A team led by NASA's Maxim Markevitch is investigating the possibility of building bigger X-ray telescope mirrors – up to thirty times as large as today's – using a plastic tape coated with a reflective material and then, just like a roll of Scotch tape, tightly rolled on itself. By studying cosmic rays and distant galaxy clusters, such large and significantly cheaper mirrors would allow us to learn more about the birth and evolution of the universe.
Images taken in the visible spectrum may be beautiful to look at, but they are often unclear, surrounded by a layer of brilliant cosmic dust. Infrared telescopy can reveal to us a different set of data, stripping away part of the haze and highlighting the heat signature of celestial bodies. But, when it comes to removing clutter to take clear pictures of very remote stars, nothing beats the imaging capabilities of X-ray telescopes.
The Orion nebula is imaged, from left to right, using visible light, infrared, and X-ray (Images: Hubble; ESO; Chandra X-ray telescope)
The X-ray observatories launched by NASA over the past few years have detected record-breaking winds and faint, diffused signals coming to us from distant galaxy clusters. However, the mirrors they employ are extremely laborious and expensive to manufacture, which puts severe constraints on their size. If we want to study the edges of the observable universe, we'll need to find a way to manufacture much larger and inexpensive X-ray mirrors.
Markevitch and his colleagues are testing a promising approach to the problem, which was inspired by a simple roll of Scotch tape. Their technique involves coating a plastic tape of varying thickness with multiple layers of a highly reflective material, and then tightly winding it into a roll.
In standard specialized X-ray mirrors, individual mirror segments must be nested inside a cylindrical optical assembly, curved, coated with highly reflective materials, and then perfectly aligned. Fabricating these rigid shells and making sure they are perfectly aligned for optimal performance is a long, laborious and expensive task.
The Chandra X-ray Observatory is performing remarkable observations, but its relatively small mirror impairs its performance (Image: NASA)
In Markevitch's concept, everything becomes much simpler: the spacing between the shells is automatically dictated by the varying thickness of the tape, as is their alignment. The collecting surface is automatic and self-supporting. Most importantly, to create a larger mirror one would simply increase the reflective area by combining multiple rolls of tape.
Today's relatively small X-ray mirrors allow us to study the low-energy "soft X-rays," but to learn more about cosmic rays – highly energetic subatomic particles – we would need to detect the higher-energy "hard X-rays." That, Markevitch says, requires mirrors at least thirty times as large. His bet is that his concept will deliver cheap, effective mirrors of just about that size.
Some X-ray telescopes, like NASA's recently launched NuSTAR, are sensitive to hard X-rays but lack in resolution (Image: NASA)
The researchers say they will conclude their analysis by next year. If they find that building such mirrors is possible, their study of cosmic rays may allow us to understand more on the origin and evolution of our universe.