Here we see how light in a conventional telescope is collected and focused at a single point. Photons of visible light from a celestial object (shown in red) enter the telescope at the left, reflect off the parabolic mirror surface and are collected at the focus. The greater the area of reflecting surface, the greater is the brightness of the image at the focus. In fact, it is this capability (light gathering power) that is the primary reason that telescopes are used, as opposed to the ability to "magnify" any given object. It is also the reason why astronomers are constantly seeking to build larger telescopes; in that way, more light can be collected from faint and distant objects.
Unfortunately, X-ray photons don't reflect as nicely as do visible photons (click on X-ray to change the photons to x-rays - shown in black). Because of the higher energy of these photons, they are absorbed at the surface of the mirror instead of reflecting so we cannot use a mirror such as this for collecting and focusing X-rays from distant sources. The Hubble Space Telescope, which uses this type of mirror, therefore cannot be used to collect and focus the X-ray emissions we'd like to study.
A special telescope design is needed to observe X-rays. How can a telescope be built which can focus the X-rays and not absorb them in the process?