Chandra X-ray Observatory optics.Launched 1999.
Space-based x-ray telescopes allow astronomers to study a wide range of energetic phenomena from neutron stars to diffuse million-degree gas permeating galaxy clusters. Unfortunately, x-rays are notoriously difficult to focus. To produce images, x-ray telescopes must use grazing incidence mirrors, which reflect incoming photons at very shallow angles. High-resolution x-ray mirrors usually use the Wolter Type I geometry—a double reflection off first a parabolic and then a hyperbolic surface (see Figure 1). As a consequence of the shallow angles (typically 1° or less), the collecting area is a small fraction of the mirror surface. To increase the telescope's effective area, multiple mirror pairs are nested. Thin mirrors can be nested in higher number and more densely than thicker mirrors, resulting in a large total collecting area, but they are more subject to manufacturing and mounting errors that degrade the angular resolution.Here we have the reflection conundrum again. X-rays go through most anything, or are stopped (absorbed), which is how we get medical X-rays. But if we can catch them at a low enough angle they can be reflected, and if they can be reflected, they can be focused, so we ought to be able to get some kind of image.
The trade-off between imaging quality and light collecting power is apparent in the most advanced current facilities. The Chandra X-ray Observatory1 employs four thick (16–23mm) mirror shells with an unparalleled angular resolution of 0.5".
All electromagnetic radiation (light, radio waves, X-rays) are composed of photons, which aren't really particles, and aren't really waves. X-ray photons are more energetic than visible light photons, which might explain the low angle of incidence needed for reflecting them.