Sub-arcsecond X-ray telescope optics and high-resolution, high-efficiency X-ray transmission grating spectrometers
A NASA-sponsored team at the MIT Space Nanotechnology Lab is developing high-performance space instrumentation for more powerful future X-ray telescopes that will study the dynamics of the high-energy universe.
In the 50 years since the launch of Uhuru, the first X-ray astronomy satellite, our knowledge about the high-energy universe has exploded. Measurements of celestial X-ray emission and absorption have taught us about supermassive black holes, neutron stars, accretion disks, and many more phenomena in the high-temperature universe. Naturally, many new questions have arisen that cannot be answered with old technology. Today, NASA’s great X-ray observatory Chandra has lasted over two decades in space, and the Agency is considering potential future strategic mission concepts with vastly improved performance such as Lynx. This improved performance relies on cutting-edge technology, such as ultra-precise X-ray mirrors and high-performance X-ray diffraction gratings. These mirrors and gratings are tens to hundreds of millimeters in size and sub-millimeter thin, but must be fabricated, shaped, and assembled with nanometer precision.
X-rays are highly energetic electromagnetic radiation, mostly known for their ability to penetrate soft organic tissue for the imaging of bones or teeth. In our universe, X-rays are generated by highly energetic processes, usually associated with temperatures in the range of millions to tens of millions of degrees. Examples of X-ray-generating environments include the coronas of stars, accretion disks in binary systems, and active galactic nuclei. In addition, gases and plasmas between these sources and Earth can absorb some of these X-rays, so X-ray observations also teach us about the dynamic interstellar and intergalactic material. Astronomers need to image X-ray sources with high angular resolution and break down the X-ray light into its finest spectral components to understand the conditions and abundance of elements at the source and the processes that generate the X-rays. A NASA-sponsored team at the Kavli Institute’s Space Nanotechnology Lab (SNL) at the Massachusetts Institute of Technology (MIT) is working to develop advanced mirrors and gratings that will enable future X-ray telescopes.
Building an X-ray telescope is challenging, mostly due to the way X-rays interact with matter. In astronomy most of the X-rays of interest are less energetic (“softer”) than medical X-rays and are more easily absorbed by matter. In addition, the index of refraction for X-rays is very close to one for most materials, which means that lenses can barely bend X-rays. Both of these reasons make it impractical to build traditional telescope lenses for imaging or prisms for spectral analysis of celestial X-ray sources. Instead, in a method akin to skipping rocks on a pond, scientists use mirrors to reflect the X-rays at very shallow angles (below the so-called critical angle) toward a focal point (see figure below). It can take hundreds of nested shells of mirrors to densely fill the aperture of a large telescope. The mirrors must be as thin as possible to avoid wasting the precious collecting area and to keep mass low. Each mirror must have a precisely bent shape and must be precisely aligned with respect to all the other mirrors. For a future mission concept such as Lynx, the requirement for the telescope angular resolution is half an arcsecond. For best reflectivity the mirrors must be coated with very thin, nanometer-thick films, typically made of metals. However, metal films are inherently stressed and can deform the thin mirrors by several micrometers, which is beyond acceptable tolerances. In addition, the amount and distribution of stress in the film is often difficult to predict or control.