Next week, NASA is planning on launching a new space-based telescope: the Nuclear Spectroscopic Telescope Array (or NuSTAR), which will start looking at all kinds of exciting and high energy astrophysical phenomena that are busily blowing up and/or tearing space to shreds.
NuSTAR is designed to image high energy X-rays, and it'll be the first space-based telescope to do so. The things out in the universe that are making high energy X-rays include billion solar-mass black holes in our galaxy and other galaxies, supernovae and supernova remanants, gamma ray bursts, and incredibly powerful relativistic particle jets.
X-rays are exciting things, but they're also difficul to get good pictures of with a telescope. When you're dealing with lower energy radiation (like visible light), you can use lenses and mirrors to push photons around and focus them at a detector, but X-rays are powerful enough to just smash straight into most optical surfaces and get absorbed instead of reflected.
The way to go about managing X-rays, it turns out, is with gentle suggestion instead of brute force: reflecting surfaces at very small incidence angles (i.e. nearly parallel to the incoming X-rays) nudge them off their path by just a few degrees at a time, and over a long enough distance the X-rays will converge at the focal plane of a detector. This is why NuSTAR has that ridiculously long 33 foot mast: if it didn't, it wouldn't be able to focus properly.
To better illustrate how an X-ray telescope works, here's a diagram of a similar design, from the EPIC camera of the XMM-Newton satellite:
You'll notice that since the reflection surfaces are at such shallow angles, the overall collection area of the telescope is very small, as opposed to a visible-light telescope which has a giant nearly flat mirror that can collect virtually every single photon that runs into it. To make up for this, X-ray telescopes use multiple nested sets of reflecting surfaces called "mirror shells." Lots of them. NuSTAR will use 130 shells, which is three times as many as XMM-Newton, which has an optical system that looked like this:
So, each one of those rings is an individual mirror surface that reflects X-rays to the detector. It's amazing how totally different this X-ray telesope is from a visible light telescope, isn't it?
NuSTAR is scheduled to launch on March 22.