Compton Gamma Ray Observatory (CGRO) The CGRO was a major step forward in gamma rayastronomy. It opened up previously unmearsurable energies in the range from 1MeV to 30 GeV. After five years of observations more than 150 sources havebeen detected by the EGRET observer aboard CGRO. The major downfall ofsatellite observation is the severely limited energy range. The masconsiderations for putting something into space make it nearly impossible toobserve energies above 50 GeV; this threshold for satellite based observationis still several years off.

The Atmospheric Cerenkov Technique (ACT) At energies above those obtainable by satellites (up to several EeV), the best method for observing gamma rays is by utilizing ground based telescopes which employ ACT. By observing the Cerenkov radiation produced by a gamma ray shower, ground based telescopes are able to achieve a very high resolution while eliminated most of the cosmic ray noise. Foremost among the ground based telescopes is the Whipple 10 m reflector.  By employing the imaging technique which they pioneered, the Whipple collaboration has made the initial observation of several AGNs which have, when combined with information from other telescopes observing different parts of the electro-magnetic spectrum, been crucial in the formation of many of the current theories of galaxy formation. The Whipple is able to observe energies ranging from 300 GeV to several TeV.

The Whipple 10 m reflector