not just the ability to produce detailed images it's also about sensitivity," he continued. "Suppose we sent aloft a 1000 square-centimetre detector on a high-altitude balloon and pointed it toward the Crab Nebula. We might collect 50 hard x-ray photons per second from the Crab. Meanwhile, the detector would also be hit each second by, perhaps, a thousand unwanted background photons. X-rays produced when cosmic rays hit Earth's atmosphere and the infrastructure of the experiment itself. That's a poor signal to noise ratio.

"The power of x-ray mirrors is that they take all the photons from a source like the Crab and focus them on one tiny spot. Signal to noise shoots way up! In fact, the better the mirror's angular resolution, the better the signal-to-noise ratio will be. That's why good mirror performance is so important."

Grazing incidence x-ray mirrors per se are nothing new. Ones on board NASA's Chandra observatory focus soft x-rays and produce images with sub-arcsecond resolution -- better even than groundbased optical telescopes can do. "Chandra's doing a superb job on the soft x-ray sky," notes Ramsey, "but the hard x-ray sky remains mostly unexplored because, until now, we've never had focusing mirrors that worked well above 10 keV."

Why not?

Ramsey explains: "At hard x-ray energies, the incidence angle for a good reflection is so small that a single mirror offers little collecting area. The only way to gather enough photons to form a good image is to use many mirrors." In years past that was expensive and difficult.

But no longer. Ramsey and his team used a replication technique to mass produce affordable high-precision x-ray mirrors, which they can nest one inside another to increase the collecting area. "We call the program High Energy Replicated Optics; or HERO for short," says Ramsey. The replication technique works like this: the mirror-builders electroplate nickel alloy onto an aluminium mold polished in the shape of an x-ray mirror. Next, they cool the nickel-coated mould. Aluminium contracts more than nickel, so the nickel shell, in the form of a HERO mirror, slides right off. Vacuum-coating the mirror with iridium, a dense metal that reflects x-rays better than other substances, is the final step in the process.