Detecting high-energy photons is different from detecting photons in lower energy bands. Overcoming the challenges of high-energy astronomical instrumentation took many years. This is why X-ray and gamma-ray astronomy are still not as far along as astronomy at other wavelengths. (Optical astronomy has been done in one form or another since pre-history, and the optical telescope was invented by Galileo in the 1600s.) This section first deals with some of the challenges which must be overcome; and then enumerates some of the ways that X-ray and gamma-ray photons are detected.