Other Problems with Detecting High-Energy Gamma-Rays
In order to make a clear image, with lots of definition, you need to
have many many photons. Imagine how poor a photograph would look if the
picture was from only a few dots of light, instead of millions and millions.
One of the things that makes high-energy gamma-ray observations
difficult is the fact that just are not that many of these highest
energy photons around. At the highest energies seen by EGRET, there may
be only one photon that hits the detector in a whole year! In order to
make a good observation, then, we have to wait a long time to collect
all of the available photons. The All-Sky map below took 18 months to
collect all of the data.