X-ray photons have a habit of passing through most things that they run into (although, the Earth's atmosphere is composed of just the right stuff to absorb them). This means that if you shoot an X-ray onto a mirror with the hopes of reflecting the photons, you'll probably be disappointed.
Though difficult, it is not impossible to focus an X-ray photon. To do it, you have to have the photon hit your mirror at a grazing incidence. In other words, you want the angle at which the photon comes into the mirror to be very shallow (see animation below).
The way grazing incidence is implemented to make X-ray telescopes will be covered in the X-ray Detector section.
In fact, to date, there has been no true focussing gamma-ray telescope. The problem is that as you go higher and higher in energy, the angle with which you can reflect a photon (as in the grazing incidence X-ray mirrors) becomes shallower and shallower. By the time you reach hard X-ray/soft gamma-ray energies, the angle is so shallow as to be impractical to get any appreciable signal.
This means that other methods must be used to determine the position in the sky of incoming gamma-rays. One method used in the current and next generation of gamma-ray detectors is a coded-aperture. This will be covered briefly in the Gamma-ray Detector section.
Further in the future, when formation-flying of spacecraft has been established, a method for focussing gamma-rays may be feasible. This telescope would employ a lens on one spacecraft which would deflect (slightly) any gamma rays which pass through it. These gamma rays would come to a focus at a detector on a second spacecraft a million kilometers (620,000 miles) away from the lens.
